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Intro to the ISA bus by Mark Sokos
THE ISA AND PC/104 BUS
IBM, IBM/XT, IBM PC, and IBM PC AT are registered trademarks of
International Business Machines Corporation.
This file is designed to give a basic overview of the bus found in
most IBM clone computers, often referred to as the XT or AT bus. The
AT version of the bus is upwardly compatible, which means that cards
designed to work on an XT bus will work on an AT bus. This bus was
produced for many years without any formal standard. In recent years,
a more formal standard called the ISA bus (Industry Standard Architecture)
has been created, with an extension called the EISA (Extended ISA) bus
also now as a standard. The EISA bus extensions will not be detailed here.
The PC/104 bus is an adaptation of the ISA bus for embedded computing
use. It uses the same signals as ISA, but uses a smaller connector and
cards that are stackable, which eliminates the need for a backplane. The
name PC/104 comes from the fact that the bus was invented for the PC,
and has 104 pins.
This file is not intended to be a thorough coverage of the standard. It
is for informational purposes only, and is intended to give designers
and hobbyists sufficient information to design their own XT and AT
compatible cards.
The IEEE P996 standard may be obtained from:
IEEE Standards Office
445 Hoes Lane
Piscataway, NJ 08854
The PC/104 standard may be obtained from:
The PC/104 Consortium
PO Box 4303
Mountain View, CA 94040
Connector Pinouts
ISA Connector
Component Side
A1: *CHKCHK B1: GND
A2: SD7 B2: RESDRV
A3: SD6 B3: +5
A4: SD5 B4: IRQ2
A5: SD4 B5: -5
A6: SD3 B6: DRQ2
A7: SD2 B7: -12
A8: SD1 B8: *NOWS [A]
A9: SD0 B9: +12
A10: CHRDY B10: GND
A11: AEN B11: *SMWTC
A12: SA19 B12: *SMRDC
A13: SA18 B13: *IOWC
A14: SA17 B14: *IORC
A15: SA16 B15: *DAK3
A16: SA15 B16: DRQ3
A17: SA14 B17: *DAK1
A18: SA13 B18: DRQ1
A19: SA12 B19: *REFRESH
A20: SA11 B20: BCLK
A21: SA10 B21: IRQ7
A22: SA9 B22: IRQ6
A23: SA8 B23: IRQ5
A24: SA7 B24: IRQ4
A25: SA6 B25: IRQ3
A26: SA5 B26: *DAK2
A27: SA4 B27: TC
A28: SA3 B28: BALE
A29: SA2 B29: +5
A30: SA1 B30: OSC
A31: SA0 B31: GND
AT Bus only:
Component Side
C1: *SBHE D1: *M16
C2: LA23 D2: *IO16
C3: LA22 D3: IRQ10
C4: LA21 D4: IRQ11
C5: LA20 D5: IRQ12
C6: LA19 D6: IRQ15 [B]
C7: LA18 D7: IRQ14
C8: LA17 D8: *DAK0
C9: *MRDC D9: DRQ0
C10: *MWTC D10: *DAK5
C11: SD8 D11: DRQ5
C12: SD9 D12: *DAK6
C13: SD10 D13: DRQ65
C14: SD11 D14: *DAK7
C15: SD12 D15: DRQ7
C16: SD13 D16: +5
C17: SD14 D17: *MASTER16
C18: SD15 D18: GND
* Active Low
[A] A signal called J8, or Card_Select was used on this pin on the IBM
XT. This signal was only active on the J8 slot on the motherboard (the
slot closest to the keyboard connector).
[B] Many texts (including earlier versions of mine) accidentally place
IRQ13 on this pin. I'm not sure how IRQ15 managed to end up on the pin
where IRQ13 fits in the sequence (IRQ10-14), but it is easy to see how
this causes numerous typographic errors.
Note that IRQ13 (coprocessor) is not present on the ISA bus connector.
