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Jones Forth fantastic simple 4th implementation written from assembly, that is remarkably readable
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\ -*- text -*- | |
\ A sometimes minimal FORTH compiler and tutorial for Linux / i386 systems. -*- asm -*- | |
\ By Richard W.M. Jones <rich@annexia.org> http://annexia.org/forth | |
\ This is PUBLIC DOMAIN (see public domain release statement below). | |
\ $Id: jonesforth.f,v 1.17 2007/10/12 20:07:44 rich Exp $ | |
\ | |
\ The first part of this tutorial is in jonesforth.S. Get if from http://annexia.org/forth | |
\ | |
\ PUBLIC DOMAIN ---------------------------------------------------------------------- | |
\ | |
\ I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide. | |
\ | |
\ In case this is not legally possible, I grant any entity the right to use this work for any purpose, | |
\ without any conditions, unless such conditions are required by law. | |
\ | |
\ SETTING UP ---------------------------------------------------------------------- | |
\ | |
\ Let's get a few housekeeping things out of the way. Firstly because I need to draw lots of | |
\ ASCII-art diagrams to explain concepts, the best way to look at this is using a window which | |
\ uses a fixed width font and is at least this wide: | |
\ | |
\<------------------------------------------------------------------------------------------------------------------------> | |
\ | |
\ Secondly make sure TABS are set to 8 characters. The following should be a vertical | |
\ line. If not, sort out your tabs. | |
\ | |
\ | | |
\ | | |
\ | | |
\ | |
\ Thirdly I assume that your screen is at least 50 characters high. | |
\ | |
\ START OF FORTH CODE ---------------------------------------------------------------------- | |
\ | |
\ We've now reached the stage where the FORTH system is running and self-hosting. All further | |
\ words can be written as FORTH itself, including words like IF, THEN, .", etc which in most | |
\ languages would be considered rather fundamental. | |
\ | |
\ Some notes about the code: | |
\ | |
\ I use indenting to show structure. The amount of whitespace has no meaning to FORTH however | |
\ except that you must use at least one whitespace character between words, and words themselves | |
\ cannot contain whitespace. | |
\ | |
\ FORTH is case-sensitive. Use capslock! | |
\ The primitive word /MOD (DIVMOD) leaves both the quotient and the remainder on the stack. (On | |
\ i386, the idivl instruction gives both anyway). Now we can define the / and MOD in terms of /MOD | |
\ and a few other primitives. | |
: / /MOD SWAP DROP ; | |
: MOD /MOD DROP ; | |
\ Define some character constants | |
: '\n' 10 ; | |
: BL 32 ; \ BL (BLank) is a standard FORTH word for space. | |
\ CR prints a carriage return | |
: CR '\n' EMIT ; | |
\ SPACE prints a space | |
: SPACE BL EMIT ; | |
\ NEGATE leaves the negative of a number on the stack. | |
: NEGATE 0 SWAP - ; | |
\ Standard words for booleans. | |
: TRUE 1 ; | |
: FALSE 0 ; | |
: NOT 0= ; | |
\ LITERAL takes whatever is on the stack and compiles LIT <foo> | |
: LITERAL IMMEDIATE | |
' LIT , \ compile LIT | |
, \ compile the literal itself (from the stack) | |
; | |
\ Now we can use [ and ] to insert literals which are calculated at compile time. (Recall that | |
\ [ and ] are the FORTH words which switch into and out of immediate mode.) | |
\ Within definitions, use [ ... ] LITERAL anywhere that '...' is a constant expression which you | |
\ would rather only compute once (at compile time, rather than calculating it each time your word runs). | |
: ':' | |
[ \ go into immediate mode (temporarily) | |
CHAR : \ push the number 58 (ASCII code of colon) on the parameter stack | |
] \ go back to compile mode | |
LITERAL \ compile LIT 58 as the definition of ':' word | |
; | |
\ A few more character constants defined the same way as above. | |
: ';' [ CHAR ; ] LITERAL ; | |
: '(' [ CHAR ( ] LITERAL ; | |
: ')' [ CHAR ) ] LITERAL ; | |
: '"' [ CHAR " ] LITERAL ; | |
: 'A' [ CHAR A ] LITERAL ; | |
: '0' [ CHAR 0 ] LITERAL ; | |
: '-' [ CHAR - ] LITERAL ; | |
: '.' [ CHAR . ] LITERAL ; | |
\ While compiling, '[COMPILE] word' compiles 'word' if it would otherwise be IMMEDIATE. | |
: [COMPILE] IMMEDIATE | |
WORD \ get the next word | |
FIND \ find it in the dictionary | |
>CFA \ get its codeword | |
, \ and compile that | |
; | |
\ RECURSE makes a recursive call to the current word that is being compiled. | |
\ | |
\ Normally while a word is being compiled, it is marked HIDDEN so that references to the | |
\ same word within are calls to the previous definition of the word. However we still have | |
\ access to the word which we are currently compiling through the LATEST pointer so we | |
\ can use that to compile a recursive call. | |
: RECURSE IMMEDIATE | |
LATEST @ \ LATEST points to the word being compiled at the moment | |
>CFA \ get the codeword | |
, \ compile it | |
; | |
\ CONTROL STRUCTURES ---------------------------------------------------------------------- | |
\ | |
\ So far we have defined only very simple definitions. Before we can go further, we really need to | |
\ make some control structures, like IF ... THEN and loops. Luckily we can define arbitrary control | |
\ structures directly in FORTH. | |
\ | |
\ Please note that the control structures as I have defined them here will only work inside compiled | |
\ words. If you try to type in expressions using IF, etc. in immediate mode, then they won't work. | |
\ Making these work in immediate mode is left as an exercise for the reader. | |
\ condition IF true-part THEN rest | |
\ -- compiles to: --> condition 0BRANCH OFFSET true-part rest | |
\ where OFFSET is the offset of 'rest' | |
\ condition IF true-part ELSE false-part THEN | |
\ -- compiles to: --> condition 0BRANCH OFFSET true-part BRANCH OFFSET2 false-part rest | |
\ where OFFSET if the offset of false-part and OFFSET2 is the offset of rest | |
\ IF is an IMMEDIATE word which compiles 0BRANCH followed by a dummy offset, and places | |
\ the address of the 0BRANCH on the stack. Later when we see THEN, we pop that address | |
\ off the stack, calculate the offset, and back-fill the offset. | |
: IF IMMEDIATE | |
' 0BRANCH , \ compile 0BRANCH | |
HERE @ \ save location of the offset on the stack | |
0 , \ compile a dummy offset | |
; | |
: THEN IMMEDIATE | |
DUP | |
HERE @ SWAP - \ calculate the offset from the address saved on the stack | |
SWAP ! \ store the offset in the back-filled location | |
; | |
: ELSE IMMEDIATE | |
' BRANCH , \ definite branch to just over the false-part | |
HERE @ \ save location of the offset on the stack | |
0 , \ compile a dummy offset | |
SWAP \ now back-fill the original (IF) offset | |
DUP \ same as for THEN word above | |
HERE @ SWAP - | |
SWAP ! | |
; | |
\ BEGIN loop-part condition UNTIL | |
\ -- compiles to: --> loop-part condition 0BRANCH OFFSET | |
\ where OFFSET points back to the loop-part | |
\ This is like do { loop-part } while (condition) in the C language | |
: BEGIN IMMEDIATE | |
HERE @ \ save location on the stack | |
; | |
: UNTIL IMMEDIATE | |
' 0BRANCH , \ compile 0BRANCH | |
HERE @ - \ calculate the offset from the address saved on the stack | |
, \ compile the offset here | |
; | |
\ BEGIN loop-part AGAIN | |
\ -- compiles to: --> loop-part BRANCH OFFSET | |
\ where OFFSET points back to the loop-part | |
\ In other words, an infinite loop which can only be returned from with EXIT | |
: AGAIN IMMEDIATE | |
' BRANCH , \ compile BRANCH | |
HERE @ - \ calculate the offset back | |
, \ compile the offset here | |
; | |
\ BEGIN condition WHILE loop-part REPEAT | |
\ -- compiles to: --> condition 0BRANCH OFFSET2 loop-part BRANCH OFFSET | |
\ where OFFSET points back to condition (the beginning) and OFFSET2 points to after the whole piece of code | |
\ So this is like a while (condition) { loop-part } loop in the C language | |
: WHILE IMMEDIATE | |
' 0BRANCH , \ compile 0BRANCH | |
HERE @ \ save location of the offset2 on the stack | |
0 , \ compile a dummy offset2 | |
; | |
: REPEAT IMMEDIATE | |
' BRANCH , \ compile BRANCH | |
SWAP \ get the original offset (from BEGIN) | |
HERE @ - , \ and compile it after BRANCH | |
DUP | |
HERE @ SWAP - \ calculate the offset2 | |
SWAP ! \ and back-fill it in the original location | |
; | |
\ UNLESS is the same as IF but the test is reversed. | |
\ | |
\ Note the use of [COMPILE]: Since IF is IMMEDIATE we don't want it to be executed while UNLESS | |
\ is compiling, but while UNLESS is running (which happens to be when whatever word using UNLESS is | |
\ being compiled -- whew!). So we use [COMPILE] to reverse the effect of marking IF as immediate. | |
\ This trick is generally used when we want to write our own control words without having to | |
\ implement them all in terms of the primitives 0BRANCH and BRANCH, but instead reusing simpler | |
\ control words like (in this instance) IF. | |
: UNLESS IMMEDIATE | |
' NOT , \ compile NOT (to reverse the test) | |
[COMPILE] IF \ continue by calling the normal IF | |
; | |
\ COMMENTS ---------------------------------------------------------------------- | |
\ | |
\ FORTH allows ( ... ) as comments within function definitions. This works by having an IMMEDIATE | |
\ word called ( which just drops input characters until it hits the corresponding ). | |
: ( IMMEDIATE | |
1 \ allowed nested parens by keeping track of depth | |
BEGIN | |
KEY \ read next character | |
DUP '(' = IF \ open paren? | |
DROP \ drop the open paren | |
1+ \ depth increases | |
ELSE | |
')' = IF \ close paren? | |
1- \ depth decreases | |
THEN | |
THEN | |
DUP 0= UNTIL \ continue until we reach matching close paren, depth 0 | |
DROP \ drop the depth counter | |
; | |
( | |
From now on we can use ( ... ) for comments. | |
STACK NOTATION ---------------------------------------------------------------------- | |
In FORTH style we can also use ( ... -- ... ) to show the effects that a word has on the | |
parameter stack. For example: | |
( n -- ) means that the word consumes an integer (n) from the parameter stack. | |
( b a -- c ) means that the word uses two integers (a and b, where a is at the top of stack) | |
and returns a single integer (c). | |
( -- ) means the word has no effect on the stack | |
) | |
( Some more complicated stack examples, showing the stack notation. ) | |
: NIP ( x y -- y ) SWAP DROP ; | |
: TUCK ( x y -- y x y ) DUP ROT ; | |
: PICK ( x_u ... x_1 x_0 u -- x_u ... x_1 x_0 x_u ) | |
1+ ( add one because of 'u' on the stack ) | |
4 * ( multiply by the word size ) | |
DSP@ + ( add to the stack pointer ) | |
@ ( and fetch ) | |
; | |
( With the looping constructs, we can now write SPACES, which writes n spaces to stdout. ) | |
: SPACES ( n -- ) | |
BEGIN | |
DUP 0> ( while n > 0 ) | |
WHILE | |
SPACE ( print a space ) | |
1- ( until we count down to 0 ) | |
REPEAT | |
DROP | |
; | |
( Standard words for manipulating BASE. ) | |
: DECIMAL ( -- ) 10 BASE ! ; | |
: HEX ( -- ) 16 BASE ! ; | |
( | |
PRINTING NUMBERS ---------------------------------------------------------------------- | |
The standard FORTH word . (DOT) is very important. It takes the number at the top | |
of the stack and prints it out. However first I'm going to implement some lower-level | |
FORTH words: | |
U.R ( u width -- ) which prints an unsigned number, padded to a certain width | |
U. ( u -- ) which prints an unsigned number | |
.R ( n width -- ) which prints a signed number, padded to a certain width. | |
For example: | |
-123 6 .R | |
will print out these characters: | |
<space> <space> - 1 2 3 | |
In other words, the number padded left to a certain number of characters. | |
The full number is printed even if it is wider than width, and this is what allows us to | |
define the ordinary functions U. and . (we just set width to zero knowing that the full | |
number will be printed anyway). | |
Another wrinkle of . and friends is that they obey the current base in the variable BASE. | |
BASE can be anything in the range 2 to 36. | |
While we're defining . &c we can also define .S which is a useful debugging tool. This | |
word prints the current stack (non-destructively) from top to bottom. | |
) | |
( This is the underlying recursive definition of U. ) | |
: U. ( u -- ) | |
BASE @ /MOD ( width rem quot ) | |
?DUP IF ( if quotient <> 0 then ) | |
RECURSE ( print the quotient ) | |
THEN | |
( print the remainder ) | |
DUP 10 < IF | |
'0' ( decimal digits 0..9 ) | |
ELSE | |
10 - ( hex and beyond digits A..Z ) | |
'A' | |
THEN | |
+ | |
EMIT | |
; | |
( | |
FORTH word .S prints the contents of the stack. It doesn't alter the stack. | |
Very useful for debugging. | |
) | |
: .S ( -- ) | |
DSP@ ( get current stack pointer ) | |
BEGIN | |
DUP S0 @ < | |
WHILE | |
DUP @ U. ( print the stack element ) | |
SPACE | |
4+ ( move up ) | |
REPEAT | |
DROP | |
; | |
( This word returns the width (in characters) of an unsigned number in the current base ) | |
: UWIDTH ( u -- width ) | |
BASE @ / ( rem quot ) | |
?DUP IF ( if quotient <> 0 then ) | |
RECURSE 1+ ( return 1+recursive call ) | |
ELSE | |
1 ( return 1 ) | |
THEN | |
; | |
: U.R ( u width -- ) | |
SWAP ( width u ) | |
DUP ( width u u ) | |
UWIDTH ( width u uwidth ) | |
-ROT ( u uwidth width ) | |
SWAP - ( u width-uwidth ) | |
( At this point if the requested width is narrower, we'll have a negative number on the stack. | |
Otherwise the number on the stack is the number of spaces to print. But SPACES won't print | |
a negative number of spaces anyway, so it's now safe to call SPACES ... ) | |
SPACES | |
( ... and then call the underlying implementation of U. ) | |
U. | |
; | |
( | |
.R prints a signed number, padded to a certain width. We can't just print the sign | |
and call U.R because we want the sign to be next to the number ('-123' instead of '- 123'). | |
) | |
: .R ( n width -- ) | |
SWAP ( width n ) | |
DUP 0< IF | |
NEGATE ( width u ) | |
1 ( save a flag to remember that it was negative | width n 1 ) | |
ROT ( 1 width u ) | |
SWAP ( 1 u width ) | |
1- ( 1 u width-1 ) | |
ELSE | |
0 ( width u 0 ) | |
ROT ( 0 width u ) | |
SWAP ( 0 u width ) | |
THEN | |
SWAP ( flag width u ) | |
DUP ( flag width u u ) | |
UWIDTH ( flag width u uwidth ) | |
-ROT ( flag u uwidth width ) | |
SWAP - ( flag u width-uwidth ) | |
SPACES ( flag u ) | |
SWAP ( u flag ) | |
IF ( was it negative? print the - character ) | |
'-' EMIT | |
THEN | |
U. | |
; | |
( Finally we can define word . in terms of .R, with a trailing space. ) | |
: . 0 .R SPACE ; | |
( The real U., note the trailing space. ) | |
: U. U. SPACE ; | |
( ? fetches the integer at an address and prints it. ) | |
: ? ( addr -- ) @ . ; | |
( c a b WITHIN returns true if a <= c and c < b ) | |
: WITHIN | |
ROT ( b c a ) | |
OVER ( b c a c ) | |
<= IF | |
> IF ( b c -- ) | |
TRUE | |
ELSE | |
FALSE | |
THEN | |
ELSE | |
2DROP ( b c -- ) | |
FALSE | |
THEN | |
; | |
( DEPTH returns the depth of the stack. ) | |
: DEPTH ( -- n ) | |
S0 @ DSP@ - | |
4- ( adjust because S0 was on the stack when we pushed DSP ) | |
; | |
( | |
ALIGNED takes an address and rounds it up (aligns it) to the next 4 byte boundary. | |
) | |
: ALIGNED ( addr -- addr ) | |
3 + 3 INVERT AND ( (addr+3) & ~3 ) | |
; | |
( | |
ALIGN aligns the HERE pointer, so the next word appended will be aligned properly. | |
) | |
: ALIGN HERE @ ALIGNED HERE ! ; | |
( | |
STRINGS ---------------------------------------------------------------------- | |
S" string" is used in FORTH to define strings. It leaves the address of the string and | |
its length on the stack, (length at the top of stack). The space following S" is the normal | |
space between FORTH words and is not a part of the string. | |
This is tricky to define because it has to do different things depending on whether | |
we are compiling or in immediate mode. (Thus the word is marked IMMEDIATE so it can | |
detect this and do different things). | |
In compile mode we append | |
LITSTRING <string length> <string rounded up 4 bytes> | |
to the current word. The primitive LITSTRING does the right thing when the current | |
word is executed. | |
In immediate mode there isn't a particularly good place to put the string, but in this | |
case we put the string at HERE (but we _don't_ change HERE). This is meant as a temporary | |
location, likely to be overwritten soon after. | |
) | |
( C, appends a byte to the current compiled word. ) | |
: C, | |
HERE @ C! ( store the character in the compiled image ) | |
1 HERE +! ( increment HERE pointer by 1 byte ) | |
; | |
: S" IMMEDIATE ( -- addr len ) | |
STATE @ IF ( compiling? ) | |
' LITSTRING , ( compile LITSTRING ) | |
HERE @ ( save the address of the length word on the stack ) | |
0 , ( dummy length - we don't know what it is yet ) | |
BEGIN | |
KEY ( get next character of the string ) | |
DUP '"' <> | |
WHILE | |
C, ( copy character ) | |
REPEAT | |
DROP ( drop the double quote character at the end ) | |
DUP ( get the saved address of the length word ) | |
HERE @ SWAP - ( calculate the length ) | |
4- ( subtract 4 (because we measured from the start of the length word) ) | |
SWAP ! ( and back-fill the length location ) | |
ALIGN ( round up to next multiple of 4 bytes for the remaining code ) | |
ELSE ( immediate mode ) | |
HERE @ ( get the start address of the temporary space ) | |
BEGIN | |
KEY | |
DUP '"' <> | |
WHILE | |
OVER C! ( save next character ) | |
1+ ( increment address ) | |
REPEAT | |
DROP ( drop the final " character ) | |
HERE @ - ( calculate the length ) | |
HERE @ ( push the start address ) | |
SWAP ( addr len ) | |
THEN | |
; | |
( | |
." is the print string operator in FORTH. Example: ." Something to print" | |
The space after the operator is the ordinary space required between words and is not | |
a part of what is printed. | |
In immediate mode we just keep reading characters and printing them until we get to | |
the next double quote. | |
In compile mode we use S" to store the string, then add TELL afterwards: | |
LITSTRING <string length> <string rounded up to 4 bytes> TELL | |
It may be interesting to note the use of [COMPILE] to turn the call to the immediate | |
word S" into compilation of that word. It compiles it into the definition of .", | |
