BF interpreter in RGBASM

bf.asm

ASCII EQUS "␀
\t\n\r !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz\{|}~"

err: MACRO
    println "error: ", \1
    ; print where the error occurred
    println "    {src}"
    print "    "
    rept src_i - 1
        print "_"
    endr
    println "^"
    fail "execution halted"
ENDM

bf: MACRO
; interpret a BF program

if !_NARG
    fail "Usage: bf source[, input]"
endc

; point to the first program char
src_i = 1
if _NARG > 0
REDEF src EQUS "\1"
src_len = STRLEN("{src}")
else
src_len = 0
endc

; point to the first input char
in_p = 1
if _NARG > 1
REDEF in EQUS "\2"
in_len = STRLEN("{in}")
else
in_len = 0
endc

; point to the first cell
mem_p = 0

; run the program
rept $7fff_ffff
if src_i > src_len
    break
endc

; get the current source code character
x = STRSUB("{src}", src_i, 1)

; increment the current cell
if x == "+"
if mem_p < 0
    err "+ outside tape"
endc
if !DEF(mem{d:mem_p})
mem{d:mem_p} = 0
endc
mem{d:mem_p} = LOW(mem{d:mem_p} + 1)

; decrement the current cell
elif x == "-"
if mem_p < 0
    err "- outside tape"
endc
if !DEF(mem{d:mem_p})
mem{d:mem_p} = 0
endc
mem{d:mem_p} = LOW(mem{d:mem_p} - 1)

; move to the next cell
elif x == ">"
mem_p = mem_p + 1

; move to the previous cell
elif x == "<"
mem_p = mem_p - 1

; output the current cell
elif x == "."
if mem_p < 0
    err ". outside tape"
endc
if !DEF(mem{d:mem_p})
mem{d:mem_p} = 0
endc
print STRSUB("{ASCII}", mem{d:mem_p} + 1, 1)

; input the current cell
elif x == ","
if mem_p < 0
    err ", outside tape"
endc
if in_p <= in_len
mem{d:mem_p} = STRSUB("\2", in_p, 1)
in_p = in_p + 1
else
mem{d:mem_p} = 0
endc

; begin while loop
elif x == "["
if mem_p < 0
    err "[ outside tape"
endc
if !DEF(mem{d:mem_p})
mem{d:mem_p} = 0
endc
if mem{d:mem_p} == 0
if !DEF(pair{d:src_i})
; find the corresponding ']' after the '['
depth = 1
for i, src_i + 1, src_len + 1
y = STRSUB("{src}", i, 1)
depth = depth + (y == "[") - (y == "]")
if depth == 0
    break
endc
endr
if i == src_len
    err "[ without ]"
endc
pair{d:src_i} = i
endc
src_i = pair{d:src_i}
endc

; end while loop
elif x == "]"
if mem_p < 0
    err "] outside tape"
endc
if !DEF(mem{d:mem_p})
mem{d:mem_p} = 0
endc
if mem{d:mem_p} != 0
if !DEF(pair{d:src_i})
; find the corresponding '[' before the ']'
depth = 1
for i, src_i - 1, 0, -1
y = STRSUB("{src}", i, 1)
depth = depth - (y == "[") + (y == "]")
if depth == 0
    break
endc
endr
if i == 0
    err "] without ["
endc
pair{d:src_i} = i
endc
src_i = pair{d:src_i}
endc

endc

; move to the next source code character
src_i = src_i + 1

endr

ENDM

; Usage: rgbasm bf.asm -Dsrc=... -Din=...
if DEF(src)
if DEF(in)
    bf {src}, /* keep leading spaces */{in},
else
    bf {src}, ,
endc
elif DEF(in)
; echo
    bf >\,[>\,]<[<]>[.>]++++++++++., /* keep leading spaces */{in},
else
; hello world
    bf ++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.+++.------.--------.>>+.>++.
endc