ISA physical dimensions:
+---- mounting bracket this end
|
|
+------------------------------------------------------------------+ <--+
| | |
| | |
| | |
| | 4.8
| | |
+------+ +------+ +------------+ |
| | | | | | | | | | | | | | | | | | | | | | |
+---------------------------+ +-----------+ <--+
|-.705-|--------3.10+-.005---------|-.220-|----1.87---|
Notes:
All demensions in inches
All demensions are +-.010 except as noted
Total length can not exceed 13.10+-.020
Longer connector has 31 gold tabs on each side 0.100+-.005
center to center 0.06+-.005 width
Shorter connector has 18 gold tabs on each side 0.100+-.005
center to center 0.06+-.005 width
PC/104 Connector:
PIN ROW A (J1) ROW B(J1) ROW C (J2) ROW D (J2)
----------------------------------------------------------------
0 GND GND
1 *CHCHK GND *SBHE *M16
2 SD7 RESDRV LA23 *IO16
3 SD6 +5V LA22 IRQ10
4 SD5 IRQ9 LA21 IRQ11
5 SD4 -5V LA20 IRQ12
6 SD3 DRQ2 LA19 IRQ15
7 SD2 -12V LA18 IRQ14
8 SD1 *ENDXFR LA17 *DACK0
9 SD0 +12V *MRDC DRQ0
10 CHRDY KEY *MWTC *DACK5
11 AEN *SMWTC SD8 DRQ5
12 SA19 *SMRDC SD9 *DACK6
13 SA18 *IOWC SD10 DRQ6
14 SA17 *IORC SD11 *DACK7
15 SA16 *DACK3 SD12 DRQ7
16 SA15 DRQ3 SD13 +5V
17 SA14 *DACK1 SD14 *MASTER16
18 SA13 DRQ1 SD15 GND
19 SA12 *REFRESH KEY GND
20 SA11 CLK
21 SA10 IRQ7
22 SA9 IRQ6
23 SA8 IRQ5
24 SA7 IRQ4
25 SA6 IRQ3
26 SA5 *DACK2
27 SA4 TC
28 SA3 BALE
29 SA2 +5V
30 SA1 OSC
31 SA0 GND
32 GND GND
Rows C and D are used for 16 bit (AT) operation.
Keys are missing pins and holes are filled.
Electrical Characteristics:
The actual drive capabilities of ISA motherboards can vary greatly.
The IEEE P996 specs 1.0 offers these guidelines:
+12V at 1.5A
-12V at 0.3A
+5V at 4.5A
-5V at 0.2A
PC/104 specifies the following:
+12V at 1.0A
+5V at 2.0A
-5V at 0.2A
-12V at 0.3A
M16, IO16, MASTER16, and ENDXFR are 20 mA max. All other signals
are 4 mA max.
Signal Descriptions:
+5, -5, +12, -12: Power supplies. -5 is often not implimented.
The PC/104 spec does not require unused voltages to be present
on the bus.
AEN: Address Enable. This is asserted when a DMAC has control of the
bus. This prevents an I/O device from responding to the I/O
command lines during a DMA transfer.
BALE: Bus Address Latch Enable. The address bus is latched on the rising
edge of this signal. The address on the SA bus is valid from the
falling edge of BALE to the end of the bus cycle. Memory devices
should latch the LA bus on the falling edge of BALE. Some references
refer to this signal as Buffered Address Latch Enable, or just
Address Latch Enable (ALE).
BCLK: Bus Clock, 33% Duty Cycle. Frequency Varies. 4.77 to 8 MHz typical.
8.3 MHz is specified as the maximum, but many systems allow this
clock to be set to 12 MHz and higher.
CHCHK: Channel Check. A low signal generates an NMI. The NMI signal
can be masked on a PC, externally to the processor (of course).
Bit 7 of port 70(hex) (enable NMI interrupts) and bit 3 of
port 61 (hex) (recognition of channel check) must both be set
to zero for an NMI to reach the cpu.
CHRDY: Channel Ready. Setting this low prevents the default ready timer
from timing out. The slave device may then set it high again
when it is ready to end the bus cycle. Holding this line low
for too long (15 microseconds, typically) can prevent RAM refresh
cycles on some systems.
This signal is called IOCHRDY (I/O Channel Ready) by some references.
CHRDY and NOWS should not be used simultaneously. This may cause
problems with some bus controllers.
DAKx: DMA Acknowledge.
DRQx: DMA Request.
ENDXFR (PC/104 only): End Transfer.