not into the definition of the word being compiled when this is running (complicated | |
enough for you?) | |
) | |
: ." IMMEDIATE ( -- ) | |
STATE @ IF ( compiling? ) | |
[COMPILE] S" ( read the string, and compile LITSTRING, etc. ) | |
' TELL , ( compile the final TELL ) | |
ELSE | |
( In immediate mode, just read characters and print them until we get | |
to the ending double quote. ) | |
BEGIN | |
KEY | |
DUP '"' = IF | |
DROP ( drop the double quote character ) | |
EXIT ( return from this function ) | |
THEN | |
EMIT | |
AGAIN | |
THEN | |
; | |
( | |
CONSTANTS AND VARIABLES ---------------------------------------------------------------------- | |
In FORTH, global constants and variables are defined like this: | |
10 CONSTANT TEN when TEN is executed, it leaves the integer 10 on the stack | |
VARIABLE VAR when VAR is executed, it leaves the address of VAR on the stack | |
Constants can be read but not written, eg: | |
TEN . CR prints 10 | |
You can read a variable (in this example called VAR) by doing: | |
VAR @ leaves the value of VAR on the stack | |
VAR @ . CR prints the value of VAR | |
VAR ? CR same as above, since ? is the same as @ . | |
and update the variable by doing: | |
20 VAR ! sets VAR to 20 | |
Note that variables are uninitialised (but see VALUE later on which provides initialised | |
variables with a slightly simpler syntax). | |
How can we define the words CONSTANT and VARIABLE? | |
The trick is to define a new word for the variable itself (eg. if the variable was called | |
'VAR' then we would define a new word called VAR). This is easy to do because we exposed | |
dictionary entry creation through the CREATE word (part of the definition of : above). | |
A call to WORD [TEN] CREATE (where [TEN] means that "TEN" is the next word in the input) | |
leaves the dictionary entry: | |
+--- HERE | |
| | |
V | |
+---------+---+---+---+---+ | |
| LINK | 3 | T | E | N | | |
+---------+---+---+---+---+ | |
len | |
For CONSTANT we can continue by appending DOCOL (the codeword), then LIT followed by | |
the constant itself and then EXIT, forming a little word definition that returns the | |
constant: | |
+---------+---+---+---+---+------------+------------+------------+------------+ | |
| LINK | 3 | T | E | N | DOCOL | LIT | 10 | EXIT | | |
+---------+---+---+---+---+------------+------------+------------+------------+ | |
len codeword | |
Notice that this word definition is exactly the same as you would have got if you had | |
written : TEN 10 ; | |
Note for people reading the code below: DOCOL is a constant word which we defined in the | |
assembler part which returns the value of the assembler symbol of the same name. | |
) | |
: CONSTANT | |
WORD ( get the name (the name follows CONSTANT) ) | |
CREATE ( make the dictionary entry ) | |
DOCOL , ( append DOCOL (the codeword field of this word) ) | |
' LIT , ( append the codeword LIT ) | |
, ( append the value on the top of the stack ) | |
' EXIT , ( append the codeword EXIT ) | |
; | |
( | |
VARIABLE is a little bit harder because we need somewhere to put the variable. There is | |
nothing particularly special about the user memory (the area of memory pointed to by HERE | |
where we have previously just stored new word definitions). We can slice off bits of this | |
memory area to store anything we want, so one possible definition of VARIABLE might create | |
this: | |
+--------------------------------------------------------------+ | |
| | | |
V | | |
+---------+---------+---+---+---+---+------------+------------+---|--------+------------+ | |
| <var> | LINK | 3 | V | A | R | DOCOL | LIT | <addr var> | EXIT | | |
+---------+---------+---+---+---+---+------------+------------+------------+------------+ | |
len codeword | |
where <var> is the place to store the variable, and <addr var> points back to it. | |
To make this more general let's define a couple of words which we can use to allocate | |
arbitrary memory from the user memory. | |
First ALLOT, where n ALLOT allocates n bytes of memory. (Note when calling this that | |
it's a very good idea to make sure that n is a multiple of 4, or at least that next time | |
a word is compiled that HERE has been left as a multiple of 4). | |
) | |
: ALLOT ( n -- addr ) | |
HERE @ SWAP ( here n ) | |
HERE +! ( adds n to HERE, after this the old value of HERE is still on the stack ) | |
; | |
( | |
Second, CELLS. In FORTH the phrase 'n CELLS ALLOT' means allocate n integers of whatever size | |
is the natural size for integers on this machine architecture. On this 32 bit machine therefore | |
CELLS just multiplies the top of stack by 4. | |
) | |
: CELLS ( n -- n ) 4 * ; | |
( | |
So now we can define VARIABLE easily in much the same way as CONSTANT above. Refer to the | |
diagram above to see what the word that this creates will look like. | |
) | |
: VARIABLE | |
1 CELLS ALLOT ( allocate 1 cell of memory, push the pointer to this memory ) | |
WORD CREATE ( make the dictionary entry (the name follows VARIABLE) ) | |
DOCOL , ( append DOCOL (the codeword field of this word) ) | |
' LIT , ( append the codeword LIT ) | |
, ( append the pointer to the new memory ) | |
' EXIT , ( append the codeword EXIT ) | |
; | |
( | |
VALUES ---------------------------------------------------------------------- | |
VALUEs are like VARIABLEs but with a simpler syntax. You would generally use them when you | |
want a variable which is read often, and written infrequently. | |
20 VALUE VAL creates VAL with initial value 20 | |
VAL pushes the value (20) directly on the stack | |
30 TO VAL updates VAL, setting it to 30 | |
VAL pushes the value (30) directly on the stack | |
Notice that 'VAL' on its own doesn't return the address of the value, but the value itself, | |
making values simpler and more obvious to use than variables (no indirection through '@'). | |
The price is a more complicated implementation, although despite the complexity there is no | |
performance penalty at runtime. | |
A naive implementation of 'TO' would be quite slow, involving a dictionary search each time. | |
But because this is FORTH we have complete control of the compiler so we can compile TO more | |
efficiently, turning: | |
TO VAL | |
into: | |
LIT <addr> ! | |
and calculating <addr> (the address of the value) at compile time. | |
Now this is the clever bit. We'll compile our value like this: | |
+---------+---+---+---+---+------------+------------+------------+------------+ | |
| LINK | 3 | V | A | L | DOCOL | LIT | <value> | EXIT | | |
+---------+---+---+---+---+------------+------------+------------+------------+ | |
len codeword | |
where <value> is the actual value itself. Note that when VAL executes, it will push the | |
value on the stack, which is what we want. | |
But what will TO use for the address <addr>? Why of course a pointer to that <value>: | |
code compiled - - - - --+------------+------------+------------+-- - - - - | |
by TO VAL | LIT | <addr> | ! | | |
- - - - --+------------+-----|------+------------+-- - - - - | |
| | |
V | |
+---------+---+---+---+---+------------+------------+------------+------------+ | |
| LINK | 3 | V | A | L | DOCOL | LIT | <value> | EXIT | | |
+---------+---+---+---+---+------------+------------+------------+------------+ | |
len codeword | |
In other words, this is a kind of self-modifying code. | |
(Note to the people who want to modify this FORTH to add inlining: values defined this | |
way cannot be inlined). | |
) | |
: VALUE ( n -- ) | |
WORD CREATE ( make the dictionary entry (the name follows VALUE) ) | |
DOCOL , ( append DOCOL ) | |
' LIT , ( append the codeword LIT ) | |
, ( append the initial value ) | |
' EXIT , ( append the codeword EXIT ) | |
; | |
: TO IMMEDIATE ( n -- ) | |
WORD ( get the name of the value ) | |
FIND ( look it up in the dictionary ) | |
>DFA ( get a pointer to the first data field (the 'LIT') ) | |
4+ ( increment to point at the value ) | |
STATE @ IF ( compiling? ) | |
' LIT , ( compile LIT ) | |
, ( compile the address of the value ) | |
' ! , ( compile ! ) | |
ELSE ( immediate mode ) | |
! ( update it straightaway ) | |
THEN | |
; | |
( x +TO VAL adds x to VAL ) | |
: +TO IMMEDIATE | |
WORD ( get the name of the value ) | |
FIND ( look it up in the dictionary ) | |
>DFA ( get a pointer to the first data field (the 'LIT') ) | |
4+ ( increment to point at the value ) | |
STATE @ IF ( compiling? ) | |
' LIT , ( compile LIT ) | |
, ( compile the address of the value ) | |
' +! , ( compile +! ) | |
ELSE ( immediate mode ) | |
+! ( update it straightaway ) | |
THEN | |
; | |
( | |
PRINTING THE DICTIONARY ---------------------------------------------------------------------- | |
ID. takes an address of a dictionary entry and prints the word's name. | |
For example: LATEST @ ID. would print the name of the last word that was defined. | |
) | |
: ID. | |
4+ ( skip over the link pointer ) | |
DUP C@ ( get the flags/length byte ) | |
F_LENMASK AND ( mask out the flags - just want the length ) | |
BEGIN | |
DUP 0> ( length > 0? ) | |
WHILE | |
SWAP 1+ ( addr len -- len addr+1 ) | |
DUP C@ ( len addr -- len addr char | get the next character) | |
EMIT ( len addr char -- len addr | and print it) | |
SWAP 1- ( len addr -- addr len-1 | subtract one from length ) | |
REPEAT | |
2DROP ( len addr -- ) | |
; | |
( | |
'WORD word FIND ?HIDDEN' returns true if 'word' is flagged as hidden. | |
'WORD word FIND ?IMMEDIATE' returns true if 'word' is flagged as immediate. | |
) | |
: ?HIDDEN | |
4+ ( skip over the link pointer ) | |
C@ ( get the flags/length byte ) | |
F_HIDDEN AND ( mask the F_HIDDEN flag and return it (as a truth value) ) | |
; | |
: ?IMMEDIATE | |
4+ ( skip over the link pointer ) | |
C@ ( get the flags/length byte ) | |
F_IMMED AND ( mask the F_IMMED flag and return it (as a truth value) ) | |
; | |
( | |
WORDS prints all the words defined in the dictionary, starting with the word defined most recently. | |
However it doesn't print hidden words. | |
The implementation simply iterates backwards from LATEST using the link pointers. | |
) | |
: WORDS | |
LATEST @ ( start at LATEST dictionary entry ) | |
BEGIN | |
?DUP ( while link pointer is not null ) | |
WHILE | |
DUP ?HIDDEN NOT IF ( ignore hidden words ) | |
DUP ID. ( but if not hidden, print the word ) | |
SPACE | |
THEN | |
@ ( dereference the link pointer - go to previous word ) | |
REPEAT | |
CR | |
; | |
( | |
FORGET ---------------------------------------------------------------------- | |
So far we have only allocated words and memory. FORTH provides a rather primitive method | |
to deallocate. | |
'FORGET word' deletes the definition of 'word' from the dictionary and everything defined | |
after it, including any variables and other memory allocated after. | |
The implementation is very simple - we look up the word (which returns the dictionary entry | |
address). Then we set HERE to point to that address, so in effect all future allocations | |
and definitions will overwrite memory starting at the word. We also need to set LATEST to | |
point to the previous word. | |
Note that you cannot FORGET built-in words (well, you can try but it will probably cause | |
a segfault). | |
XXX: Because we wrote VARIABLE to store the variable in memory allocated before the word, | |
in the current implementation VARIABLE FOO FORGET FOO will leak 1 cell of memory. | |
) | |
: FORGET | |
WORD FIND ( find the word, gets the dictionary entry address ) | |
DUP @ LATEST ! ( set LATEST to point to the previous word ) | |
HERE ! ( and store HERE with the dictionary address ) | |
; | |
( | |
DUMP ---------------------------------------------------------------------- | |
DUMP is used to dump out the contents of memory, in the 'traditional' hexdump format. | |
Notice that the parameters to DUMP (address, length) are compatible with string words | |
such as WORD and S". | |
You can dump out the raw code for the last word you defined by doing something like: | |
LATEST @ 128 DUMP | |
) | |
: DUMP ( addr len -- ) | |
BASE @ ROT ( save the current BASE at the bottom of the stack ) | |
HEX ( and switch to hexadecimal mode ) | |
BEGIN | |
?DUP ( while len > 0 ) | |
WHILE | |
OVER 8 U.R ( print the address ) | |
SPACE | |
( print up to 16 words on this line ) | |
2DUP ( addr len addr len ) | |
1- 15 AND 1+ ( addr len addr linelen ) | |
BEGIN | |
?DUP ( while linelen > 0 ) | |
WHILE | |
SWAP ( addr len linelen addr ) | |
DUP C@ ( addr len linelen addr byte ) | |
2 .R SPACE ( print the byte ) | |
1+ SWAP 1- ( addr len linelen addr -- addr len addr+1 linelen-1 ) | |
REPEAT | |
DROP ( addr len ) | |
( print the ASCII equivalents ) | |
2DUP 1- 15 AND 1+ ( addr len addr linelen ) | |
BEGIN | |
?DUP ( while linelen > 0) | |
WHILE | |
SWAP ( addr len linelen addr ) | |
DUP C@ ( addr len linelen addr byte ) | |
DUP 32 128 WITHIN IF ( 32 <= c < 128? ) | |
EMIT | |
ELSE | |
DROP '.' EMIT | |
THEN | |
1+ SWAP 1- ( addr len linelen addr -- addr len addr+1 linelen-1 ) | |
REPEAT | |
DROP ( addr len ) | |
CR | |
DUP 1- 15 AND 1+ ( addr len linelen ) | |
DUP ( addr len linelen linelen ) | |
ROT ( addr linelen len linelen ) | |
- ( addr linelen len-linelen ) | |
ROT ( len-linelen addr linelen ) | |
+ ( len-linelen addr+linelen ) | |
SWAP ( addr-linelen len-linelen ) | |
REPEAT | |
DROP ( restore stack ) | |
BASE ! ( restore saved BASE ) | |
; | |
( | |
CASE ---------------------------------------------------------------------- | |
CASE...ENDCASE is how we do switch statements in FORTH. There is no generally | |
agreed syntax for this, so I've gone for the syntax mandated by the ISO standard | |
FORTH (ANS-FORTH). | |
( some value on the stack ) | |
CASE | |
test1 OF ... ENDOF | |
test2 OF ... ENDOF | |
testn OF ... ENDOF | |
... ( default case ) | |
ENDCASE | |
The CASE statement tests the value on the stack by comparing it for equality with | |
test1, test2, ..., testn and executes the matching piece of code within OF ... ENDOF. | |
If none of the test values match then the default case is executed. Inside the ... of | |
the default case, the value is still at the top of stack (it is implicitly DROP-ed | |
by ENDCASE). When ENDOF is executed it jumps after ENDCASE (ie. there is no "fall-through" | |
and no need for a break statement like in C). | |
The default case may be omitted. In fact the tests may also be omitted so that you | |
just have a default case, although this is probably not very useful. | |
An example (assuming that 'q', etc. are words which push the ASCII value of the letter | |
on the stack): | |
0 VALUE QUIT | |
0 VALUE SLEEP | |
KEY CASE | |
'q' OF 1 TO QUIT ENDOF | |
's' OF 1 TO SLEEP ENDOF | |
( default case: ) | |
." Sorry, I didn't understand key <" DUP EMIT ." >, try again." CR | |
ENDCASE | |
(In some versions of FORTH, more advanced tests are supported, such as ranges, etc. | |
Other versions of FORTH need you to write OTHERWISE to indicate the default case. | |
As I said above, this FORTH tries to follow the ANS FORTH standard). | |
The implementation of CASE...ENDCASE is somewhat non-trivial. I'm following the | |
implementations from here: | |
http://www.uni-giessen.de/faq/archiv/forthfaq.case_endcase/msg00000.html | |
The general plan is to compile the code as a series of IF statements: | |
CASE (push 0 on the immediate-mode parameter stack) | |
test1 OF ... ENDOF test1 OVER = IF DROP ... ELSE | |
test2 OF ... ENDOF test2 OVER = IF DROP ... ELSE | |
testn OF ... ENDOF testn OVER = IF DROP ... ELSE | |
... ( default case ) ... | |
ENDCASE DROP THEN [THEN [THEN ...]] | |
The CASE statement pushes 0 on the immediate-mode parameter stack, and that number | |
is used to count how many THEN statements we need when we get to ENDCASE so that each | |
IF has a matching THEN. The counting is done implicitly. If you recall from the | |
implementation above of IF, each IF pushes a code address on the immediate-mode stack, | |
and these addresses are non-zero, so by the time we get to ENDCASE the stack contains | |
some number of non-zeroes, followed by a zero. The number of non-zeroes is how many | |
times IF has been called, so how many times we need to match it with THEN. | |
This code uses [COMPILE] so that we compile calls to IF, ELSE, THEN instead of | |
actually calling them while we're compiling the words below. | |
As is the case with all of our control structures, they only work within word | |
definitions, not in immediate mode. | |
) | |
: CASE IMMEDIATE | |
0 ( push 0 to mark the bottom of the stack ) | |
; | |
: OF IMMEDIATE | |
' OVER , ( compile OVER ) | |
' = , ( compile = ) | |
[COMPILE] IF ( compile IF ) | |
' DROP , ( compile DROP ) | |
; | |
: ENDOF IMMEDIATE | |
[COMPILE] ELSE ( ENDOF is the same as ELSE ) | |
; | |
: ENDCASE IMMEDIATE | |
' DROP , ( compile DROP ) | |
( keep compiling THEN until we get to our zero marker ) | |
BEGIN | |
?DUP | |
WHILE | |
[COMPILE] THEN | |
REPEAT | |
; | |
( | |
DECOMPILER ---------------------------------------------------------------------- | |
CFA> is the opposite of >CFA. It takes a codeword and tries to find the matching | |
dictionary definition. (In truth, it works with any pointer into a word, not just | |
the codeword pointer, and this is needed to do stack traces). | |
In this FORTH this is not so easy. In fact we have to search through the dictionary | |
because we don't have a convenient back-pointer (as is often the case in other versions | |
of FORTH). Because of this search, CFA> should not be used when performance is critical, | |
so it is only used for debugging tools such as the decompiler and printing stack | |
traces. | |
This word returns 0 if it doesn't find a match. | |
) | |
: CFA> | |
LATEST @ ( start at LATEST dictionary entry ) | |
BEGIN | |
?DUP ( while link pointer is not null ) | |
WHILE | |
2DUP SWAP ( cfa curr curr cfa ) | |
< IF ( current dictionary entry < cfa? ) | |
NIP ( leave curr dictionary entry on the stack ) | |
EXIT | |
THEN | |
@ ( follow link pointer back ) | |
REPEAT | |
DROP ( restore stack ) | |
0 ( sorry, nothing found ) | |
; | |
( | |
SEE decompiles a FORTH word. | |
We search for the dictionary entry of the word, then search again for the next | |
word (effectively, the end of the compiled word). This results in two pointers: | |
+---------+---+---+---+---+------------+------------+------------+------------+ | |
| LINK | 3 | T | E | N | DOCOL | LIT | 10 | EXIT | | |
+---------+---+---+---+---+------------+------------+------------+------------+ | |
^ ^ | |
| | | |
Start of word End of word | |
With this information we can have a go at decompiling the word. We need to | |
recognise "meta-words" like LIT, LITSTRING, BRANCH, etc. and treat those separately. | |
) | |
: SEE | |
WORD FIND ( find the dictionary entry to decompile ) | |
( Now we search again, looking for the next word in the dictionary. This gives us | |
the length of the word that we will be decompiling. (Well, mostly it does). ) | |
HERE @ ( address of the end of the last compiled word ) | |
LATEST @ ( word last curr ) | |
BEGIN | |
2 PICK ( word last curr word ) | |
OVER ( word last curr word curr ) | |
<> ( word last curr word<>curr? ) | |
WHILE ( word last curr ) | |
NIP ( word curr ) | |
DUP @ ( word curr prev (which becomes: word last curr) ) | |
REPEAT | |
DROP ( at this point, the stack is: start-of-word end-of-word ) | |
SWAP ( end-of-word start-of-word ) | |
( begin the definition with : NAME [IMMEDIATE] ) | |
':' EMIT SPACE DUP ID. SPACE | |
DUP ?IMMEDIATE IF ." IMMEDIATE " THEN | |
>DFA ( get the data address, ie. points after DOCOL | end-of-word start-of-data ) | |
( now we start decompiling until we hit the end of the word ) | |
BEGIN ( end start ) | |
2DUP > | |
WHILE | |
DUP @ ( end start codeword ) | |
CASE | |
' LIT OF ( is it LIT ? ) | |
4 + DUP @ ( get next word which is the integer constant ) | |
. ( and print it ) | |
ENDOF | |
' LITSTRING OF ( is it LITSTRING ? ) | |
[ CHAR S ] LITERAL EMIT '"' EMIT SPACE ( print S"<space> ) | |
4 + DUP @ ( get the length word ) | |
SWAP 4 + SWAP ( end start+4 length ) | |
2DUP TELL ( print the string ) | |
'"' EMIT SPACE ( finish the string with a final quote ) | |
+ ALIGNED ( end start+4+len, aligned ) | |
4 - ( because we're about to add 4 below ) | |
ENDOF | |
' 0BRANCH OF ( is it 0BRANCH ? ) | |
." 0BRANCH ( " | |
4 + DUP @ ( print the offset ) | |
. | |
." ) " | |
ENDOF | |
' BRANCH OF ( is it BRANCH ? ) | |
." BRANCH ( " | |
4 + DUP @ ( print the offset ) | |
. | |
." ) " | |
ENDOF | |
' ' OF ( is it ' (TICK) ? ) | |
[ CHAR ' ] LITERAL EMIT SPACE | |
4 + DUP @ ( get the next codeword ) | |
CFA> ( and force it to be printed as a dictionary entry ) | |
ID. SPACE | |
ENDOF | |
' EXIT OF ( is it EXIT? ) | |
( We expect the last word to be EXIT, and if it is then we don't print it | |
because EXIT is normally implied by ;. EXIT can also appear in the middle | |
of words, and then it needs to be printed. ) | |
2DUP ( end start end start ) | |
4 + ( end start end start+4 ) | |
<> IF ( end start | we're not at the end ) | |
." EXIT " | |
THEN | |
ENDOF | |
( default case: ) | |
DUP ( in the default case we always need to DUP before using ) | |
CFA> ( look up the codeword to get the dictionary entry ) | |
ID. SPACE ( and print it ) | |
ENDCASE | |
4 + ( end start+4 ) | |
REPEAT | |
';' EMIT CR | |
2DROP ( restore stack ) | |
; | |
( | |
EXECUTION TOKENS ---------------------------------------------------------------------- | |
Standard FORTH defines a concept called an 'execution token' (or 'xt') which is very | |
similar to a function pointer in C. We map the execution token to a codeword address. | |
execution token of DOUBLE is the address of this codeword | |
| | |
V | |
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ | |
| LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | | |
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ | |
len pad codeword ^ | |
There is one assembler primitive for execution tokens, EXECUTE ( xt -- ), which runs them. | |
You can make an execution token for an existing word the long way using >CFA, | |
ie: WORD [foo] FIND >CFA will push the xt for foo onto the stack where foo is the | |
next word in input. So a very slow way to run DOUBLE might be: | |
: DOUBLE DUP + ; | |
: SLOW WORD FIND >CFA EXECUTE ; | |
5 SLOW DOUBLE . CR \ prints 10 | |
We also offer a simpler and faster way to get the execution token of any word FOO: | |
['] FOO | |
(Exercises for readers: (1) What is the difference between ['] FOO and ' FOO? | |
(2) What is the relationship between ', ['] and LIT?) | |
More useful is to define anonymous words and/or to assign xt's to variables. | |
To define an anonymous word (and push its xt on the stack) use :NONAME ... ; as in this | |
example: | |
:NONAME ." anon word was called" CR ; \ pushes xt on the stack | |
DUP EXECUTE EXECUTE \ executes the anon word twice | |
Stack parameters work as expected: | |
:NONAME ." called with parameter " . CR ; | |
DUP | |
10 SWAP EXECUTE \ prints 'called with parameter 10' | |
20 SWAP EXECUTE \ prints 'called with parameter 20' | |
Notice that the above code has a memory leak: the anonymous word is still compiled | |
into the data segment, so even if you lose track of the xt, the word continues to | |
occupy memory. A good way to keep track of the xt and thus avoid the memory leak is | |
to assign it to a CONSTANT, VARIABLE or VALUE: | |
0 VALUE ANON | |
:NONAME ." anon word was called" CR ; TO ANON | |
ANON EXECUTE | |
ANON EXECUTE | |
Another use of :NONAME is to create an array of functions which can be called quickly | |
(think: fast switch statement). This example is adapted from the ANS FORTH standard: | |
10 CELLS ALLOT CONSTANT CMD-TABLE | |
: SET-CMD CELLS CMD-TABLE + ! ; | |
: CALL-CMD CELLS CMD-TABLE + @ EXECUTE ; | |
:NONAME ." alternate 0 was called" CR ; 0 SET-CMD | |
:NONAME ." alternate 1 was called" CR ; 1 SET-CMD | |
\ etc... | |
:NONAME ." alternate 9 was called" CR ; 9 SET-CMD | |
0 CALL-CMD | |
1 CALL-CMD | |
) | |
: :NONAME | |
0 0 CREATE ( create a word with no name - we need a dictionary header because ; expects it ) | |
HERE @ ( current HERE value is the address of the codeword, ie. the xt ) | |
DOCOL , ( compile DOCOL (the codeword) ) | |
] ( go into compile mode ) | |
; | |
: ['] IMMEDIATE | |
' LIT , ( compile LIT ) | |
; | |
( | |
EXCEPTIONS ---------------------------------------------------------------------- | |
Amazingly enough, exceptions can be implemented directly in FORTH, in fact rather easily. | |
The general usage is as follows: | |
: FOO ( n -- ) THROW ; | |
: TEST-EXCEPTIONS | |
25 ['] FOO CATCH \ execute 25 FOO, catching any exception | |
?DUP IF | |
." called FOO and it threw exception number: " | |
. CR | |
DROP \ we have to drop the argument of FOO (25) | |
THEN | |
; | |
\ prints: called FOO and it threw exception number: 25 | |
CATCH runs an execution token and detects whether it throws any exception or not. The | |
stack signature of CATCH is rather complicated: | |
( a_n-1 ... a_1 a_0 xt -- r_m-1 ... r_1 r_0 0 ) if xt did NOT throw an exception | |
( a_n-1 ... a_1 a_0 xt -- ?_n-1 ... ?_1 ?_0 e ) if xt DID throw exception 'e' | |
where a_i and r_i are the (arbitrary number of) argument and return stack contents | |
before and after xt is EXECUTEd. Notice in particular the case where an exception | |
is thrown, the stack pointer is restored so that there are n of _something_ on the | |
stack in the positions where the arguments a_i used to be. We don't really guarantee | |
what is on the stack -- perhaps the original arguments, and perhaps other nonsense -- | |
it largely depends on the implementation of the word that was executed. | |
THROW, ABORT and a few others throw exceptions. | |
Exception numbers are non-zero integers. By convention the positive numbers can be used | |
for app-specific exceptions and the negative numbers have certain meanings defined in | |
the ANS FORTH standard. (For example, -1 is the exception thrown by ABORT). | |
0 THROW does nothing. This is the stack signature of THROW: | |
( 0 -- ) | |
( * e -- ?_n-1 ... ?_1 ?_0 e ) the stack is restored to the state from the corresponding CATCH | |
The implementation hangs on the definitions of CATCH and THROW and the state shared | |
between them. | |
Up to this point, the return stack has consisted merely of a list of return addresses, | |
with the top of the return stack being the return address where we will resume executing | |
when the current word EXITs. However CATCH will push a more complicated 'exception stack | |
frame' on the return stack. The exception stack frame records some things about the | |
state of execution at the time that CATCH was called. | |
When called, THROW walks up the return stack (the process is called 'unwinding') until | |
it finds the exception stack frame. It then uses the data in the exception stack frame | |
to restore the state allowing execution to continue after the matching CATCH. (If it | |
unwinds the stack and doesn't find the exception stack frame then it prints a message | |
and drops back to the prompt, which is also normal behaviour for so-called 'uncaught | |
exceptions'). | |
This is what the exception stack frame looks like. (As is conventional, the return stack | |
is shown growing downwards from higher to lower memory addresses). | |
+------------------------------+ | |
| return address from CATCH | Notice this is already on the | |
| | return stack when CATCH is called. | |
+------------------------------+ | |
| original parameter stack | | |
| pointer | | |
+------------------------------+ ^ | |
| exception stack marker | | | |
| (EXCEPTION-MARKER) | | Direction of stack | |
+------------------------------+ | unwinding by THROW. | |
| | |
| | |
The EXCEPTION-MARKER marks the entry as being an exception stack frame rather than an | |
ordinary return address, and it is this which THROW "notices" as it is unwinding the | |
stack. (If you want to implement more advanced exceptions such as TRY...WITH then | |
you'll need to use a different value of marker if you want the old and new exception stack | |
frame layouts to coexist). | |
What happens if the executed word doesn't throw an exception? It will eventually | |
return and call EXCEPTION-MARKER, so EXCEPTION-MARKER had better do something sensible | |
without us needing to modify EXIT. This nicely gives us a suitable definition of | |
EXCEPTION-MARKER, namely a function that just drops the stack frame and itself | |
returns (thus "returning" from the original CATCH). | |
One thing to take from this is that exceptions are a relatively lightweight mechanism | |
in FORTH. | |
) | |
: EXCEPTION-MARKER | |
RDROP ( drop the original parameter stack pointer ) | |
0 ( there was no exception, this is the normal return path ) | |
; | |
: CATCH ( xt -- exn? ) | |
DSP@ 4+ >R ( save parameter stack pointer (+4 because of xt) on the return stack ) | |
' EXCEPTION-MARKER 4+ ( push the address of the RDROP inside EXCEPTION-MARKER ... ) | |
>R ( ... on to the return stack so it acts like a return address ) | |
EXECUTE ( execute the nested function ) | |
; | |
: THROW ( n -- ) | |
?DUP IF ( only act if the exception code <> 0 ) | |
RSP@ ( get return stack pointer ) | |
BEGIN | |
DUP R0 4- < ( RSP < R0 ) | |
WHILE | |
DUP @ ( get the return stack entry ) | |
' EXCEPTION-MARKER 4+ = IF ( found the EXCEPTION-MARKER on the return stack ) | |
4+ ( skip the EXCEPTION-MARKER on the return stack ) | |
RSP! ( restore the return stack pointer ) | |
( Restore the parameter stack. ) | |
DUP DUP DUP ( reserve some working space so the stack for this word | |
doesn't coincide with the part of the stack being restored ) | |
R> ( get the saved parameter stack pointer | n dsp ) | |
4- ( reserve space on the stack to store n ) | |
SWAP OVER ( dsp n dsp ) | |
! ( write n on the stack ) | |
DSP! EXIT ( restore the parameter stack pointer, immediately exit ) | |
THEN | |
4+ | |
REPEAT | |
( No matching catch - print a message and restart the INTERPRETer. ) | |
DROP | |
CASE | |
0 1- OF ( ABORT ) | |
." ABORTED" CR | |
ENDOF | |
( default case ) | |
." UNCAUGHT THROW " | |
DUP . CR | |
ENDCASE | |
QUIT | |
THEN | |
; | |
: ABORT ( -- ) | |
0 1- THROW | |
; | |
( Print a stack trace by walking up the return stack. ) | |
: PRINT-STACK-TRACE | |
RSP@ ( start at caller of this function ) | |
BEGIN | |
DUP R0 4- < ( RSP < R0 ) | |
WHILE | |
DUP @ ( get the return stack entry ) | |
CASE | |
' EXCEPTION-MARKER 4+ OF ( is it the exception stack frame? ) | |
." CATCH ( DSP=" | |
4+ DUP @ U. ( print saved stack pointer ) | |
." ) " | |
ENDOF | |
( default case ) | |
DUP | |
CFA> ( look up the codeword to get the dictionary entry ) | |
?DUP IF ( and print it ) | |
2DUP ( dea addr dea ) | |
ID. ( print word from dictionary entry ) | |
[ CHAR + ] LITERAL EMIT | |
SWAP >DFA 4+ - . ( print offset ) | |
THEN | |
ENDCASE | |
4+ ( move up the stack ) | |
REPEAT | |
DROP | |
CR | |
; | |
( | |
C STRINGS ---------------------------------------------------------------------- | |
FORTH strings are represented by a start address and length kept on the stack or in memory. | |
Most FORTHs don't handle C strings, but we need them in order to access the process arguments | |
and environment left on the stack by the Linux kernel, and to make some system calls. | |
Operation Input Output FORTH word Notes | |
---------------------------------------------------------------------- | |
Create FORTH string addr len S" ..." | |
Create C string c-addr Z" ..." | |
C -> FORTH c-addr addr len DUP STRLEN | |
FORTH -> C addr len c-addr CSTRING Allocated in a temporary buffer, so | |
should be consumed / copied immediately. | |
FORTH string should not contain NULs. | |
For example, DUP STRLEN TELL prints a C string. | |
) | |
( | |
Z" .." is like S" ..." except that the string is terminated by an ASCII NUL character. | |
To make it more like a C string, at runtime Z" just leaves the address of the string | |
on the stack (not address & length as with S"). To implement this we need to add the | |
extra NUL to the string and also a DROP instruction afterwards. Apart from that the | |
implementation just a modified S". | |
) | |
: Z" IMMEDIATE | |
STATE @ IF ( compiling? ) | |
' LITSTRING , ( compile LITSTRING ) | |
HERE @ ( save the address of the length word on the stack ) | |
0 , ( dummy length - we don't know what it is yet ) | |
BEGIN | |
KEY ( get next character of the string ) | |
DUP '"' <> | |
WHILE | |
HERE @ C! ( store the character in the compiled image ) | |
1 HERE +! ( increment HERE pointer by 1 byte ) | |
REPEAT | |
0 HERE @ C! ( add the ASCII NUL byte ) | |
1 HERE +! | |
DROP ( drop the double quote character at the end ) | |
DUP ( get the saved address of the length word ) | |
HERE @ SWAP - ( calculate the length ) | |
4- ( subtract 4 (because we measured from the start of the length word) ) | |
SWAP ! ( and back-fill the length location ) | |
ALIGN ( round up to next multiple of 4 bytes for the remaining code ) | |
' DROP , ( compile DROP (to drop the length) ) | |
ELSE ( immediate mode ) | |
HERE @ ( get the start address of the temporary space ) | |
BEGIN | |
KEY | |
DUP '"' <> | |
WHILE | |
OVER C! ( save next character ) | |
1+ ( increment address ) | |
REPEAT | |
DROP ( drop the final " character ) | |
0 SWAP C! ( store final ASCII NUL ) | |
HERE @ ( push the start address ) | |
THEN | |
; | |
: STRLEN ( str -- len ) | |
DUP ( save start address ) | |
BEGIN | |
DUP C@ 0<> ( zero byte found? ) | |
WHILE | |
1+ | |
REPEAT | |
SWAP - ( calculate the length ) | |
; | |
: CSTRING ( addr len -- c-addr ) | |
SWAP OVER ( len saddr len ) | |
HERE @ SWAP ( len saddr daddr len ) | |
CMOVE ( len ) | |
HERE @ + ( daddr+len ) | |
0 SWAP C! ( store terminating NUL char ) | |
HERE @ ( push start address ) | |
; | |
( | |
THE ENVIRONMENT ---------------------------------------------------------------------- | |
Linux makes the process arguments and environment available to us on the stack. | |
The top of stack pointer is saved by the early assembler code when we start up in the FORTH | |
variable S0, and starting at this pointer we can read out the command line arguments and the | |
environment. | |
Starting at S0, S0 itself points to argc (the number of command line arguments). | |
S0+4 points to argv[0], S0+8 points to argv[1] etc up to argv[argc-1]. | |
argv[argc] is a NULL pointer. | |
After that the stack contains environment variables, a set of pointers to strings of the | |
form NAME=VALUE and on until we get to another NULL pointer. | |
The first word that we define, ARGC, pushes the number of command line arguments (note that | |
as with C argc, this includes the name of the command). | |
) | |
: ARGC | |
S0 @ @ | |
; | |
( | |
n ARGV gets the nth command line argument. | |
For example to print the command name you would do: | |
0 ARGV TELL CR | |
) | |
: ARGV ( n -- str u ) | |
1+ CELLS S0 @ + ( get the address of argv[n] entry ) | |
@ ( get the address of the string ) | |
DUP STRLEN ( and get its length / turn it into a FORTH string ) | |
; | |
( | |
ENVIRON returns the address of the first environment string. The list of strings ends | |
with a NULL pointer. | |
For example to print the first string in the environment you could do: | |
ENVIRON @ DUP STRLEN TELL | |
) | |
: ENVIRON ( -- addr ) | |
ARGC ( number of command line parameters on the stack to skip ) | |
2 + ( skip command line count and NULL pointer after the command line args ) | |
CELLS ( convert to an offset ) | |
S0 @ + ( add to base stack address ) | |
; | |
( | |
SYSTEM CALLS AND FILES ---------------------------------------------------------------------- | |
Miscellaneous words related to system calls, and standard access to files. | |
) | |
( BYE exits by calling the Linux exit(2) syscall. ) | |
: BYE ( -- ) | |
0 ( return code (0) ) | |
SYS_EXIT ( system call number ) | |
SYSCALL1 | |
; | |
( | |
UNUSED returns the number of cells remaining in the user memory (data segment). | |
For our implementation we will use Linux brk(2) system call to find out the end | |
of the data segment and subtract HERE from it. | |
) | |
: GET-BRK ( -- brkpoint ) | |
0 SYS_BRK SYSCALL1 ( call brk(0) ) | |
; | |
: UNUSED ( -- n ) | |
GET-BRK ( get end of data segment according to the kernel ) | |
HERE @ ( get current position in data segment ) | |
- | |
4 / ( returns number of cells ) | |
; | |
( | |
MORECORE increases the data segment by the specified number of (4 byte) cells. | |
NB. The number of cells requested should normally be a multiple of 1024. The | |
reason is that Linux can't extend the data segment by less than a single page | |
(4096 bytes or 1024 cells). | |