GND: Ground (0 volts).
IO16: I/O size 16. Generated by a 16 bit slave when addressed by a
bus master.
IORC: I/O Read Command line.
IOWC: I/O Write Command line.
IRQx: Interrupt Request. IRQ2 has the highest priority. IRQ 10-15 are
only available on AT machines, and are higher priority than
IRQ 3-7.
LAxx: Latchable Address lines. Combine with the lower address lines to
form a 24 bit address space (16 MB)
MASTER16: 16 bit bus master. Generated by the ISA bus master when
initiating a bus cycle.
M16: Memory Access, 16 bit
MRDC: Memory Read Command line.
MWTC: Memory Write Command line.
NOWS: No Wait State. Used to shorten the number of wait states generated
by the default ready timer. This causes the bus cycle to end
more quickly, since wait states will not be inserted. Most
systems will ignore NOWS if CHRDY is active (low). However,
this may cause problems with some bus controllers, and both
signals should not be active simultaneously.
OSC: Oscillator, 14.318 MHz, 50% Duty Cycle. Frequency varies.
This was originally divided by 3 to provide the 4.77 MHz cpu
clock of early PCs, and divided by 12 to produce the 1.19 MHz
system clock. Some references have placed this signal as low
as 1 MHz (possibly referencing the system clock), but most modern
systems use 14.318 MHz.
This frequency (14.318 MHz) is four times the television colorburst
frequency.
Refresh timing on many PC's is based on OSC/18, or approximately
one refresh cycle every 15 microseconds. Many modern motherboards
allow this rate to be changed, which frees up some bus cycles for
use by software, but also can cause memory errors if the system
RAM cannot handle the slower refresh rates.
REFRESH: Refresh. Generated when the refresh logic is bus master.
An ISA device acting as bus master may also use this signal to
initiate a refresh cycle.
RESDRV: This signal goes momentarily high when the machine is powered up.
Driving it high will force a system reset.
SA0-SA19: System Address Lines, tri-state.
SBHE: System Bus High Enable, tristate. Indicates a 16 bit data transfer.
This may also indicate an 8 bit transfer using the upper half
of the data bus (if an odd address is present).
SD0-SD15: System Data lines, or Standard Data Lines. They are bidrectional
and tri-state. On most systems, the data lines float high when not
driven.
SMRDC: Standard Memory Read Command line. Indicates a memory read in the
lower 1 MB area.
SMWTC: Standard Memory Write Commmand line. Indicates a memory write in
the lower 1 MB area.
TC: Terminal Count. Notifies the cpu that that the last DMA data transfer
operation is complete.
8 Bit Memory or I/O Transfer Timing Diagram (4 wait states shown)
__ __ __ __ __ __ __
BCLK ___| |___| |___| |__| |___| |___| |___| |__
W1 W2 W3 W4
__
BALE _______| |_______________________________________
AEN __________________________________________________
______________________________________
SA0-SA19 ---------<______________________________________>-
_____________ _____
Command Line |______________________________|
(IORC,IOWC,
SMRDC, or SMWTC)
_____
SD0-SD7 ---------------------------------------<_____>----
(READ)
___________________________________
SD0-SD7 ---------<___________________________________>----
(WRITE)
Note: W1 through W4 indicate wait cycles.
BALE is placed high, and the address is latched on the SA bus. The
slave device may safely sample the address during the falling edge
of BALE, and the address on the SA bus remains valid until the end
of the transfer cycle. Note that AEN remains low throughout the entire
transfer cycle.
The command line is then pulled low (IORC or IOWC for I/O commands,
SMRDSC or SMWTC for memory commands, read and write respectively).
For write operations, the data remaines on the SD bus for the remainder
of the transfer cycle. For read operations, the data must be valid
on the falling edge of the last cycle.
NOWS is sampled at the midpoint of each wait cycle. If it is low, the
transfer cycle terminates without further wait states.
CHRDY is sampled during the first half of the clock cycle. If it is
low, further wait cycles will be inserted.
The default for 8 bit transfers is 4 wait states. Some computers allow
the number of default wait states to be changed.
16 Bit Memory or I/O Transfer Timing Diagram (1 wait state shown)
__ __ __ __ __ __
BCLK ___| |___| |___| |__| |___| |___| |_
AEN [2] __________________________________________
_____________
LA17-LA23 -------<_____________>-[1]-----------------
__
BALE ______________| |________________________
________________ _______
SBHE |__________________|
__________________
SA0-SA19 ---------------<__________________>-------
_________________ ____________________
M16 |____|
* * [4]
_________________ ___________
IO16 [3] |_____________|
*
_________________ ___________
Command Line |____________|
(IORC,IOWC,
MRDC, or MWTC)
____
SD0-SD15 ---------------------------<____>---------
(READ) *
______________
SD0-SD15 -----------------<______________>---------
(WRITE)
An asterisk (*) denotes the point where the signal is sampled.
[1] The portion of the address on the LA bus for the NEXT cycle
may now be placed on the bus. This is used so that cards may begin
decoding the address early. Address pipelining must be active.
[2] AEN remains low throughout the entire transfer cycle, indicating
that a normal (non-DMA) transfer is occuring.
[3] Some bus controllers sample this signal during the same clock cycle
as M16, instead of during the first wait state, as shown above.
In this case, IO16 needs to be pulled low as soon as the address is
decoded, which is before the I/O command lines are active.
[4] M16 is sampled a second time, in case the adapter card did not
activate the signal in time for the first sample (usually because the
memory device is not monitoring the LA bus for early address
information, or is waiting for the falling edge of BALE).
16 bit transfers follow the same basic timing as 8 bit transfers.
A valid address may appear on the LA bus prior to the beginning of the
transfer cycle. Unlike the SA bus, the LA bus is not latched, and is
not valid for the entire transfer cycle (on most computers). The LA bus
should be latched on the falling edge of BALE. Note that on some systems,
the LA bus signals will follow the same timing as the SA bus. On
either type of system, a valid address is present on the falling edge
of BALE.
I/O adapter cards do not need to monitor the LA bus or BALE, since I/O
addresses are always within the address space of the SA bus.
SBHE will be pulled low by the system board, and the adapter card must
respond with IO16 or M16 at the appropriate time, or else the transfer
will be split into two seperate 8 bit transfers. Many systems expect
IO16 or M16 before the command lines are valid. This requires that
IO16 or M16 be pulled low as soon as the address is decoded (before it
is known whether the cycle is I/O or Memory). If the system is starting
a memory cycle, it will ignore IO16 (and vice-versa for I/O cycles
and M16).
For read operations, the data is sampled on the rising edge of the
last clock cycle. For write operations, valid data appears on the bus
before the end of the cycle, as shown in the timing diagram. While
the timing diagram indicates that the data needs to be sampled on the
rising clock, on most systems it remains valid for the entire clock
cycle.
The default for 16 bit transfers is 1 wait state. This may be shortened
or lengthened in the same manner as 8 bit transfers, via NOWS and CHRDY.
Many systems only allow 16 bit memory devices (and not I/O devices) to
transfer using 0 wait states (NOWS has no effect on 16 bit I/O cycles).
SMRDC/SMWTC follow the same timing as MRDC/MWTC respectively when
the address is within the lower 1 MB. If the address is not within the
lower 1 MB boundary, SMRDC/SMWTC will remain high during the entire
cycle.
It is also possible for an 8 bit bus cycle to use the upper portion
of the bus. In this case, the timing will be similar to a 16 bit
cycle, but an odd address will be present on the bus. This means that
the bus is transferring 8 bits using the upper data bits (SD8-SD15).
Shortening or Lengthening the bus cycle:
BCLK W W W W
_ __ __ __ __ __ __ __ __ __ __ __
|__| |__| |__| |__| |__| |__| |__| |__| |__| |__| |__| |__
|--Transfer 1-----|----Transfer 2---------|----Transfer 3---|
BALE
__ __ __ __
________| |______________| |____________________| |______________|
SBHE
_________ _______________________
|__________________|__________________|
SA0-SA19
_________________ _____________________ _________________
----------<_________________><_____________________><_________________>
IO16
___________ ___ ___________________________
|_____________| |_____________|
* *
CHRDY
________________________________ _______________________________
|______|
* * * [1]
NOWS
______________________________________________________ _____
|__________|
* [2]
IORC
______________ _______ _______ ____
|_________| |_______________| |_________|
SD0-SD15
____ ____ ____
--------------------<____>------------------<____>------------<____>---
* * *
An asterisk (*) denotes the point where the signal is sampled.