This FORTH doesn't automatically increase the size of the data segment "on demand" | |
(ie. when , (COMMA), ALLOT, CREATE, and so on are used). Instead the programmer | |
needs to be aware of how much space a large allocation will take, check UNUSED, and | |
call MORECORE if necessary. A simple programming exercise is to change the | |
implementation of the data segment so that MORECORE is called automatically if | |
the program needs more memory. | |
) | |
: BRK ( brkpoint -- ) | |
SYS_BRK SYSCALL1 | |
; | |
: MORECORE ( cells -- ) | |
CELLS GET-BRK + BRK | |
; | |
( | |
Standard FORTH provides some simple file access primitives which we model on | |
top of Linux syscalls. | |
The main complication is converting FORTH strings (address & length) into C | |
strings for the Linux kernel. | |
Notice there is no buffering in this implementation. | |
) | |
: R/O ( -- fam ) O_RDONLY ; | |
: R/W ( -- fam ) O_RDWR ; | |
: OPEN-FILE ( addr u fam -- fd 0 (if successful) | c-addr u fam -- fd errno (if there was an error) ) | |
ROT ( fam addr u ) | |
CSTRING ( fam cstring ) | |
SYS_OPEN SYSCALL2 ( open (filename, flags) ) | |
DUP ( fd fd ) | |
DUP 0< IF ( errno? ) | |
NEGATE ( fd errno ) | |
ELSE | |
DROP 0 ( fd 0 ) | |
THEN | |
; | |
: CREATE-FILE ( addr u fam -- fd 0 (if successful) | c-addr u fam -- fd errno (if there was an error) ) | |
O_CREAT OR | |
O_TRUNC OR | |
ROT ( fam addr u ) | |
CSTRING ( fam cstring ) | |
420 ROT ( 0644 fam cstring ) | |
SYS_OPEN SYSCALL3 ( open (filename, flags|O_TRUNC|O_CREAT, 0644) ) | |
DUP ( fd fd ) | |
DUP 0< IF ( errno? ) | |
NEGATE ( fd errno ) | |
ELSE | |
DROP 0 ( fd 0 ) | |
THEN | |
; | |
: CLOSE-FILE ( fd -- 0 (if successful) | fd -- errno (if there was an error) ) | |
SYS_CLOSE SYSCALL1 | |
NEGATE | |
; | |
: READ-FILE ( addr u fd -- u2 0 (if successful) | addr u fd -- 0 0 (if EOF) | addr u fd -- u2 errno (if error) ) | |
ROT SWAP -ROT ( u addr fd ) | |
SYS_READ SYSCALL3 | |
DUP ( u2 u2 ) | |
DUP 0< IF ( errno? ) | |
NEGATE ( u2 errno ) | |
ELSE | |
DROP 0 ( u2 0 ) | |
THEN | |
; | |
( | |
PERROR prints a message for an errno, similar to C's perror(3) but we don't have the extensive | |
list of strerror strings available, so all we can do is print the errno. | |
) | |
: PERROR ( errno addr u -- ) | |
TELL | |
':' EMIT SPACE | |
." ERRNO=" | |
. CR | |
; | |
( | |
ASSEMBLER CODE ---------------------------------------------------------------------- | |
This is just the outline of a simple assembler, allowing you to write FORTH primitives | |
in assembly language. | |
Assembly primitives begin ': NAME' in the normal way, but are ended with ;CODE. ;CODE | |
updates the header so that the codeword isn't DOCOL, but points instead to the assembled | |
code (in the DFA part of the word). | |
We provide a convenience macro NEXT (you guessed what it does). However you don't need to | |
use it because ;CODE will put a NEXT at the end of your word. | |
The rest consists of some immediate words which expand into machine code appended to the | |
definition of the word. Only a very tiny part of the i386 assembly space is covered, just | |
enough to write a few assembler primitives below. | |
) | |
HEX | |
( Equivalent to the NEXT macro ) | |
: NEXT IMMEDIATE AD C, FF C, 20 C, ; | |
: ;CODE IMMEDIATE | |
[COMPILE] NEXT ( end the word with NEXT macro ) | |
ALIGN ( machine code is assembled in bytes so isn't necessarily aligned at the end ) | |
LATEST @ DUP | |
HIDDEN ( unhide the word ) | |
DUP >DFA SWAP >CFA ! ( change the codeword to point to the data area ) | |
[COMPILE] [ ( go back to immediate mode ) | |
; | |
( The i386 registers ) | |
: EAX IMMEDIATE 0 ; | |
: ECX IMMEDIATE 1 ; | |
: EDX IMMEDIATE 2 ; | |
: EBX IMMEDIATE 3 ; | |
: ESP IMMEDIATE 4 ; | |
: EBP IMMEDIATE 5 ; | |
: ESI IMMEDIATE 6 ; | |
: EDI IMMEDIATE 7 ; | |
( i386 stack instructions ) | |
: PUSH IMMEDIATE 50 + C, ; | |
: POP IMMEDIATE 58 + C, ; | |
( RDTSC instruction ) | |
: RDTSC IMMEDIATE 0F C, 31 C, ; | |
DECIMAL | |
( | |
RDTSC is an assembler primitive which reads the Pentium timestamp counter (a very fine- | |
grained counter which counts processor clock cycles). Because the TSC is 64 bits wide | |
we have to push it onto the stack in two slots. | |
) | |
: RDTSC ( -- lsb msb ) | |
RDTSC ( writes the result in %edx:%eax ) | |
EAX PUSH ( push lsb ) | |
EDX PUSH ( push msb ) | |
;CODE | |
( | |
INLINE can be used to inline an assembler primitive into the current (assembler) | |
word. | |
For example: | |
: 2DROP INLINE DROP INLINE DROP ;CODE | |
will build an efficient assembler word 2DROP which contains the inline assembly code | |
for DROP followed by DROP (eg. two 'pop %eax' instructions in this case). | |
Another example. Consider this ordinary FORTH definition: | |
: C@++ ( addr -- addr+1 byte ) DUP 1+ SWAP C@ ; | |
(it is equivalent to the C operation '*p++' where p is a pointer to char). If we | |
notice that all of the words used to define C@++ are in fact assembler primitives, | |
then we can write a faster (but equivalent) definition like this: | |
: C@++ INLINE DUP INLINE 1+ INLINE SWAP INLINE C@ ;CODE | |
One interesting point to note is that this "concatenative" style of programming | |
allows you to write assembler words portably. The above definition would work | |
for any CPU architecture. | |
There are several conditions that must be met for INLINE to be used successfully: | |
(1) You must be currently defining an assembler word (ie. : ... ;CODE). | |
(2) The word that you are inlining must be known to be an assembler word. If you try | |
to inline a FORTH word, you'll get an error message. | |
(3) The assembler primitive must be position-independent code and must end with a | |
single NEXT macro. | |
Exercises for the reader: (a) Generalise INLINE so that it can inline FORTH words when | |
building FORTH words. (b) Further generalise INLINE so that it does something sensible | |
when you try to inline FORTH into assembler and vice versa. | |
The implementation of INLINE is pretty simple. We find the word in the dictionary, | |
check it's an assembler word, then copy it into the current definition, byte by byte, | |
until we reach the NEXT macro (which is not copied). | |
) | |
HEX | |
: =NEXT ( addr -- next? ) | |
DUP C@ AD <> IF DROP FALSE EXIT THEN | |
1+ DUP C@ FF <> IF DROP FALSE EXIT THEN | |
1+ C@ 20 <> IF FALSE EXIT THEN | |
TRUE | |
; | |
DECIMAL | |
( (INLINE) is the lowlevel inline function. ) | |
: (INLINE) ( cfa -- ) | |
@ ( remember codeword points to the code ) | |
BEGIN ( copy bytes until we hit NEXT macro ) | |
DUP =NEXT NOT | |
WHILE | |
DUP C@ C, | |
1+ | |
REPEAT | |
DROP | |
; | |
: INLINE IMMEDIATE | |
WORD FIND ( find the word in the dictionary ) | |
>CFA ( codeword ) | |
DUP @ DOCOL = IF ( check codeword <> DOCOL (ie. not a FORTH word) ) | |
." Cannot INLINE FORTH words" CR ABORT | |
THEN | |
(INLINE) | |
; | |
HIDE =NEXT | |
( | |
NOTES ---------------------------------------------------------------------- | |
DOES> isn't possible to implement with this FORTH because we don't have a separate | |
data pointer. | |
) | |
( | |
WELCOME MESSAGE ---------------------------------------------------------------------- | |
Print the version and OK prompt. | |
) | |
: WELCOME | |
S" TEST-MODE" FIND NOT IF | |
." JONESFORTH VERSION " VERSION . CR | |
UNUSED . ." CELLS REMAINING" CR | |
." OK " | |
THEN | |
; | |
WELCOME | |
HIDE WELCOME |
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/* A sometimes minimal FORTH compiler and tutorial for Linux / i386 systems. -*- asm -*- | |
By Richard W.M. Jones <rich@annexia.org> http://annexia.org/forth | |
This is PUBLIC DOMAIN (see public domain release statement below). | |
$Id: jonesforth.S,v 1.45 2007/10/22 18:53:13 rich Exp $ | |
gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S | |
*/ | |
.set JONES_VERSION,45 | |
/* | |
INTRODUCTION ---------------------------------------------------------------------- | |
FORTH is one of those alien languages which most working programmers regard in the same | |
way as Haskell, LISP, and so on. Something so strange that they'd rather any thoughts | |
of it just go away so they can get on with writing this paying code. But that's wrong | |
and if you care at all about programming then you should at least understand all these | |
languages, even if you will never use them. | |
LISP is the ultimate high-level language, and features from LISP are being added every | |
decade to the more common languages. But FORTH is in some ways the ultimate in low level | |
programming. Out of the box it lacks features like dynamic memory management and even | |
strings. In fact, at its primitive level it lacks even basic concepts like IF-statements | |
and loops. | |
Why then would you want to learn FORTH? There are several very good reasons. First | |
and foremost, FORTH is minimal. You really can write a complete FORTH in, say, 2000 | |
lines of code. I don't just mean a FORTH program, I mean a complete FORTH operating | |
system, environment and language. You could boot such a FORTH on a bare PC and it would | |
come up with a prompt where you could start doing useful work. The FORTH you have here | |
isn't minimal and uses a Linux process as its 'base PC' (both for the purposes of making | |
it a good tutorial). It's possible to completely understand the system. Who can say they | |
completely understand how Linux works, or gcc? | |
Secondly FORTH has a peculiar bootstrapping property. By that I mean that after writing | |
a little bit of assembly to talk to the hardware and implement a few primitives, all the | |
rest of the language and compiler is written in FORTH itself. Remember I said before | |
that FORTH lacked IF-statements and loops? Well of course it doesn't really because | |
such a lanuage would be useless, but my point was rather that IF-statements and loops are | |
written in FORTH itself. | |
Now of course this is common in other languages as well, and in those languages we call | |
them 'libraries'. For example in C, 'printf' is a library function written in C. But | |
in FORTH this goes way beyond mere libraries. Can you imagine writing C's 'if' in C? | |
And that brings me to my third reason: If you can write 'if' in FORTH, then why restrict | |
yourself to the usual if/while/for/switch constructs? You want a construct that iterates | |
over every other element in a list of numbers? You can add it to the language. What | |
about an operator which pulls in variables directly from a configuration file and makes | |
them available as FORTH variables? Or how about adding Makefile-like dependencies to | |
the language? No problem in FORTH. How about modifying the FORTH compiler to allow | |
complex inlining strategies -- simple. This concept isn't common in programming languages, | |
but it has a name (in fact two names): "macros" (by which I mean LISP-style macros, not | |
the lame C preprocessor) and "domain specific languages" (DSLs). | |
This tutorial isn't about learning FORTH as the language. I'll point you to some references | |
you should read if you're not familiar with using FORTH. This tutorial is about how to | |
write FORTH. In fact, until you understand how FORTH is written, you'll have only a very | |
superficial understanding of how to use it. | |
So if you're not familiar with FORTH or want to refresh your memory here are some online | |
references to read: | |
http://en.wikipedia.org/wiki/Forth_%28programming_language%29 | |
http://galileo.phys.virginia.edu/classes/551.jvn.fall01/primer.htm | |
http://wiki.laptop.org/go/Forth_Lessons | |
http://www.albany.net/~hello/simple.htm | |
Here is another "Why FORTH?" essay: http://www.jwdt.com/~paysan/why-forth.html | |
Discussion and criticism of this FORTH here: http://lambda-the-ultimate.org/node/2452 | |
ACKNOWLEDGEMENTS ---------------------------------------------------------------------- | |
This code draws heavily on the design of LINA FORTH (http://home.hccnet.nl/a.w.m.van.der.horst/lina.html) | |
by Albert van der Horst. Any similarities in the code are probably not accidental. | |
Some parts of this FORTH are also based on this IOCCC entry from 1992: | |
http://ftp.funet.fi/pub/doc/IOCCC/1992/buzzard.2.design. | |
I was very proud when Sean Barrett, the original author of the IOCCC entry, commented in the LtU thread | |
http://lambda-the-ultimate.org/node/2452#comment-36818 about this FORTH. | |
And finally I'd like to acknowledge the (possibly forgotten?) authors of ARTIC FORTH because their | |
original program which I still have on original cassette tape kept nagging away at me all these years. | |
http://en.wikipedia.org/wiki/Artic_Software | |
PUBLIC DOMAIN ---------------------------------------------------------------------- | |
I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide. | |
In case this is not legally possible, I grant any entity the right to use this work for any purpose, | |
without any conditions, unless such conditions are required by law. | |
SETTING UP ---------------------------------------------------------------------- | |
Let's get a few housekeeping things out of the way. Firstly because I need to draw lots of | |
ASCII-art diagrams to explain concepts, the best way to look at this is using a window which | |
uses a fixed width font and is at least this wide: | |
<------------------------------------------------------------------------------------------------------------------------> | |
Secondly make sure TABS are set to 8 characters. The following should be a vertical | |
line. If not, sort out your tabs. | |
| | |
| | |
| | |
Thirdly I assume that your screen is at least 50 characters high. | |
ASSEMBLING ---------------------------------------------------------------------- | |
If you want to actually run this FORTH, rather than just read it, you will need Linux on an | |
i386. Linux because instead of programming directly to the hardware on a bare PC which I | |
could have done, I went for a simpler tutorial by assuming that the 'hardware' is a Linux | |
process with a few basic system calls (read, write and exit and that's about all). i386 | |
is needed because I had to write the assembly for a processor, and i386 is by far the most | |
common. (Of course when I say 'i386', any 32- or 64-bit x86 processor will do. I'm compiling | |
this on a 64 bit AMD Opteron). | |
Again, to assemble this you will need gcc and gas (the GNU assembler). The commands to | |
assemble and run the code (save this file as 'jonesforth.S') are: | |
gcc -m32 -nostdlib -static -Wl,-Ttext,0 -Wl,--build-id=none -o jonesforth jonesforth.S | |
cat jonesforth.f - | ./jonesforth | |
If you want to run your own FORTH programs you can do: | |
cat jonesforth.f myprog.f | ./jonesforth | |
If you want to load your own FORTH code and then continue reading user commands, you can do: | |
cat jonesforth.f myfunctions.f - | ./jonesforth | |
ASSEMBLER ---------------------------------------------------------------------- | |
(You can just skip to the next section -- you don't need to be able to read assembler to | |
follow this tutorial). | |
However if you do want to read the assembly code here are a few notes about gas (the GNU assembler): | |
(1) Register names are prefixed with '%', so %eax is the 32 bit i386 accumulator. The registers | |
available on i386 are: %eax, %ebx, %ecx, %edx, %esi, %edi, %ebp and %esp, and most of them | |
have special purposes. | |
(2) Add, mov, etc. take arguments in the form SRC,DEST. So mov %eax,%ecx moves %eax -> %ecx | |
(3) Constants are prefixed with '$', and you mustn't forget it! If you forget it then it | |
causes a read from memory instead, so: | |
mov $2,%eax moves number 2 into %eax | |
mov 2,%eax reads the 32 bit word from address 2 into %eax (ie. most likely a mistake) | |
(4) gas has a funky syntax for local labels, where '1f' (etc.) means label '1:' "forwards" | |
and '1b' (etc.) means label '1:' "backwards". Notice that these labels might be mistaken | |
for hex numbers (eg. you might confuse 1b with $0x1b). | |
(5) 'ja' is "jump if above", 'jb' for "jump if below", 'je' "jump if equal" etc. | |
(6) gas has a reasonably nice .macro syntax, and I use them a lot to make the code shorter and | |
less repetitive. | |
For more help reading the assembler, do "info gas" at the Linux prompt. | |
Now the tutorial starts in earnest. | |
THE DICTIONARY ---------------------------------------------------------------------- | |
In FORTH as you will know, functions are called "words", and just as in other languages they | |
have a name and a definition. Here are two FORTH words: | |
: DOUBLE DUP + ; \ name is "DOUBLE", definition is "DUP +" | |
: QUADRUPLE DOUBLE DOUBLE ; \ name is "QUADRUPLE", definition is "DOUBLE DOUBLE" | |
Words, both built-in ones and ones which the programmer defines later, are stored in a dictionary | |
which is just a linked list of dictionary entries. | |
<--- DICTIONARY ENTRY (HEADER) -----------------------> | |
+------------------------+--------+---------- - - - - +----------- - - - - | |
| LINK POINTER | LENGTH/| NAME | DEFINITION | |
| | FLAGS | | | |
+--- (4 bytes) ----------+- byte -+- n bytes - - - - +----------- - - - - | |
I'll come to the definition of the word later. For now just look at the header. The first | |
4 bytes are the link pointer. This points back to the previous word in the dictionary, or, for | |
the first word in the dictionary it is just a NULL pointer. Then comes a length/flags byte. | |
The length of the word can be up to 31 characters (5 bits used) and the top three bits are used | |
for various flags which I'll come to later. This is followed by the name itself, and in this | |
implementation the name is rounded up to a multiple of 4 bytes by padding it with zero bytes. | |
That's just to ensure that the definition starts on a 32 bit boundary. | |
A FORTH variable called LATEST contains a pointer to the most recently defined word, in | |
other words, the head of this linked list. | |
DOUBLE and QUADRUPLE might look like this: | |
pointer to previous word | |
^ | |
| | |
+--|------+---+---+---+---+---+---+---+---+------------- - - - - | |
| LINK | 6 | D | O | U | B | L | E | 0 | (definition ...) | |
+---------+---+---+---+---+---+---+---+---+------------- - - - - | |
^ len padding | |
| | |
+--|------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - - | |
| LINK | 9 | Q | U | A | D | R | U | P | L | E | 0 | 0 | (definition ...) | |
+---------+---+---+---+---+---+---+---+---+---+---+---+---+------------- - - - - | |
^ len padding | |
| | |
| | |
LATEST | |
You should be able to see from this how you might implement functions to find a word in | |
the dictionary (just walk along the dictionary entries starting at LATEST and matching | |
the names until you either find a match or hit the NULL pointer at the end of the dictionary); | |
and add a word to the dictionary (create a new definition, set its LINK to LATEST, and set | |
LATEST to point to the new word). We'll see precisely these functions implemented in | |
assembly code later on. | |
One interesting consequence of using a linked list is that you can redefine words, and | |
a newer definition of a word overrides an older one. This is an important concept in | |
FORTH because it means that any word (even "built-in" or "standard" words) can be | |
overridden with a new definition, either to enhance it, to make it faster or even to | |
disable it. However because of the way that FORTH words get compiled, which you'll | |
understand below, words defined using the old definition of a word continue to use | |
the old definition. Only words defined after the new definition use the new definition. | |
DIRECT THREADED CODE ---------------------------------------------------------------------- | |
Now we'll get to the really crucial bit in understanding FORTH, so go and get a cup of tea | |
or coffee and settle down. It's fair to say that if you don't understand this section, then you | |
won't "get" how FORTH works, and that would be a failure on my part for not explaining it well. | |
So if after reading this section a few times you don't understand it, please email me | |
(rich@annexia.org). | |
Let's talk first about what "threaded code" means. Imagine a peculiar version of C where | |
you are only allowed to call functions without arguments. (Don't worry for now that such a | |
language would be completely useless!) So in our peculiar C, code would look like this: | |
f () | |
{ | |
a (); | |
b (); | |
c (); | |
} | |
and so on. How would a function, say 'f' above, be compiled by a standard C compiler? | |
Probably into assembly code like this. On the right hand side I've written the actual | |
i386 machine code. | |
f: | |
CALL a E8 08 00 00 00 | |
CALL b E8 1C 00 00 00 | |
CALL c E8 2C 00 00 00 | |
; ignore the return from the function for now | |
"E8" is the x86 machine code to "CALL" a function. In the first 20 years of computing | |
memory was hideously expensive and we might have worried about the wasted space being used | |
by the repeated "E8" bytes. We can save 20% in code size (and therefore, in expensive memory) | |
by compressing this into just: | |
08 00 00 00 Just the function addresses, without | |
1C 00 00 00 the CALL prefix. | |
2C 00 00 00 | |
On a 16-bit machine like the ones which originally ran FORTH the savings are even greater - 33%. | |
[Historical note: If the execution model that FORTH uses looks strange from the following | |
paragraphs, then it was motivated entirely by the need to save memory on early computers. | |
This code compression isn't so important now when our machines have more memory in their L1 | |
caches than those early computers had in total, but the execution model still has some | |
useful properties]. | |
Of course this code won't run directly on the CPU any more. Instead we need to write an | |
interpreter which takes each set of bytes and calls it. | |
On an i386 machine it turns out that we can write this interpreter rather easily, in just | |
two assembly instructions which turn into just 3 bytes of machine code. Let's store the | |
pointer to the next word to execute in the %esi register: | |
08 00 00 00 <- We're executing this one now. %esi is the _next_ one to execute. | |
%esi -> 1C 00 00 00 | |
2C 00 00 00 | |
The all-important i386 instruction is called LODSL (or in Intel manuals, LODSW). It does | |
two things. Firstly it reads the memory at %esi into the accumulator (%eax). Secondly it | |
increments %esi by 4 bytes. So after LODSL, the situation now looks like this: | |
08 00 00 00 <- We're still executing this one | |
1C 00 00 00 <- %eax now contains this address (0x0000001C) | |
%esi -> 2C 00 00 00 | |
Now we just need to jump to the address in %eax. This is again just a single x86 instruction | |
written JMP *(%eax). And after doing the jump, the situation looks like: | |
08 00 00 00 | |
1C 00 00 00 <- Now we're executing this subroutine. | |
%esi -> 2C 00 00 00 | |
To make this work, each subroutine is followed by the two instructions 'LODSL; JMP *(%eax)' | |
which literally make the jump to the next subroutine. | |
And that brings us to our first piece of actual code! Well, it's a macro. | |
*/ | |
/* NEXT macro. */ | |
.macro NEXT | |
lodsl | |
jmp *(%eax) | |
.endm | |
/* The macro is called NEXT. That's a FORTH-ism. It expands to those two instructions. | |
Every FORTH primitive that we write has to be ended by NEXT. Think of it kind of like | |
a return. | |
The above describes what is known as direct threaded code. | |
To sum up: We compress our function calls down to a list of addresses and use a somewhat | |
magical macro to act as a "jump to next function in the list". We also use one register (%esi) | |
to act as a kind of instruction pointer, pointing to the next function in the list. | |
I'll just give you a hint of what is to come by saying that a FORTH definition such as: | |
: QUADRUPLE DOUBLE DOUBLE ; | |
actually compiles (almost, not precisely but we'll see why in a moment) to a list of | |
function addresses for DOUBLE, DOUBLE and a special function called EXIT to finish off. | |
At this point, REALLY EAGLE-EYED ASSEMBLY EXPERTS are saying "JONES, YOU'VE MADE A MISTAKE!". | |
I lied about JMP *(%eax). | |
INDIRECT THREADED CODE ---------------------------------------------------------------------- | |
It turns out that direct threaded code is interesting but only if you want to just execute | |
a list of functions written in assembly language. So QUADRUPLE would work only if DOUBLE | |
was an assembly language function. In the direct threaded code, QUADRUPLE would look like: | |
+------------------+ | |
| addr of DOUBLE --------------------> (assembly code to do the double) | |
+------------------+ NEXT | |
%esi -> | addr of DOUBLE | | |
+------------------+ | |
We can add an extra indirection to allow us to run both words written in assembly language | |
(primitives written for speed) and words written in FORTH themselves as lists of addresses. | |
The extra indirection is the reason for the brackets in JMP *(%eax). | |
Let's have a look at how QUADRUPLE and DOUBLE really look in FORTH: | |
: QUADRUPLE DOUBLE DOUBLE ; | |
+------------------+ | |
| codeword | : DOUBLE DUP + ; | |
+------------------+ | |
| addr of DOUBLE ---------------> +------------------+ | |
+------------------+ | codeword | | |
| addr of DOUBLE | +------------------+ | |
+------------------+ | addr of DUP --------------> +------------------+ | |
| addr of EXIT | +------------------+ | codeword -------+ | |
+------------------+ %esi -> | addr of + --------+ +------------------+ | | |
+------------------+ | | assembly to <-----+ | |
| addr of EXIT | | | implement DUP | | |
+------------------+ | | .. | | |
| | .. | | |
| | NEXT | | |
| +------------------+ | |
| | |
+-----> +------------------+ | |
| codeword -------+ | |
+------------------+ | | |
| assembly to <------+ | |
| implement + | | |
| .. | | |
| .. | | |
| NEXT | | |
+------------------+ | |
This is the part where you may need an extra cup of tea/coffee/favourite caffeinated | |
beverage. What has changed is that I've added an extra pointer to the beginning of | |
the definitions. In FORTH this is sometimes called the "codeword". The codeword is | |
a pointer to the interpreter to run the function. For primitives written in | |
assembly language, the "interpreter" just points to the actual assembly code itself. | |
They don't need interpreting, they just run. | |
In words written in FORTH (like QUADRUPLE and DOUBLE), the codeword points to an interpreter | |
function. | |
I'll show you the interpreter function shortly, but let's recall our indirect | |
JMP *(%eax) with the "extra" brackets. Take the case where we're executing DOUBLE | |
as shown, and DUP has been called. Note that %esi is pointing to the address of + | |
The assembly code for DUP eventually does a NEXT. That: | |
(1) reads the address of + into %eax %eax points to the codeword of + | |
(2) increments %esi by 4 | |
(3) jumps to the indirect %eax jumps to the address in the codeword of +, | |
ie. the assembly code to implement + | |
+------------------+ | |
| codeword | | |
+------------------+ | |
| addr of DOUBLE ---------------> +------------------+ | |
+------------------+ | codeword | | |
| addr of DOUBLE | +------------------+ | |
+------------------+ | addr of DUP --------------> +------------------+ | |
| addr of EXIT | +------------------+ | codeword -------+ | |
+------------------+ | addr of + --------+ +------------------+ | | |
+------------------+ | | assembly to <-----+ | |
%esi -> | addr of EXIT | | | implement DUP | | |
+------------------+ | | .. | | |
| | .. | | |
| | NEXT | | |
| +------------------+ | |
| | |
+-----> +------------------+ | |
| codeword -------+ | |
+------------------+ | | |
now we're | assembly to <-----+ | |
executing | implement + | | |
this | .. | | |
function | .. | | |
| NEXT | | |
+------------------+ | |
So I hope that I've convinced you that NEXT does roughly what you'd expect. This is | |
indirect threaded code. | |
I've glossed over four things. I wonder if you can guess without reading on what they are? | |
. | |
. | |
. | |
My list of four things are: (1) What does "EXIT" do? (2) which is related to (1) is how do | |
you call into a function, ie. how does %esi start off pointing at part of QUADRUPLE, but | |
then point at part of DOUBLE. (3) What goes in the codeword for the words which are written | |
in FORTH? (4) How do you compile a function which does anything except call other functions | |
ie. a function which contains a number like : DOUBLE 2 * ; ? | |
THE INTERPRETER AND RETURN STACK ------------------------------------------------------------ | |
Going at these in no particular order, let's talk about issues (3) and (2), the interpreter | |
and the return stack. | |
Words which are defined in FORTH need a codeword which points to a little bit of code to | |
give them a "helping hand" in life. They don't need much, but they do need what is known | |
as an "interpreter", although it doesn't really "interpret" in the same way that, say, | |
Java bytecode used to be interpreted (ie. slowly). This interpreter just sets up a few | |
machine registers so that the word can then execute at full speed using the indirect | |
threaded model above. | |
One of the things that needs to happen when QUADRUPLE calls DOUBLE is that we save the old | |
%esi ("instruction pointer") and create a new one pointing to the first word in DOUBLE. | |
Because we will need to restore the old %esi at the end of DOUBLE (this is, after all, like | |
a function call), we will need a stack to store these "return addresses" (old values of %esi). | |
As you will have seen in the background documentation, FORTH has two stacks, an ordinary | |
stack for parameters, and a return stack which is a bit more mysterious. But our return | |
stack is just the stack I talked about in the previous paragraph, used to save %esi when | |
calling from a FORTH word into another FORTH word. | |
In this FORTH, we are using the normal stack pointer (%esp) for the parameter stack. | |
We will use the i386's "other" stack pointer (%ebp, usually called the "frame pointer") | |
for our return stack. | |
I've got two macros which just wrap up the details of using %ebp for the return stack. | |
You use them as for example "PUSHRSP %eax" (push %eax on the return stack) or "POPRSP %ebx" | |
(pop top of return stack into %ebx). | |
*/ | |
/* Macros to deal with the return stack. */ | |
.macro PUSHRSP reg | |
lea -4(%ebp),%ebp // push reg on to return stack | |
movl \reg,(%ebp) | |
.endm | |
.macro POPRSP reg | |
mov (%ebp),\reg // pop top of return stack to reg | |
lea 4(%ebp),%ebp | |
.endm | |
/* | |
And with that we can now talk about the interpreter. | |
In FORTH the interpreter function is often called DOCOL (I think it means "DO COLON" because | |
all FORTH definitions start with a colon, as in : DOUBLE DUP + ; | |
The "interpreter" (it's not really "interpreting") just needs to push the old %esi on the | |
stack and set %esi to the first word in the definition. Remember that we jumped to the | |
function using JMP *(%eax)? Well a consequence of that is that conveniently %eax contains | |
the address of this codeword, so just by adding 4 to it we get the address of the first | |
data word. Finally after setting up %esi, it just does NEXT which causes that first word | |
to run. | |
*/ | |
/* DOCOL - the interpreter! */ | |
.text | |
.align 4 | |
DOCOL: | |
PUSHRSP %esi // push %esi on to the return stack | |
addl $4,%eax // %eax points to codeword, so make | |
movl %eax,%esi // %esi point to first data word | |
NEXT | |
/* | |
Just to make this absolutely clear, let's see how DOCOL works when jumping from QUADRUPLE | |
into DOUBLE: | |
QUADRUPLE: | |
+------------------+ | |
| codeword | | |
+------------------+ DOUBLE: | |
| addr of DOUBLE ---------------> +------------------+ | |
+------------------+ %eax -> | addr of DOCOL | | |
%esi -> | addr of DOUBLE | +------------------+ | |
+------------------+ | addr of DUP | | |
| addr of EXIT | +------------------+ | |
+------------------+ | etc. | | |
First, the call to DOUBLE calls DOCOL (the codeword of DOUBLE). DOCOL does this: It | |
pushes the old %esi on the return stack. %eax points to the codeword of DOUBLE, so we | |
just add 4 on to it to get our new %esi: | |
QUADRUPLE: | |
+------------------+ | |
| codeword | | |
+------------------+ DOUBLE: | |
| addr of DOUBLE ---------------> +------------------+ | |
top of return +------------------+ %eax -> | addr of DOCOL | | |
stack points -> | addr of DOUBLE | + 4 = +------------------+ | |
+------------------+ %esi -> | addr of DUP | | |
| addr of EXIT | +------------------+ | |
+------------------+ | etc. | | |
Then we do NEXT, and because of the magic of threaded code that increments %esi again | |
and calls DUP. | |
Well, it seems to work. | |
One minor point here. Because DOCOL is the first bit of assembly actually to be defined | |
in this file (the others were just macros), and because I usually compile this code with the | |
text segment starting at address 0, DOCOL has address 0. So if you are disassembling the | |
code and see a word with a codeword of 0, you will immediately know that the word is | |
written in FORTH (it's not an assembler primitive) and so uses DOCOL as the interpreter. | |
STARTING UP ---------------------------------------------------------------------- | |
Now let's get down to nuts and bolts. When we start the program we need to set up | |
a few things like the return stack. But as soon as we can, we want to jump into FORTH | |
code (albeit much of the "early" FORTH code will still need to be written as | |
assembly language primitives). | |
This is what the set up code does. Does a tiny bit of house-keeping, sets up the | |
separate return stack (NB: Linux gives us the ordinary parameter stack already), then | |
immediately jumps to a FORTH word called QUIT. Despite its name, QUIT doesn't quit | |
anything. It resets some internal state and starts reading and interpreting commands. | |
(The reason it is called QUIT is because you can call QUIT from your own FORTH code | |
to "quit" your program and go back to interpreting). | |
*/ | |
/* Assembler entry point. */ | |
.text | |
.globl _start | |
_start: | |
cld | |
mov %esp,var_S0 // Save the initial data stack pointer in FORTH variable S0. | |
mov $return_stack_top,%ebp // Initialise the return stack. | |
call set_up_data_segment | |
mov $cold_start,%esi // Initialise interpreter. | |
NEXT // Run interpreter! | |
.section .rodata | |
cold_start: // High-level code without a codeword. | |
.int QUIT | |
/* | |
BUILT-IN WORDS ---------------------------------------------------------------------- | |
Remember our dictionary entries (headers)? Let's bring those together with the codeword | |
and data words to see how : DOUBLE DUP + ; really looks in memory. | |
pointer to previous word | |
^ | |
| | |
+--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ | |
| LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | | |
+---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+ | |
^ len pad codeword | | |
| V | |
LINK in next word points to codeword of DUP | |
Initially we can't just write ": DOUBLE DUP + ;" (ie. that literal string) here because we | |
don't yet have anything to read the string, break it up at spaces, parse each word, etc. etc. | |
So instead we will have to define built-in words using the GNU assembler data constructors | |
(like .int, .byte, .string, .ascii and so on -- look them up in the gas info page if you are | |
unsure of them). | |
The long way would be: | |
.int <link to previous word> | |
.byte 6 // len | |
.ascii "DOUBLE" // string | |
.byte 0 // padding | |
DOUBLE: .int DOCOL // codeword | |
.int DUP // pointer to codeword of DUP | |
.int PLUS // pointer to codeword of + | |
.int EXIT // pointer to codeword of EXIT | |
That's going to get quite tedious rather quickly, so here I define an assembler macro | |
so that I can just write: | |
defword "DOUBLE",6,,DOUBLE | |
.int DUP,PLUS,EXIT | |
and I'll get exactly the same effect. | |
Don't worry too much about the exact implementation details of this macro - it's complicated! | |
*/ | |
/* Flags - these are discussed later. */ | |
.set F_IMMED,0x80 | |
.set F_HIDDEN,0x20 | |
.set F_LENMASK,0x1f // length mask | |
// Store the chain of links. | |
.set link,0 | |
.macro defword name, namelen, flags=0, label | |
.section .rodata | |
.align 4 | |
.globl name_\label | |
name_\label : | |
.int link // link | |
.set link,name_\label | |