W=Wait Cycle
This timing diagram shows three different transfer cycles. The first
is a 16 bit standard I/O read. This is followed by an almost identical
16 bit I/O read, with one wait state inserted. The I/O device pulls
CHRDY low to indicate that it is not ready to complete the transfer
(see [1]). This inserts a wait cycle, and CHRDY is again sampled. At this
second sample, the I/O device has completed its operation and released
CHRDY, and the bus cycle now terminates.
The third cycle is an 8 bit transfer, which is shortened to 1 wait state
(the default is 4) by the use of NOWS.
I/O Port Addresses
Note: Only the first 10 address lines are decoded for I/O operations.
This limits the I/O address space to address 3FF (hex) and lower.
Some systems allow for 16 bit I/O address space, but may be limited due
to some I/O cards only decoding 10 of these 16 bits.
Port Address Assignments:
Port (hex)
000-00F DMA Controller
010-01F DMA Controller (PS/2)
020-02F Master Programmable Interrupt Controller (PIC)
030-03F Slave PIC
040-05F Programmable Interval Timer (PIT)
060-06F Keyboard Controller
070-071 Real Time Clock
080-083 DMA Page Registers
090-097 Programmable Option Select (PS/2)
0A0-0AF PIC #2
0C0-0CF DMAC #2
0E0-0EF reserved
0F0-0FF Math coprocessor, PCJr Disk Controller
100-10F Programmable Option Select (PS/2)
110-16F AVAILABLE
170-17F Hard Drive 1 (AT)
180-1EF AVAILABLE
1F0-1FF Hard Drive 0 (AT)
200-20F Game Adapter
210-217 Expansion Card Ports
220-26F AVAILABLE
270-27F Parallel Port 3
280-2A1 AVAILABLE
2A2-2A3 clock
2B0-2DF EGA/Video
2E2-2E3 Data Acquisition Adapter (AT)
2E8-2EF Serial Port COM4
2F0-2F7 Reserved
2F8-2FF Serial Port COM2
300-31F Prototype Adapter, Periscope Hardware Debugger
320-32F AVAILABLE
330-33F Reserved for XT/370
340-35F AVAILABLE
360-36F Network
370-377 Floppy Disk Controller
378-37F Parallel Port 2
380-38F SDLC Adapter
390-39F Cluster Adapter
3A0-3AF reserved
3B0-3BB Monochome Adapter
3BC-3BF Parallel Port 1
3C0-3CF EGA/VGA
3D0-3DF Color Graphics Adapter (CGA)
3E0-3EF Serial Port COM3
3F0-3F7 Floppy Disk Controller
3F8-3FF Serial Port COM1
Soundblaster cards usually use I/O ports 220-22F.
Data acquisition cards frequently use 300-320.
Memory Addresses
00000-9FFFF System RAM (640k)
A0000-AFFFF EGA/VGA Video RAM
B0000-BFFFF Hercules/Mono/CGA Video RAM
C0000-C7FFF Video ROM
C8000-CFFFF Hard drive adapter BIOS ROM
D0000-D7FFF I/O Expansion ROM (unused on most systems)
D8000-DFFFF PC JR Cartridge (unused on most systems)
E0000-EFFFF Expansion ROM (unused on some systems)
F0000-FFFFF System ROM
100000+ System RAM (extended memory)
There is very little memory space available for ISA cards to use. Generally,
most computers will let you have the D segment (or at least the lower half
of the D segment), and maybe the E segment, but nothing else. On newer
systems, the D and E segment will usually contain RAM that must be disabled
in the BIOS setup before these memory areas may be used.
It should be obvious from the above memory map that, unlike I/O addresses,
all memory address lines must be decoded to prevent bus collisions with
other memory mapped devices.
To create a rom that is recognized by the BIOS, the first two bytes must
be 55 AA. The third byte must be the number of 512 byte blocks of address
space used by the ROM. The eight bit checksum for the ROM must also be zero.
If all of these conditions are met, then your ROM will be executed when
the system starts, beginning execution at the byte immediately following
the 512 byte block count.
DMA Read and Write
The ISA bus uses two DMA controllers (DMAC) cascaded together.
The slave DMAC connects to the master DMAC via DMA channel 4 (channel
0 on the master DMAC). The slave therefore gains control of the
bus through the master DMAC. On the ISA bus, the DMAC is programmed
to use fixed priority (channel 0 always has the highest priority),
which means that channel 0-4 from the slave have the highest priority
(since they connect to the master channel 0), followed by channels
5-7 (which are channel 1-3 on the master).
The DMAC can be programmed for read transfers (data is read from memory
and written to the I/O device), write transfers (data is read from the
I/O device and written to memory), or verify transfers (neither a
read or a write - this was used by DMA CH0 for DRAM refresh on early
PCs).
Before a DMA transfer can take place, the DMA Controller (DMAC) must
be programmed. This is done by writing the start address and the number
of bytes to transfer (called the transfer count) and the direction of
the transfer to the DMAC.
After the DMAC has been programmed, the device may activate the
appropriate DMA request (DRQx) line.
Slave DMA Controller:
I/O Port
0000 DMA CH0 Memory Address Register
Contains the lower 16 bits of the memory address, written as
two consecutive bytes.
0001 DMA CH0 Transfer Count
Contains the lower 16 bits of the transfer count, written as
two consecutive bytes.
0002 DMA CH1 Memory Address Register
0003 DMA CH1 Transfer Count
0004 DMA CH2 Memory Address Register
0005 DMA CH2 Transfer Count
0006 DMA CH3 Memory Address Register
0007 DMA CH3 Transfer Count
0008 DMAC Status/Control Register
Status (I/O read) bits 0-3: Terminal Count, CH 0-3
bits 4-7: Request CH0-3
Control (write)
bit 0: Mem to mem enable (1 = enabled)
bit 1: ch0 address hold enable (1 = enabled)
bit 2: controller disable (1 = disabled)
bit 3: timing (0 = normal, 1 = compressed)
bit 4: priority (0 = fixed, 1 = rotating)
bit 5: write selection (0 = late, 1 = extended)
bit 6: DRQx sense asserted (0 = high, 1 = low)
bit 7: DAKn sense asserted (0 = low, 1 = high)
0009 Software DRQn Request
bits 0-1: channel select (CH0-3)
bit 2: request bit (0 = reset, 1 = set)
000A DMA mask register
bits 0-1: channel select (CH0-3)
bit 2: mask bit (0 = reset, 1 = set)
000B DMA Mode Register
bits 0-1: channel select (CH0-3)
bits 2-3: 00 = verify transfer, 01 = write transfer, 10 = read
transfer, 11 = reserved
bit 4: Auto init (0 = disabled, 1 = enabled)
bit 5: Address (0 = increment, 1 = decrement)
bits 6-7: 00 = demand transfer mode, 01 = single transfer mode
10 = block transfer mode, 11 = cascade mode
000C DMA Clear Byte Pointer - writing to this causes the DMAC to clear
the pointer used to keep track of 16 bit data transfers into
and out of the DMAC for hi/low byte sequencing.
000D DMA Master Clear (Hardware Reset)
000E DMA Reset Mask Register - clears the mask register
000F DMA Mask Register
bits 0-3: mask bits for CH0-3 (0 = not masked, 1 = masked)
0081 DMA CH2 Page Register (address bits A16-A23)
0082 DMA CH3 Page Register
0083 DMA CH1 Page Register
0087 DMA CH0 Page Register
0089 DMA CH6 Page Register
008A DMA CH7 Page Register
008B DMA CH5 Page Register
Master DMA Controller:
I/O Port
00C0 DMA CH4 Memory Address Register
Contains the lower 16 bits of the memory address, written as
two consecutive bytes.
00C2 DMA CH4 Transfer Count
Contains the lower 16 bits of the transfer count, written as
two consecutive bytes.