.byte \flags+\namelen // flags + length byte | |
.ascii "\name" // the name | |
.align 4 // padding to next 4 byte boundary | |
.globl \label | |
\label : | |
.int DOCOL // codeword - the interpreter | |
// list of word pointers follow | |
.endm | |
/* | |
Similarly I want a way to write words written in assembly language. There will quite a few | |
of these to start with because, well, everything has to start in assembly before there's | |
enough "infrastructure" to be able to start writing FORTH words, but also I want to define | |
some common FORTH words in assembly language for speed, even though I could write them in FORTH. | |
This is what DUP looks like in memory: | |
pointer to previous word | |
^ | |
| | |
+--|------+---+---+---+---+------------+ | |
| LINK | 3 | D | U | P | code_DUP ---------------------> points to the assembly | |
+---------+---+---+---+---+------------+ code used to write DUP, | |
^ len codeword which ends with NEXT. | |
| | |
LINK in next word | |
Again, for brevity in writing the header I'm going to write an assembler macro called defcode. | |
As with defword above, don't worry about the complicated details of the macro. | |
*/ | |
.macro defcode name, namelen, flags=0, label | |
.section .rodata | |
.align 4 | |
.globl name_\label | |
name_\label : | |
.int link // link | |
.set link,name_\label | |
.byte \flags+\namelen // flags + length byte | |
.ascii "\name" // the name | |
.align 4 // padding to next 4 byte boundary | |
.globl \label | |
\label : | |
.int code_\label // codeword | |
.text | |
//.align 4 | |
.globl code_\label | |
code_\label : // assembler code follows | |
.endm | |
/* | |
Now some easy FORTH primitives. These are written in assembly for speed. If you understand | |
i386 assembly language then it is worth reading these. However if you don't understand assembly | |
you can skip the details. | |
*/ | |
defcode "DROP",4,,DROP | |
pop %eax // drop top of stack | |
NEXT | |
defcode "SWAP",4,,SWAP | |
pop %eax // swap top two elements on stack | |
pop %ebx | |
push %eax | |
push %ebx | |
NEXT | |
defcode "DUP",3,,DUP | |
mov (%esp),%eax // duplicate top of stack | |
push %eax | |
NEXT | |
defcode "OVER",4,,OVER | |
mov 4(%esp),%eax // get the second element of stack | |
push %eax // and push it on top | |
NEXT | |
defcode "ROT",3,,ROT | |
pop %eax | |
pop %ebx | |
pop %ecx | |
push %eax | |
push %ecx | |
push %ebx | |
NEXT | |
defcode "-ROT",4,,NROT | |
pop %eax | |
pop %ebx | |
pop %ecx | |
push %ebx | |
push %eax | |
push %ecx | |
NEXT | |
defcode "2DROP",5,,TWODROP // drop top two elements of stack | |
pop %eax | |
pop %eax | |
NEXT | |
defcode "2DUP",4,,TWODUP // duplicate top two elements of stack | |
mov (%esp),%eax | |
mov 4(%esp),%ebx | |
push %ebx | |
push %eax | |
NEXT | |
defcode "2SWAP",5,,TWOSWAP // swap top two pairs of elements of stack | |
pop %eax | |
pop %ebx | |
pop %ecx | |
pop %edx | |
push %ebx | |
push %eax | |
push %edx | |
push %ecx | |
NEXT | |
defcode "?DUP",4,,QDUP // duplicate top of stack if non-zero | |
movl (%esp),%eax | |
test %eax,%eax | |
jz 1f | |
push %eax | |
1: NEXT | |
defcode "1+",2,,INCR | |
incl (%esp) // increment top of stack | |
NEXT | |
defcode "1-",2,,DECR | |
decl (%esp) // decrement top of stack | |
NEXT | |
defcode "4+",2,,INCR4 | |
addl $4,(%esp) // add 4 to top of stack | |
NEXT | |
defcode "4-",2,,DECR4 | |
subl $4,(%esp) // subtract 4 from top of stack | |
NEXT | |
defcode "+",1,,ADD | |
pop %eax // get top of stack | |
addl %eax,(%esp) // and add it to next word on stack | |
NEXT | |
defcode "-",1,,SUB | |
pop %eax // get top of stack | |
subl %eax,(%esp) // and subtract it from next word on stack | |
NEXT | |
defcode "*",1,,MUL | |
pop %eax | |
pop %ebx | |
imull %ebx,%eax | |
push %eax // ignore overflow | |
NEXT | |
/* | |
In this FORTH, only /MOD is primitive. Later we will define the / and MOD words in | |
terms of the primitive /MOD. The design of the i386 assembly instruction idiv which | |
leaves both quotient and remainder makes this the obvious choice. | |
*/ | |
defcode "/MOD",4,,DIVMOD | |
xor %edx,%edx | |
pop %ebx | |
pop %eax | |
idivl %ebx | |
push %edx // push remainder | |
push %eax // push quotient | |
NEXT | |
/* | |
Lots of comparison operations like =, <, >, etc.. | |
ANS FORTH says that the comparison words should return all (binary) 1's for | |
TRUE and all 0's for FALSE. However this is a bit of a strange convention | |
so this FORTH breaks it and returns the more normal (for C programmers ...) | |
1 meaning TRUE and 0 meaning FALSE. | |
*/ | |
defcode "=",1,,EQU // top two words are equal? | |
pop %eax | |
pop %ebx | |
cmp %ebx,%eax | |
sete %al | |
movzbl %al,%eax | |
pushl %eax | |
NEXT | |
defcode "<>",2,,NEQU // top two words are not equal? | |
pop %eax | |
pop %ebx | |
cmp %ebx,%eax | |
setne %al | |
movzbl %al,%eax | |
pushl %eax | |
NEXT | |
defcode "<",1,,LT | |
pop %eax | |
pop %ebx | |
cmp %eax,%ebx | |
setl %al | |
movzbl %al,%eax | |
pushl %eax | |
NEXT | |
defcode ">",1,,GT | |
pop %eax | |
pop %ebx | |
cmp %eax,%ebx | |
setg %al | |
movzbl %al,%eax | |
pushl %eax | |
NEXT | |
defcode "<=",2,,LE | |
pop %eax | |
pop %ebx | |
cmp %eax,%ebx | |
setle %al | |
movzbl %al,%eax | |
pushl %eax | |
NEXT | |
defcode ">=",2,,GE | |
pop %eax | |
pop %ebx | |
cmp %eax,%ebx | |
setge %al | |
movzbl %al,%eax | |
pushl %eax | |
NEXT | |
defcode "0=",2,,ZEQU // top of stack equals 0? | |
pop %eax | |
test %eax,%eax | |
setz %al | |
movzbl %al,%eax | |
pushl %eax | |
NEXT | |
defcode "0<>",3,,ZNEQU // top of stack not 0? | |
pop %eax | |
test %eax,%eax | |
setnz %al | |
movzbl %al,%eax | |
pushl %eax | |
NEXT | |
defcode "0<",2,,ZLT // comparisons with 0 | |
pop %eax | |
test %eax,%eax | |
setl %al | |
movzbl %al,%eax | |
pushl %eax | |
NEXT | |
defcode "0>",2,,ZGT | |
pop %eax | |
test %eax,%eax | |
setg %al | |
movzbl %al,%eax | |
pushl %eax | |
NEXT | |
defcode "0<=",3,,ZLE | |
pop %eax | |
test %eax,%eax | |
setle %al | |
movzbl %al,%eax | |
pushl %eax | |
NEXT | |
defcode "0>=",3,,ZGE | |
pop %eax | |
test %eax,%eax | |
setge %al | |
movzbl %al,%eax | |
pushl %eax | |
NEXT | |
defcode "AND",3,,AND // bitwise AND | |
pop %eax | |
andl %eax,(%esp) | |
NEXT | |
defcode "OR",2,,OR // bitwise OR | |
pop %eax | |
orl %eax,(%esp) | |
NEXT | |
defcode "XOR",3,,XOR // bitwise XOR | |
pop %eax | |
xorl %eax,(%esp) | |
NEXT | |
defcode "INVERT",6,,INVERT // this is the FORTH bitwise "NOT" function (cf. NEGATE and NOT) | |
notl (%esp) | |
NEXT | |
/* | |
RETURNING FROM FORTH WORDS ---------------------------------------------------------------------- | |
Time to talk about what happens when we EXIT a function. In this diagram QUADRUPLE has called | |
DOUBLE, and DOUBLE is about to exit (look at where %esi is pointing): | |
QUADRUPLE | |
+------------------+ | |
| codeword | | |
+------------------+ DOUBLE | |
| addr of DOUBLE ---------------> +------------------+ | |
+------------------+ | codeword | | |
| addr of DOUBLE | +------------------+ | |
+------------------+ | addr of DUP | | |
| addr of EXIT | +------------------+ | |
+------------------+ | addr of + | | |
+------------------+ | |
%esi -> | addr of EXIT | | |
+------------------+ | |
What happens when the + function does NEXT? Well, the following code is executed. | |
*/ | |
defcode "EXIT",4,,EXIT | |
POPRSP %esi // pop return stack into %esi | |
NEXT | |
/* | |
EXIT gets the old %esi which we saved from before on the return stack, and puts it in %esi. | |
So after this (but just before NEXT) we get: | |
QUADRUPLE | |
+------------------+ | |
| codeword | | |
+------------------+ DOUBLE | |
| addr of DOUBLE ---------------> +------------------+ | |
+------------------+ | codeword | | |
%esi -> | addr of DOUBLE | +------------------+ | |
+------------------+ | addr of DUP | | |
| addr of EXIT | +------------------+ | |
+------------------+ | addr of + | | |
+------------------+ | |
| addr of EXIT | | |
+------------------+ | |
And NEXT just completes the job by, well, in this case just by calling DOUBLE again :-) | |
LITERALS ---------------------------------------------------------------------- | |
The final point I "glossed over" before was how to deal with functions that do anything | |
apart from calling other functions. For example, suppose that DOUBLE was defined like this: | |
: DOUBLE 2 * ; | |
It does the same thing, but how do we compile it since it contains the literal 2? One way | |
would be to have a function called "2" (which you'd have to write in assembler), but you'd need | |
a function for every single literal that you wanted to use. | |
FORTH solves this by compiling the function using a special word called LIT: | |
+---------------------------+-------+-------+-------+-------+-------+ | |
| (usual header of DOUBLE) | DOCOL | LIT | 2 | * | EXIT | | |
+---------------------------+-------+-------+-------+-------+-------+ | |
LIT is executed in the normal way, but what it does next is definitely not normal. It | |
looks at %esi (which now points to the number 2), grabs it, pushes it on the stack, then | |
manipulates %esi in order to skip the number as if it had never been there. | |
What's neat is that the whole grab/manipulate can be done using a single byte single | |
i386 instruction, our old friend LODSL. Rather than me drawing more ASCII-art diagrams, | |
see if you can find out how LIT works: | |
*/ | |
defcode "LIT",3,,LIT | |
// %esi points to the next command, but in this case it points to the next | |
// literal 32 bit integer. Get that literal into %eax and increment %esi. | |
// On x86, it's a convenient single byte instruction! (cf. NEXT macro) | |
lodsl | |
push %eax // push the literal number on to stack | |
NEXT | |
/* | |
MEMORY ---------------------------------------------------------------------- | |
As important point about FORTH is that it gives you direct access to the lowest levels | |
of the machine. Manipulating memory directly is done frequently in FORTH, and these are | |
the primitive words for doing it. | |
*/ | |
defcode "!",1,,STORE | |
pop %ebx // address to store at | |
pop %eax // data to store there | |
mov %eax,(%ebx) // store it | |
NEXT | |
defcode "@",1,,FETCH | |
pop %ebx // address to fetch | |
mov (%ebx),%eax // fetch it | |
push %eax // push value onto stack | |
NEXT | |
defcode "+!",2,,ADDSTORE | |
pop %ebx // address | |
pop %eax // the amount to add | |
addl %eax,(%ebx) // add it | |
NEXT | |
defcode "-!",2,,SUBSTORE | |
pop %ebx // address | |
pop %eax // the amount to subtract | |
subl %eax,(%ebx) // add it | |
NEXT | |
/* | |
! and @ (STORE and FETCH) store 32-bit words. It's also useful to be able to read and write bytes | |
so we also define standard words C@ and C!. | |
Byte-oriented operations only work on architectures which permit them (i386 is one of those). | |
*/ | |
defcode "C!",2,,STOREBYTE | |
pop %ebx // address to store at | |
pop %eax // data to store there | |
movb %al,(%ebx) // store it | |
NEXT | |
defcode "C@",2,,FETCHBYTE | |
pop %ebx // address to fetch | |
xor %eax,%eax | |
movb (%ebx),%al // fetch it | |
push %eax // push value onto stack | |
NEXT | |
/* C@C! is a useful byte copy primitive. */ | |
defcode "C@C!",4,,CCOPY | |
movl 4(%esp),%ebx // source address | |
movb (%ebx),%al // get source character | |
pop %edi // destination address | |
stosb // copy to destination | |
push %edi // increment destination address | |
incl 4(%esp) // increment source address | |
NEXT | |
/* and CMOVE is a block copy operation. */ | |
defcode "CMOVE",5,,CMOVE | |
mov %esi,%edx // preserve %esi | |
pop %ecx // length | |
pop %edi // destination address | |
pop %esi // source address | |
rep movsb // copy source to destination | |
mov %edx,%esi // restore %esi | |
NEXT | |
/* | |
BUILT-IN VARIABLES ---------------------------------------------------------------------- | |
These are some built-in variables and related standard FORTH words. Of these, the only one that we | |
have discussed so far was LATEST, which points to the last (most recently defined) word in the | |
FORTH dictionary. LATEST is also a FORTH word which pushes the address of LATEST (the variable) | |
on to the stack, so you can read or write it using @ and ! operators. For example, to print | |
the current value of LATEST (and this can apply to any FORTH variable) you would do: | |
LATEST @ . CR | |
To make defining variables shorter, I'm using a macro called defvar, similar to defword and | |
defcode above. (In fact the defvar macro uses defcode to do the dictionary header). | |
*/ | |
.macro defvar name, namelen, flags=0, label, initial=0 | |
defcode \name,\namelen,\flags,\label | |
push $var_\name | |
NEXT | |
.data | |
.align 4 | |
var_\name : | |
.int \initial | |
.endm | |
/* | |
The built-in variables are: | |
STATE Is the interpreter executing code (0) or compiling a word (non-zero)? | |
LATEST Points to the latest (most recently defined) word in the dictionary. | |
HERE Points to the next free byte of memory. When compiling, compiled words go here. | |
S0 Stores the address of the top of the parameter stack. | |
BASE The current base for printing and reading numbers. | |
*/ | |
defvar "STATE",5,,STATE | |
defvar "HERE",4,,HERE | |
defvar "LATEST",6,,LATEST,name_SYSCALL0 // SYSCALL0 must be last in built-in dictionary | |
defvar "S0",2,,SZ | |
defvar "BASE",4,,BASE,10 | |
/* | |
BUILT-IN CONSTANTS ---------------------------------------------------------------------- | |
It's also useful to expose a few constants to FORTH. When the word is executed it pushes a | |
constant value on the stack. | |
The built-in constants are: | |
VERSION Is the current version of this FORTH. | |
R0 The address of the top of the return stack. | |
DOCOL Pointer to DOCOL. | |
F_IMMED The IMMEDIATE flag's actual value. | |
F_HIDDEN The HIDDEN flag's actual value. | |
F_LENMASK The length mask in the flags/len byte. | |
SYS_* and the numeric codes of various Linux syscalls (from <asm/unistd.h>) | |
*/ | |
//#include <asm-i386/unistd.h> // you might need this instead | |
#include <asm/unistd.h> | |
.macro defconst name, namelen, flags=0, label, value | |
defcode \name,\namelen,\flags,\label | |
push $\value | |
NEXT | |
.endm | |
defconst "VERSION",7,,VERSION,JONES_VERSION | |
defconst "R0",2,,RZ,return_stack_top | |
defconst "DOCOL",5,,__DOCOL,DOCOL | |
defconst "F_IMMED",7,,__F_IMMED,F_IMMED | |
defconst "F_HIDDEN",8,,__F_HIDDEN,F_HIDDEN | |
defconst "F_LENMASK",9,,__F_LENMASK,F_LENMASK | |
defconst "SYS_EXIT",8,,SYS_EXIT,__NR_exit | |
defconst "SYS_OPEN",8,,SYS_OPEN,__NR_open | |
defconst "SYS_CLOSE",9,,SYS_CLOSE,__NR_close | |
defconst "SYS_READ",8,,SYS_READ,__NR_read | |
defconst "SYS_WRITE",9,,SYS_WRITE,__NR_write | |
defconst "SYS_CREAT",9,,SYS_CREAT,__NR_creat | |
defconst "SYS_BRK",7,,SYS_BRK,__NR_brk | |
defconst "O_RDONLY",8,,__O_RDONLY,0 | |
defconst "O_WRONLY",8,,__O_WRONLY,1 | |
defconst "O_RDWR",6,,__O_RDWR,2 | |
defconst "O_CREAT",7,,__O_CREAT,0100 | |
defconst "O_EXCL",6,,__O_EXCL,0200 | |
defconst "O_TRUNC",7,,__O_TRUNC,01000 | |
defconst "O_APPEND",8,,__O_APPEND,02000 | |
defconst "O_NONBLOCK",10,,__O_NONBLOCK,04000 | |
/* | |
RETURN STACK ---------------------------------------------------------------------- | |
These words allow you to access the return stack. Recall that the register %ebp always points to | |
the top of the return stack. | |
*/ | |
defcode ">R",2,,TOR | |
pop %eax // pop parameter stack into %eax | |
PUSHRSP %eax // push it on to the return stack | |
NEXT | |
defcode "R>",2,,FROMR | |
POPRSP %eax // pop return stack on to %eax | |
push %eax // and push on to parameter stack | |
NEXT | |
defcode "RSP@",4,,RSPFETCH | |
push %ebp | |
NEXT | |
defcode "RSP!",4,,RSPSTORE | |
pop %ebp | |
NEXT | |
defcode "RDROP",5,,RDROP | |
addl $4,%ebp // pop return stack and throw away | |
NEXT | |
/* | |
PARAMETER (DATA) STACK ---------------------------------------------------------------------- | |
These functions allow you to manipulate the parameter stack. Recall that Linux sets up the parameter | |
stack for us, and it is accessed through %esp. | |
*/ | |
defcode "DSP@",4,,DSPFETCH | |
mov %esp,%eax | |
push %eax | |
NEXT | |
defcode "DSP!",4,,DSPSTORE | |
pop %esp | |
NEXT | |
/* | |
INPUT AND OUTPUT ---------------------------------------------------------------------- | |
These are our first really meaty/complicated FORTH primitives. I have chosen to write them in | |
assembler, but surprisingly in "real" FORTH implementations these are often written in terms | |
of more fundamental FORTH primitives. I chose to avoid that because I think that just obscures | |
the implementation. After all, you may not understand assembler but you can just think of it | |
as an opaque block of code that does what it says. | |
Let's discuss input first. | |
The FORTH word KEY reads the next byte from stdin (and pushes it on the parameter stack). | |
So if KEY is called and someone hits the space key, then the number 32 (ASCII code of space) | |
is pushed on the stack. | |
In FORTH there is no distinction between reading code and reading input. We might be reading | |
and compiling code, we might be reading words to execute, we might be asking for the user | |
to type their name -- ultimately it all comes in through KEY. | |
The implementation of KEY uses an input buffer of a certain size (defined at the end of this | |
file). It calls the Linux read(2) system call to fill this buffer and tracks its position | |
in the buffer using a couple of variables, and if it runs out of input buffer then it refills | |
it automatically. The other thing that KEY does is if it detects that stdin has closed, it | |
exits the program, which is why when you hit ^D the FORTH system cleanly exits. | |
buffer bufftop | |
| | | |
V V | |
+-------------------------------+--------------------------------------+ | |
| INPUT READ FROM STDIN ....... | unused part of the buffer | | |
+-------------------------------+--------------------------------------+ | |
^ | |
| | |
currkey (next character to read) | |
<---------------------- BUFFER_SIZE (4096 bytes) ----------------------> | |
*/ | |
defcode "KEY",3,,KEY | |
call _KEY | |
push %eax // push return value on stack | |
NEXT | |
_KEY: | |
mov (currkey),%ebx | |
cmp (bufftop),%ebx | |
jge 1f // exhausted the input buffer? | |
xor %eax,%eax | |
mov (%ebx),%al // get next key from input buffer | |
inc %ebx | |
mov %ebx,(currkey) // increment currkey | |
ret | |
1: // Out of input; use read(2) to fetch more input from stdin. | |
xor %ebx,%ebx // 1st param: stdin | |
mov $buffer,%ecx // 2nd param: buffer | |
mov %ecx,currkey | |
mov $BUFFER_SIZE,%edx // 3rd param: max length | |
mov $__NR_read,%eax // syscall: read | |
int $0x80 | |
test %eax,%eax // If %eax <= 0, then exit. | |
jbe 2f | |
addl %eax,%ecx // buffer+%eax = bufftop | |
mov %ecx,bufftop | |
jmp _KEY | |
2: // Error or end of input: exit the program. | |
xor %ebx,%ebx | |
mov $__NR_exit,%eax // syscall: exit | |
int $0x80 | |
.data | |
.align 4 | |
currkey: | |
.int buffer // Current place in input buffer (next character to read). | |
bufftop: | |
.int buffer // Last valid data in input buffer + 1. | |
/* | |
By contrast, output is much simpler. The FORTH word EMIT writes out a single byte to stdout. | |
This implementation just uses the write system call. No attempt is made to buffer output, but | |
it would be a good exercise to add it. | |
*/ | |
defcode "EMIT",4,,EMIT | |
pop %eax | |
call _EMIT | |
NEXT | |
_EMIT: | |
mov $1,%ebx // 1st param: stdout | |
// write needs the address of the byte to write | |
mov %al,emit_scratch | |
mov $emit_scratch,%ecx // 2nd param: address | |
mov $1,%edx // 3rd param: nbytes = 1 | |
mov $__NR_write,%eax // write syscall | |
int $0x80 | |
ret | |
.data // NB: easier to fit in the .data section | |
emit_scratch: | |
.space 1 // scratch used by EMIT | |
/* | |
Back to input, WORD is a FORTH word which reads the next full word of input. | |
What it does in detail is that it first skips any blanks (spaces, tabs, newlines and so on). | |
Then it calls KEY to read characters into an internal buffer until it hits a blank. Then it | |
calculates the length of the word it read and returns the address and the length as | |
two words on the stack (with the length at the top of stack). | |
Notice that WORD has a single internal buffer which it overwrites each time (rather like | |
a static C string). Also notice that WORD's internal buffer is just 32 bytes long and | |
there is NO checking for overflow. 31 bytes happens to be the maximum length of a | |
FORTH word that we support, and that is what WORD is used for: to read FORTH words when | |
we are compiling and executing code. The returned strings are not NUL-terminated. | |
Start address+length is the normal way to represent strings in FORTH (not ending in an | |
ASCII NUL character as in C), and so FORTH strings can contain any character including NULs | |
and can be any length. | |
WORD is not suitable for just reading strings (eg. user input) because of all the above | |
peculiarities and limitations. | |
Note that when executing, you'll see: | |
WORD FOO | |
which puts "FOO" and length 3 on the stack, but when compiling: | |
: BAR WORD FOO ; | |
is an error (or at least it doesn't do what you might expect). Later we'll talk about compiling | |
and immediate mode, and you'll understand why. | |
*/ | |
defcode "WORD",4,,WORD | |
call _WORD | |
push %edi // push base address | |
push %ecx // push length | |
NEXT | |
_WORD: | |
/* Search for first non-blank character. Also skip \ comments. */ | |
1: | |
call _KEY // get next key, returned in %eax | |
cmpb $'\\',%al // start of a comment? | |
je 3f // if so, skip the comment | |
cmpb $' ',%al | |
jbe 1b // if so, keep looking | |
/* Search for the end of the word, storing chars as we go. */ | |
mov $word_buffer,%edi // pointer to return buffer | |
2: | |
stosb // add character to return buffer | |
call _KEY // get next key, returned in %al | |
cmpb $' ',%al // is blank? | |
ja 2b // if not, keep looping | |
/* Return the word (well, the static buffer) and length. */ | |
sub $word_buffer,%edi | |
mov %edi,%ecx // return length of the word | |
mov $word_buffer,%edi // return address of the word | |
ret | |
/* Code to skip \ comments to end of the current line. */ | |
3: | |
call _KEY | |
cmpb $'\n',%al // end of line yet? | |
jne 3b | |
jmp 1b | |
.data // NB: easier to fit in the .data section | |
// A static buffer where WORD returns. Subsequent calls | |
// overwrite this buffer. Maximum word length is 32 chars. | |
word_buffer: | |
.space 32 | |
/* | |
As well as reading in words we'll need to read in numbers and for that we are using a function | |
called NUMBER. This parses a numeric string such as one returned by WORD and pushes the | |
number on the parameter stack. | |
The function uses the variable BASE as the base (radix) for conversion, so for example if | |
BASE is 2 then we expect a binary number. Normally BASE is 10. | |
If the word starts with a '-' character then the returned value is negative. | |
If the string can't be parsed as a number (or contains characters outside the current BASE) | |
then we need to return an error indication. So NUMBER actually returns two items on the stack. | |
At the top of stack we return the number of unconverted characters (ie. if 0 then all characters | |
were converted, so there is no error). Second from top of stack is the parsed number or a | |
partial value if there was an error. | |
*/ | |
defcode "NUMBER",6,,NUMBER | |
pop %ecx // length of string | |
pop %edi // start address of string | |
call _NUMBER | |
push %eax // parsed number | |
push %ecx // number of unparsed characters (0 = no error) | |
NEXT | |
_NUMBER: | |
xor %eax,%eax | |
xor %ebx,%ebx | |
test %ecx,%ecx // trying to parse a zero-length string is an error, but will return 0. | |
jz 5f | |
movl var_BASE,%edx // get BASE (in %dl) | |
// Check if first character is '-'. | |
movb (%edi),%bl // %bl = first character in string | |
inc %edi | |
push %eax // push 0 on stack | |
cmpb $'-',%bl // negative number? | |
jnz 2f | |
pop %eax | |
push %ebx // push <> 0 on stack, indicating negative | |
dec %ecx | |
jnz 1f | |
pop %ebx // error: string is only '-'. | |
movl $1,%ecx | |
ret | |
// Loop reading digits. | |
1: imull %edx,%eax // %eax *= BASE | |
movb (%edi),%bl // %bl = next character in string | |
inc %edi | |
// Convert 0-9, A-Z to a number 0-35. | |
2: subb $'0',%bl // < '0'? | |
jb 4f | |
cmp $10,%bl // <= '9'? | |
jb 3f | |
subb $17,%bl // < 'A'? (17 is 'A'-'0') | |
jb 4f | |
addb $10,%bl | |
3: cmp %dl,%bl // >= BASE? | |
jge 4f | |
// OK, so add it to %eax and loop. | |
add %ebx,%eax | |
dec %ecx | |
jnz 1b | |
// Negate the result if first character was '-' (saved on the stack). | |
4: pop %ebx | |
test %ebx,%ebx | |
jz 5f | |
neg %eax | |
5: ret | |
/* | |
DICTIONARY LOOK UPS ---------------------------------------------------------------------- | |
We're building up to our prelude on how FORTH code is compiled, but first we need yet more infrastructure. | |
The FORTH word FIND takes a string (a word as parsed by WORD -- see above) and looks it up in the | |
dictionary. What it actually returns is the address of the dictionary header, if it finds it, | |
or 0 if it didn't. | |
So if DOUBLE is defined in the dictionary, then WORD DOUBLE FIND returns the following pointer: | |
pointer to this | |
| | |
| | |
V | |
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ | |
| LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | | |
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ | |
See also >CFA and >DFA. | |
FIND doesn't find dictionary entries which are flagged as HIDDEN. See below for why. | |
*/ | |
defcode "FIND",4,,FIND | |
pop %ecx // %ecx = length | |
pop %edi // %edi = address | |
call _FIND | |
push %eax // %eax = address of dictionary entry (or NULL) | |
NEXT | |
_FIND: | |
push %esi // Save %esi so we can use it in string comparison. | |
// Now we start searching backwards through the dictionary for this word. | |
mov var_LATEST,%edx // LATEST points to name header of the latest word in the dictionary | |
1: test %edx,%edx // NULL pointer? (end of the linked list) | |
je 4f | |
// Compare the length expected and the length of the word. | |
// Note that if the F_HIDDEN flag is set on the word, then by a bit of trickery | |
// this won't pick the word (the length will appear to be wrong). | |
xor %eax,%eax | |
movb 4(%edx),%al // %al = flags+length field | |
andb $(F_HIDDEN|F_LENMASK),%al // %al = name length | |
cmpb %cl,%al // Length is the same? | |
jne 2f | |
// Compare the strings in detail. | |
push %ecx // Save the length | |
push %edi // Save the address (repe cmpsb will move this pointer) | |
lea 5(%edx),%esi // Dictionary string we are checking against. | |
repe cmpsb // Compare the strings. | |
pop %edi | |
pop %ecx | |
jne 2f // Not the same. | |
// The strings are the same - return the header pointer in %eax | |
pop %esi | |
mov %edx,%eax | |
ret | |
2: mov (%edx),%edx // Move back through the link field to the previous word | |
jmp 1b // .. and loop. | |
4: // Not found. | |
pop %esi | |
xor %eax,%eax // Return zero to indicate not found. | |
ret | |
/* | |
FIND returns the dictionary pointer, but when compiling we need the codeword pointer (recall | |
that FORTH definitions are compiled into lists of codeword pointers). The standard FORTH | |
word >CFA turns a dictionary pointer into a codeword pointer. | |
The example below shows the result of: | |
WORD DOUBLE FIND >CFA | |
FIND returns a pointer to this | |
| >CFA converts it to a pointer to this | |
| | | |
V V | |
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ | |
| LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | | |
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ | |
codeword | |
Notes: | |
Because names vary in length, this isn't just a simple increment. | |
In this FORTH you cannot easily turn a codeword pointer back into a dictionary entry pointer, but | |
that is not true in most FORTH implementations where they store a back pointer in the definition | |
(with an obvious memory/complexity cost). The reason they do this is that it is useful to be | |
able to go backwards (codeword -> dictionary entry) in order to decompile FORTH definitions | |
quickly. | |
What does CFA stand for? My best guess is "Code Field Address". | |
*/ | |
defcode ">CFA",4,,TCFA | |
pop %edi | |
call _TCFA | |
push %edi | |
NEXT | |
_TCFA: | |
xor %eax,%eax | |
add $4,%edi // Skip link pointer. | |
movb (%edi),%al // Load flags+len into %al. | |
inc %edi // Skip flags+len byte. | |
andb $F_LENMASK,%al // Just the length, not the flags. | |
add %eax,%edi // Skip the name. | |
addl $3,%edi // The codeword is 4-byte aligned. | |
andl $~3,%edi | |
ret | |
/* | |
Related to >CFA is >DFA which takes a dictionary entry address as returned by FIND and | |
returns a pointer to the first data field. | |
FIND returns a pointer to this | |
| >CFA converts it to a pointer to this | |
| | | |
| | >DFA converts it to a pointer to this | |
| | | | |
V V V | |
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ | |
| LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | | |
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ | |
codeword | |
(Note to those following the source of FIG-FORTH / ciforth: My >DFA definition is | |
different from theirs, because they have an extra indirection). | |
You can see that >DFA is easily defined in FORTH just by adding 4 to the result of >CFA. | |
*/ | |
defword ">DFA",4,,TDFA | |
.int TCFA // >CFA (get code field address) | |
.int INCR4 // 4+ (add 4 to it to get to next word) | |
.int EXIT // EXIT (return from FORTH word) | |
/* | |
COMPILING ---------------------------------------------------------------------- | |
Now we'll talk about how FORTH compiles words. Recall that a word definition looks like this: | |
: DOUBLE DUP + ; | |
and we have to turn this into: | |
pointer to previous word | |
^ | |
| | |
+--|------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ | |
| LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | | |
+---------+---+---+---+---+---+---+---+---+------------+--|---------+------------+------------+ | |
^ len pad codeword | | |
| V | |
LATEST points here points to codeword of DUP | |
There are several problems to solve. Where to put the new word? How do we read words? How | |
do we define the words : (COLON) and ; (SEMICOLON)? | |
FORTH solves this rather elegantly and as you might expect in a very low-level way which | |
allows you to change how the compiler works on your own code. | |
FORTH has an INTERPRET function (a true interpreter this time, not DOCOL) which runs in a | |
loop, reading words (using WORD), looking them up (using FIND), turning them into codeword | |
pointers (using >CFA) and deciding what to do with them. | |
What it does depends on the mode of the interpreter (in variable STATE). | |
When STATE is zero, the interpreter just runs each word as it looks them up. This is known as | |
immediate mode. | |
The interesting stuff happens when STATE is non-zero -- compiling mode. In this mode the | |
interpreter appends the codeword pointer to user memory (the HERE variable points to the next | |
free byte of user memory -- see DATA SEGMENT section below). | |
So you may be able to see how we could define : (COLON). The general plan is: | |
(1) Use WORD to read the name of the function being defined. | |
(2) Construct the dictionary entry -- just the header part -- in user memory: | |
pointer to previous word (from LATEST) +-- Afterwards, HERE points here, where | |
^ | the interpreter will start appending | |
| V codewords. | |
+--|------+---+---+---+---+---+---+---+---+------------+ | |
| LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | | |
+---------+---+---+---+---+---+---+---+---+------------+ | |
len pad codeword | |
(3) Set LATEST to point to the newly defined word, ... | |
(4) .. and most importantly leave HERE pointing just after the new codeword. This is where | |
the interpreter will append codewords. | |
(5) Set STATE to 1. This goes into compile mode so the interpreter starts appending codewords to | |
our partially-formed header. | |
After : has run, our input is here: | |
: DOUBLE DUP + ; | |
^ | |
| | |
Next byte returned by KEY will be the 'D' character of DUP | |
so the interpreter (now it's in compile mode, so I guess it's really the compiler) reads "DUP", | |
looks it up in the dictionary, gets its codeword pointer, and appends it: | |
+-- HERE updated to point here. | |
| | |
V | |
+---------+---+---+---+---+---+---+---+---+------------+------------+ | |
| LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | | |
+---------+---+---+---+---+---+---+---+---+------------+------------+ | |
len pad codeword | |
Next we read +, get the codeword pointer, and append it: | |
+-- HERE updated to point here. | |
| | |
V | |
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+ | |
| LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | | |
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+ | |
len pad codeword | |
The issue is what happens next. Obviously what we _don't_ want to happen is that we | |
read ";" and compile it and go on compiling everything afterwards. | |
At this point, FORTH uses a trick. Remember the length byte in the dictionary definition | |
isn't just a plain length byte, but can also contain flags. One flag is called the | |
IMMEDIATE flag (F_IMMED in this code). If a word in the dictionary is flagged as | |
IMMEDIATE then the interpreter runs it immediately _even if it's in compile mode_. | |
This is how the word ; (SEMICOLON) works -- as a word flagged in the dictionary as IMMEDIATE. | |
And all it does is append the codeword for EXIT on to the current definition and switch | |
back to immediate mode (set STATE back to 0). Shortly we'll see the actual definition | |
of ; and we'll see that it's really a very simple definition, declared IMMEDIATE. | |
After the interpreter reads ; and executes it 'immediately', we get this: | |
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ | |
| LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | DUP | + | EXIT | | |
+---------+---+---+---+---+---+---+---+---+------------+------------+------------+------------+ | |
len pad codeword ^ | |
| | |
HERE | |
STATE is set to 0. | |
And that's it, job done, our new definition is compiled, and we're back in immediate mode | |
just reading and executing words, perhaps including a call to test our new word DOUBLE. | |
The only last wrinkle in this is that while our word was being compiled, it was in a | |
half-finished state. We certainly wouldn't want DOUBLE to be called somehow during | |
this time. There are several ways to stop this from happening, but in FORTH what we | |
do is flag the word with the HIDDEN flag (F_HIDDEN in this code) just while it is | |
being compiled. This prevents FIND from finding it, and thus in theory stops any | |
chance of it being called. | |
The above explains how compiling, : (COLON) and ; (SEMICOLON) works and in a moment I'm | |
going to define them. The : (COLON) function can be made a little bit more general by writing | |
it in two parts. The first part, called CREATE, makes just the header: | |
+-- Afterwards, HERE points here. | |
| | |
V | |
+---------+---+---+---+---+---+---+---+---+ | |
| LINK | 6 | D | O | U | B | L | E | 0 | | |
+---------+---+---+---+---+---+---+---+---+ | |
len pad | |
and the second part, the actual definition of : (COLON), calls CREATE and appends the | |
DOCOL codeword, so leaving: | |
+-- Afterwards, HERE points here. | |
| | |
V | |
+---------+---+---+---+---+---+---+---+---+------------+ | |
| LINK | 6 | D | O | U | B | L | E | 0 | DOCOL | | |
+---------+---+---+---+---+---+---+---+---+------------+ | |
len pad codeword | |
CREATE is a standard FORTH word and the advantage of this split is that we can reuse it to | |
create other types of words (not just ones which contain code, but words which contain variables, | |
constants and other data). | |
*/ | |
defcode "CREATE",6,,CREATE | |
// Get the name length and address. | |
pop %ecx // %ecx = length | |
pop %ebx // %ebx = address of name | |
// Link pointer. | |
movl var_HERE,%edi // %edi is the address of the header | |
movl var_LATEST,%eax // Get link pointer | |
stosl // and store it in the header. | |
// Length byte and the word itself. | |
mov %cl,%al // Get the length. | |
stosb // Store the length/flags byte. | |
push %esi | |
mov %ebx,%esi // %esi = word | |
rep movsb // Copy the word | |
pop %esi | |