00C4 DMA CH5 Memory Address Register
00C6 DMA CH5 Transfer Count
00C8 DMA CH6 Memory Address Register
00CA DMA CH6 Transfer Count
00CC DMA CH7 Memory Address Register
00CE DMA CH7 Transfer Count
00D0 DMAC Status/Control Register
Status (I/O read) bits 0-3: Terminal Count, CH 4-7
bits 4-7: Request CH4-7
Control (write)
bit 0: Mem to mem enable (1 = enabled)
bit 1: ch0 address hold enable (1 = enabled)
bit 2: controller disable (1 = disabled)
bit 3: timing (0 = normal, 1 = compressed)
bit 4: priority (0 = fixed, 1 = rotating)
bit 5: write selection (0 = late, 1 = extended)
bit 6: DRQx sense asserted (0 = high, 1 = low)
bit 7: DAKn sense asserted (0 = low, 1 = high)
00D2 Software DRQn Request
bits 0-1: channel select (CH4-7)
bit 2: request bit (0 = reset, 1 = set)
00D4 DMA mask register
bits 0-1: channel select (CH4-7)
bit 2: mask bit (0 = reset, 1 = set)
00D6 DMA Mode Register
bits 0-1: channel select (CH4-7)
bits 2-3: 00 = verify transfer, 01 = write transfer, 10 = read
transfer, 11 = reserved
bit 4: Auto init (0 = disabled, 1 = enabled)
bit 5: Address (0 = increment, 1 = decrement)
bits 6-7: 00 = demand transfer mode, 01 = single transfer mode
10 = block transfer mode, 11 = cascade mode
00D8 DMA Clear Byte Pointer - writing to this causes the DMAC to clear
the pointer used to keep track of 16 bit data transfers into
and out of the DMAC for hi/low byte sequencing.
00DA DMA Master Clear (Hardware Reset)
00DC DMA Reset Mask Register - clears the mask register
00DE DMA Mask Register
bits 0-3: mask bits for CH4-7 (0 = not masked, 1 = masked)
Single Transfer Mode:
The DMAC is programmed for transfer.
The DMA device requests a transfer by driving the appropriate
DRQ line high. The DMAC responds by asserting AEN and acknowledges
the DMA request through the appropriate DAK line. The I/O and
memory command lines are also asserted. When the DMA device sees
the DAK signal, it drops the DRQ line.
The DMAC places the memory address on the SA bus (at the same time
as the command lines are asserted), and the device either reads from
or writes to memory, depending on the type of transfer. The transfer
count is incrimented, and the address incrimented/decrimented. DAK
is de-asserted. The cpu now once again has control of the bus, and
continues execution until the I/O device is once again ready for
transfer. The DMA device repeats the procedure, driving DRQ high and
waiting for DAK, then transferring data. This continues for a number
of cycles equal to the transfer count. When this has been completed,
the DMAC signals the cpu that the DMA transfer is complete via the TC
(terminal count) signal.
__ __ __ __ __ __
BCLK ___| |___| |___| |__| |___| |___| |___
_______
DRQx _| |___________________________________
______________________________
AEN ____| |________
_______ ________
DAKx |___________________________|
____________________________
SA0-SA15 -------<____________________________>-------
___________ ____________
Command Line |___________________|
(IORC, MRDC)
_____________
SD0-SD7 ----------------------<_____________>-------
(READ)
____________________________
SD0-SD7 -------<____________________________>-------
(WRITE)
Block Transfer Mode:
The DMAC is programmed for transfer.
The device attempting DMA transfer drives the appropriate DRQ line
high. The motherboard responds by driving AEN high and DAK low.
This indicates that the DMA device is now the bus master.
In response to the DAK signal, the DMA device drops DRQ. The DMAC
places the address for DMA transfer on the address bus. Both the
memory and I/O command lines are asserted (since DMA involves both an
I/O and a memory device). AEN prevents I/O devices from responding to
the I/O command lines, which would not result in proper operation since
the I/O lines are active, but a memory address is on the address bus.
The data transfer is now done (memory read or write), and the DMAC
incriments/decriments the address and begins another cycle.
This continues for a number of cycles equal to the DMAC transfer count.
When this has been completed, the terminal count signal (TC) is generated
by the DMAC to inform the cpu that the DMA transfer has been completed.
Note: Block transfer must be used carefully. The bus cannot be used
for other things (like RAM refresh) while block mode transfers are
being done.
Demand Transfer Mode:
The DMAC is programmed for transfer.
The device attempting DMA transfer drives the appropriate DRQ line
high. The motherboard responds by driving AEN high and DAK low.
This indicates that the DMA device is now the bus master.
Unlike single transfer and block transfer, the DMA device does not
drop DRQ in response to DAK. The DMA device transfers data in the
same manner as for block transfers. The DMAC will continue to generate
DMA cycles as long as the I/O device asserts DRQ. When the I/O device
is unable to continue the transfer (if it no longer had data ready to
transfer, for example), it drops DRQ and the cpu once again has control
of the bus. Control is returned to the DMAC by once again asserting DRQ.
This continues until the terminal count has been reached, and the TC
signal informs the cpu that the transfer has been completed.
Interrupts on the ISA bus:
Interrupt
(Hex)
NMI 2 Parity Error, Mem Refresh
IRQ0 8 8253 Channel 0 (System Timer)
IRQ1 9 Keyboard
IRQ2 A Cascade from slave PIC
IRQ3 B COM2
IRQ4 C COM1
IRQ5 D LPT2
IRQ6 E Floppy Drive Controller
IRQ7 F LPT1
IRQ8 F Real Time Clock
IRQ9 F Redirection to IRQ2
IRQ10 F Reserved
IRQ11 F Reserved
IRQ12 F Mouse Interface
IRQ13 F Coprocessor
IRQ14 F Hard Drive Controller
IRQ15 F Reserved
IRQ0,1,2,8, and 13 are not available on the ISA bus.
The IBM PC and XT had only a single 8259 interrupt controller. The
AT and later machines have a second interrupt controller, and the
two are used in a master/slave combination. IRQ2 and IRQ9 are the
same pin on most ISA systems.
Interrupts on most systems may be either edge triggered or level
triggered. The default is usually edge triggered, and active high
(low to high transition).
The interrupt level must be held high until the first interrupt
acknowledge cycle (two interrupt acknowledge bus cycles are generated
in response to an interrupt request).
The software aspects of interrupts and interrupt handlers is intentionally
omitted from this document, due to the numerous syntactical differences
in software tools and the fact that adequate documentation of this topic
is usually provided with developement software.
Bus Mastering:
An ISA device may take control of the bus, but this must be done with
caution. There are no safety mechanisms involved, and so it is easily
possible to crash the entire system by incorrectly taking control of
the bus. For example, most systems require bus cycles for DRAM refresh.
If the ISA bus master does not relinquish control of the bus or generate
its own DRAM refresh cycles every 15 microseconds, the system RAM can
become corrupted. The ISA adapter card can generate refresh cycles without
relinquishing control of the bus by asserting REFRESH. MRDC can be then
monitored to determine when the refresh cycle ends.
To take control of the bus, the device first asserts its DRQ line.
The DMAC sends a hold request to the cpu, and when the DMAC receives
a hold acknowledge, it asserts the appropriate DAK line corresponding
to the DRQ line asserted. The device is now the bus master.
AEN is asserted, so if the device wishes to access I/O devices, it
must assert MASTER16 to release AEN. Control of the bus is returned to
the system board by releasing DRQ.
This file is (C) Copyright 1996-2000 by Mark Sokos. It may be freely
copied and distributed, provided that no fee is charged.
All of the information in this file is provided "as-is".
I have attempted to provide as accurate information as is possible,
however, it should be noted that some references offer conflicting
or incomplete information. Please let me know if any mistakes are
found. Some slight differences may be attributed to the lack of a
formal standard while many pieces of equipment were manufactured.
Send questions or comments to Mark Sokos (sokos@desupernet.net).
The latest version of this file may be found at:
http://users.desupernet.net/sokos/
References:
"ISA System Architecture, 3rd Edition" by Tom Shanley and Don Anderson
ISBN 0-201-40996-8
"Eisa System Architecture, 2nd Edition" by Tom Shanley and Don Anderson
ISBN 0-201-40995-X
"Microcomputer Busses" by R.M. Cram ISBN 0-12-196155-9
HelpPC 2.10 Quick Reference Utility, by David Jurgens
"PC/104 Specification Version 2.3" by the PC/104 Consortium
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