addl $3,%edi // Align to next 4 byte boundary. | |
andl $~3,%edi | |
// Update LATEST and HERE. | |
movl var_HERE,%eax | |
movl %eax,var_LATEST | |
movl %edi,var_HERE | |
NEXT | |
/* | |
Because I want to define : (COLON) in FORTH, not assembler, we need a few more FORTH words | |
to use. | |
The first is , (COMMA) which is a standard FORTH word which appends a 32 bit integer to the user | |
memory pointed to by HERE, and adds 4 to HERE. So the action of , (COMMA) is: | |
previous value of HERE | |
| | |
V | |
+---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+ | |
| LINK | 6 | D | O | U | B | L | E | 0 | | <data> | | |
+---------+---+---+---+---+---+---+---+---+-- - - - - --+------------+ | |
len pad ^ | |
| | |
new value of HERE | |
and <data> is whatever 32 bit integer was at the top of the stack. | |
, (COMMA) is quite a fundamental operation when compiling. It is used to append codewords | |
to the current word that is being compiled. | |
*/ | |
defcode ",",1,,COMMA | |
pop %eax // Code pointer to store. | |
call _COMMA | |
NEXT | |
_COMMA: | |
movl var_HERE,%edi // HERE | |
stosl // Store it. | |
movl %edi,var_HERE // Update HERE (incremented) | |
ret | |
/* | |
Our definitions of : (COLON) and ; (SEMICOLON) will need to switch to and from compile mode. | |
Immediate mode vs. compile mode is stored in the global variable STATE, and by updating this | |
variable we can switch between the two modes. | |
For various reasons which may become apparent later, FORTH defines two standard words called | |
[ and ] (LBRAC and RBRAC) which switch between modes: | |
Word Assembler Action Effect | |
[ LBRAC STATE := 0 Switch to immediate mode. | |
] RBRAC STATE := 1 Switch to compile mode. | |
[ (LBRAC) is an IMMEDIATE word. The reason is as follows: If we are in compile mode and the | |
interpreter saw [ then it would compile it rather than running it. We would never be able to | |
switch back to immediate mode! So we flag the word as IMMEDIATE so that even in compile mode | |
the word runs immediately, switching us back to immediate mode. | |
*/ | |
defcode "[",1,F_IMMED,LBRAC | |
xor %eax,%eax | |
movl %eax,var_STATE // Set STATE to 0. | |
NEXT | |
defcode "]",1,,RBRAC | |
movl $1,var_STATE // Set STATE to 1. | |
NEXT | |
/* | |
Now we can define : (COLON) using CREATE. It just calls CREATE, appends DOCOL (the codeword), sets | |
the word HIDDEN and goes into compile mode. | |
*/ | |
defword ":",1,,COLON | |
.int WORD // Get the name of the new word | |
.int CREATE // CREATE the dictionary entry / header | |
.int LIT, DOCOL, COMMA // Append DOCOL (the codeword). | |
.int LATEST, FETCH, HIDDEN // Make the word hidden (see below for definition). | |
.int RBRAC // Go into compile mode. | |
.int EXIT // Return from the function. | |
/* | |
; (SEMICOLON) is also elegantly simple. Notice the F_IMMED flag. | |
*/ | |
defword ";",1,F_IMMED,SEMICOLON | |
.int LIT, EXIT, COMMA // Append EXIT (so the word will return). | |
.int LATEST, FETCH, HIDDEN // Toggle hidden flag -- unhide the word (see below for definition). | |
.int LBRAC // Go back to IMMEDIATE mode. | |
.int EXIT // Return from the function. | |
/* | |
EXTENDING THE COMPILER ---------------------------------------------------------------------- | |
Words flagged with IMMEDIATE (F_IMMED) aren't just for the FORTH compiler to use. You can define | |
your own IMMEDIATE words too, and this is a crucial aspect when extending basic FORTH, because | |
it allows you in effect to extend the compiler itself. Does gcc let you do that? | |
Standard FORTH words like IF, WHILE, ." and so on are all written as extensions to the basic | |
compiler, and are all IMMEDIATE words. | |
The IMMEDIATE word toggles the F_IMMED (IMMEDIATE flag) on the most recently defined word, | |
or on the current word if you call it in the middle of a definition. | |
Typical usage is: | |
: MYIMMEDWORD IMMEDIATE | |
...definition... | |
; | |
but some FORTH programmers write this instead: | |
: MYIMMEDWORD | |
...definition... | |
; IMMEDIATE | |
The two usages are equivalent, to a first approximation. | |
*/ | |
defcode "IMMEDIATE",9,F_IMMED,IMMEDIATE | |
movl var_LATEST,%edi // LATEST word. | |
addl $4,%edi // Point to name/flags byte. | |
xorb $F_IMMED,(%edi) // Toggle the IMMED bit. | |
NEXT | |
/* | |
'addr HIDDEN' toggles the hidden flag (F_HIDDEN) of the word defined at addr. To hide the | |
most recently defined word (used above in : and ; definitions) you would do: | |
LATEST @ HIDDEN | |
'HIDE word' toggles the flag on a named 'word'. | |
Setting this flag stops the word from being found by FIND, and so can be used to make 'private' | |
words. For example, to break up a large word into smaller parts you might do: | |
: SUB1 ... subword ... ; | |
: SUB2 ... subword ... ; | |
: SUB3 ... subword ... ; | |
: MAIN ... defined in terms of SUB1, SUB2, SUB3 ... ; | |
HIDE SUB1 | |
HIDE SUB2 | |
HIDE SUB3 | |
After this, only MAIN is 'exported' or seen by the rest of the program. | |
*/ | |
defcode "HIDDEN",6,,HIDDEN | |
pop %edi // Dictionary entry. | |
addl $4,%edi // Point to name/flags byte. | |
xorb $F_HIDDEN,(%edi) // Toggle the HIDDEN bit. | |
NEXT | |
defword "HIDE",4,,HIDE | |
.int WORD // Get the word (after HIDE). | |
.int FIND // Look up in the dictionary. | |
.int HIDDEN // Set F_HIDDEN flag. | |
.int EXIT // Return. | |
/* | |
' (TICK) is a standard FORTH word which returns the codeword pointer of the next word. | |
The common usage is: | |
' FOO , | |
which appends the codeword of FOO to the current word we are defining (this only works in compiled code). | |
You tend to use ' in IMMEDIATE words. For example an alternate (and rather useless) way to define | |
a literal 2 might be: | |
: LIT2 IMMEDIATE | |
' LIT , \ Appends LIT to the currently-being-defined word | |
2 , \ Appends the number 2 to the currently-being-defined word | |
; | |
So you could do: | |
: DOUBLE LIT2 * ; | |
(If you don't understand how LIT2 works, then you should review the material about compiling words | |
and immediate mode). | |
This definition of ' uses a cheat which I copied from buzzard92. As a result it only works in | |
compiled code. It is possible to write a version of ' based on WORD, FIND, >CFA which works in | |
immediate mode too. | |
*/ | |
defcode "'",1,,TICK | |
lodsl // Get the address of the next word and skip it. | |
pushl %eax // Push it on the stack. | |
NEXT | |
/* | |
BRANCHING ---------------------------------------------------------------------- | |
It turns out that all you need in order to define looping constructs, IF-statements, etc. | |
are two primitives. | |
BRANCH is an unconditional branch. 0BRANCH is a conditional branch (it only branches if the | |
top of stack is zero). | |
The diagram below shows how BRANCH works in some imaginary compiled word. When BRANCH executes, | |
%esi starts by pointing to the offset field (compare to LIT above): | |
+---------------------+-------+---- - - ---+------------+------------+---- - - - ----+------------+ | |
| (Dictionary header) | DOCOL | | BRANCH | offset | (skipped) | word | | |
+---------------------+-------+---- - - ---+------------+-----|------+---- - - - ----+------------+ | |
^ | ^ | |
| | | | |
| +-----------------------+ | |
%esi added to offset | |
The offset is added to %esi to make the new %esi, and the result is that when NEXT runs, execution | |
continues at the branch target. Negative offsets work as expected. | |
0BRANCH is the same except the branch happens conditionally. | |
Now standard FORTH words such as IF, THEN, ELSE, WHILE, REPEAT, etc. can be implemented entirely | |
in FORTH. They are IMMEDIATE words which append various combinations of BRANCH or 0BRANCH | |
into the word currently being compiled. | |
As an example, code written like this: | |
condition-code IF true-part THEN rest-code | |
compiles to: | |
condition-code 0BRANCH OFFSET true-part rest-code | |
| ^ | |
| | | |
+-------------+ | |
*/ | |
defcode "BRANCH",6,,BRANCH | |
add (%esi),%esi // add the offset to the instruction pointer | |
NEXT | |
defcode "0BRANCH",7,,ZBRANCH | |
pop %eax | |
test %eax,%eax // top of stack is zero? | |
jz code_BRANCH // if so, jump back to the branch function above | |
lodsl // otherwise we need to skip the offset | |
NEXT | |
/* | |
LITERAL STRINGS ---------------------------------------------------------------------- | |
LITSTRING is a primitive used to implement the ." and S" operators (which are written in | |
FORTH). See the definition of those operators later. | |
TELL just prints a string. It's more efficient to define this in assembly because we | |
can make it a single Linux syscall. | |
*/ | |
defcode "LITSTRING",9,,LITSTRING | |
lodsl // get the length of the string | |
push %esi // push the address of the start of the string | |
push %eax // push it on the stack | |
addl %eax,%esi // skip past the string | |
addl $3,%esi // but round up to next 4 byte boundary | |
andl $~3,%esi | |
NEXT | |
defcode "TELL",4,,TELL | |
mov $1,%ebx // 1st param: stdout | |
pop %edx // 3rd param: length of string | |
pop %ecx // 2nd param: address of string | |
mov $__NR_write,%eax // write syscall | |
int $0x80 | |
NEXT | |
/* | |
QUIT AND INTERPRET ---------------------------------------------------------------------- | |
QUIT is the first FORTH function called, almost immediately after the FORTH system "boots". | |
As explained before, QUIT doesn't "quit" anything. It does some initialisation (in particular | |
it clears the return stack) and it calls INTERPRET in a loop to interpret commands. The | |
reason it is called QUIT is because you can call it from your own FORTH words in order to | |
"quit" your program and start again at the user prompt. | |
INTERPRET is the FORTH interpreter ("toploop", "toplevel" or "REPL" might be a more accurate | |
description -- see: http://en.wikipedia.org/wiki/REPL). | |
*/ | |
// QUIT must not return (ie. must not call EXIT). | |
defword "QUIT",4,,QUIT | |
.int RZ,RSPSTORE // R0 RSP!, clear the return stack | |
.int INTERPRET // interpret the next word | |
.int BRANCH,-8 // and loop (indefinitely) | |
/* | |
This interpreter is pretty simple, but remember that in FORTH you can always override | |
it later with a more powerful one! | |
*/ | |
defcode "INTERPRET",9,,INTERPRET | |
call _WORD // Returns %ecx = length, %edi = pointer to word. | |
// Is it in the dictionary? | |
xor %eax,%eax | |
movl %eax,interpret_is_lit // Not a literal number (not yet anyway ...) | |
call _FIND // Returns %eax = pointer to header or 0 if not found. | |
test %eax,%eax // Found? | |
jz 1f | |
// In the dictionary. Is it an IMMEDIATE codeword? | |
mov %eax,%edi // %edi = dictionary entry | |
movb 4(%edi),%al // Get name+flags. | |
push %ax // Just save it for now. | |
call _TCFA // Convert dictionary entry (in %edi) to codeword pointer. | |
pop %ax | |
andb $F_IMMED,%al // Is IMMED flag set? | |
mov %edi,%eax | |
jnz 4f // If IMMED, jump straight to executing. | |
jmp 2f | |
1: // Not in the dictionary (not a word) so assume it's a literal number. | |
incl interpret_is_lit | |
call _NUMBER // Returns the parsed number in %eax, %ecx > 0 if error | |
test %ecx,%ecx | |
jnz 6f | |
mov %eax,%ebx | |
mov $LIT,%eax // The word is LIT | |
2: // Are we compiling or executing? | |
movl var_STATE,%edx | |
test %edx,%edx | |
jz 4f // Jump if executing. | |
// Compiling - just append the word to the current dictionary definition. | |
call _COMMA | |
mov interpret_is_lit,%ecx // Was it a literal? | |
test %ecx,%ecx | |
jz 3f | |
mov %ebx,%eax // Yes, so LIT is followed by a number. | |
call _COMMA | |
3: NEXT | |
4: // Executing - run it! | |
mov interpret_is_lit,%ecx // Literal? | |
test %ecx,%ecx // Literal? | |
jnz 5f | |
// Not a literal, execute it now. This never returns, but the codeword will | |
// eventually call NEXT which will reenter the loop in QUIT. | |
jmp *(%eax) | |
5: // Executing a literal, which means push it on the stack. | |
push %ebx | |
NEXT | |
6: // Parse error (not a known word or a number in the current BASE). | |
// Print an error message followed by up to 40 characters of context. | |
mov $2,%ebx // 1st param: stderr | |
mov $errmsg,%ecx // 2nd param: error message | |
mov $errmsgend-errmsg,%edx // 3rd param: length of string | |
mov $__NR_write,%eax // write syscall | |
int $0x80 | |
mov (currkey),%ecx // the error occurred just before currkey position | |
mov %ecx,%edx | |
sub $buffer,%edx // %edx = currkey - buffer (length in buffer before currkey) | |
cmp $40,%edx // if > 40, then print only 40 characters | |
jle 7f | |
mov $40,%edx | |
7: sub %edx,%ecx // %ecx = start of area to print, %edx = length | |
mov $__NR_write,%eax // write syscall | |
int $0x80 | |
mov $errmsgnl,%ecx // newline | |
mov $1,%edx | |
mov $__NR_write,%eax // write syscall | |
int $0x80 | |
NEXT | |
.section .rodata | |
errmsg: .ascii "PARSE ERROR: " | |
errmsgend: | |
errmsgnl: .ascii "\n" | |
.data // NB: easier to fit in the .data section | |
.align 4 | |
interpret_is_lit: | |
.int 0 // Flag used to record if reading a literal | |
/* | |
ODDS AND ENDS ---------------------------------------------------------------------- | |
CHAR puts the ASCII code of the first character of the following word on the stack. For example | |
CHAR A puts 65 on the stack. | |
EXECUTE is used to run execution tokens. See the discussion of execution tokens in the | |
FORTH code for more details. | |
SYSCALL0, SYSCALL1, SYSCALL2, SYSCALL3 make a standard Linux system call. (See <asm/unistd.h> | |
for a list of system call numbers). As their name suggests these forms take between 0 and 3 | |
syscall parameters, plus the system call number. | |
In this FORTH, SYSCALL0 must be the last word in the built-in (assembler) dictionary because we | |
initialise the LATEST variable to point to it. This means that if you want to extend the assembler | |
part, you must put new words before SYSCALL0, or else change how LATEST is initialised. | |
*/ | |
defcode "CHAR",4,,CHAR | |
call _WORD // Returns %ecx = length, %edi = pointer to word. | |
xor %eax,%eax | |
movb (%edi),%al // Get the first character of the word. | |
push %eax // Push it onto the stack. | |
NEXT | |
defcode "EXECUTE",7,,EXECUTE | |
pop %eax // Get xt into %eax | |
jmp *(%eax) // and jump to it. | |
// After xt runs its NEXT will continue executing the current word. | |
defcode "SYSCALL3",8,,SYSCALL3 | |
pop %eax // System call number (see <asm/unistd.h>) | |
pop %ebx // First parameter. | |
pop %ecx // Second parameter | |
pop %edx // Third parameter | |
int $0x80 | |
push %eax // Result (negative for -errno) | |
NEXT | |
defcode "SYSCALL2",8,,SYSCALL2 | |
pop %eax // System call number (see <asm/unistd.h>) | |
pop %ebx // First parameter. | |
pop %ecx // Second parameter | |
int $0x80 | |
push %eax // Result (negative for -errno) | |
NEXT | |
defcode "SYSCALL1",8,,SYSCALL1 | |
pop %eax // System call number (see <asm/unistd.h>) | |
pop %ebx // First parameter. | |
int $0x80 | |
push %eax // Result (negative for -errno) | |
NEXT | |
defcode "SYSCALL0",8,,SYSCALL0 | |
pop %eax // System call number (see <asm/unistd.h>) | |
int $0x80 | |
push %eax // Result (negative for -errno) | |
NEXT | |
/* | |
DATA SEGMENT ---------------------------------------------------------------------- | |
Here we set up the Linux data segment, used for user definitions and variously known as just | |
the 'data segment', 'user memory' or 'user definitions area'. It is an area of memory which | |
grows upwards and stores both newly-defined FORTH words and global variables of various | |
sorts. | |
It is completely analogous to the C heap, except there is no generalised 'malloc' and 'free' | |
(but as with everything in FORTH, writing such functions would just be a Simple Matter | |
Of Programming). Instead in normal use the data segment just grows upwards as new FORTH | |
words are defined/appended to it. | |
There are various "features" of the GNU toolchain which make setting up the data segment | |
more complicated than it really needs to be. One is the GNU linker which inserts a random | |
"build ID" segment. Another is Address Space Randomization which means we can't tell | |
where the kernel will choose to place the data segment (or the stack for that matter). | |
Therefore writing this set_up_data_segment assembler routine is a little more complicated | |
than it really needs to be. We ask the Linux kernel where it thinks the data segment starts | |
using the brk(2) system call, then ask it to reserve some initial space (also using brk(2)). | |
You don't need to worry about this code. | |
*/ | |
.text | |
.set INITIAL_DATA_SEGMENT_SIZE,65536 | |
set_up_data_segment: | |
xor %ebx,%ebx // Call brk(0) | |
movl $__NR_brk,%eax | |
int $0x80 | |
movl %eax,var_HERE // Initialise HERE to point at beginning of data segment. | |
addl $INITIAL_DATA_SEGMENT_SIZE,%eax // Reserve nn bytes of memory for initial data segment. | |
movl %eax,%ebx // Call brk(HERE+INITIAL_DATA_SEGMENT_SIZE) | |
movl $__NR_brk,%eax | |
int $0x80 | |
ret | |
/* | |
We allocate static buffers for the return static and input buffer (used when | |
reading in files and text that the user types in). | |
*/ | |
.set RETURN_STACK_SIZE,8192 | |
.set BUFFER_SIZE,4096 | |
.bss | |
/* FORTH return stack. */ | |
.align 4096 | |
return_stack: | |
.space RETURN_STACK_SIZE | |
return_stack_top: // Initial top of return stack. | |
/* This is used as a temporary input buffer when reading from files or the terminal. */ | |
.align 4096 | |
buffer: | |
.space BUFFER_SIZE | |
/* | |
START OF FORTH CODE ---------------------------------------------------------------------- | |
We've now reached the stage where the FORTH system is running and self-hosting. All further | |
words can be written as FORTH itself, including words like IF, THEN, .", etc which in most | |
languages would be considered rather fundamental. | |
I used to append this here in the assembly file, but I got sick of fighting against gas's | |
crack-smoking (lack of) multiline string syntax. So now that is in a separate file called | |
jonesforth.f | |
If you don't already have that file, download it from http://annexia.org/forth in order | |
to continue the tutorial. | |
*/ | |
/* END OF jonesforth.S */ |
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