Which IRQ can a device using an 8-bit ISA bus NOT use?

Answer 1

Your question is a bit vague..... a wee bit more detail would help get you a clearer response.
There was only one answer, " 2" which is a good guess, but not the only answer, and in many cases, not even the correct answer...

If the computer is a true 8 bit ISA computer, then the answer " 2 " is completely wrong ! There is a great deal of history in the ISA bus needed to explain this...

The IBM first PC had no floppy and no harddrive. There were TWO DIN type plugs that were identical to the keyboard 5 pin plug on the back. The first was the keyboard, the second was used to plug in the Audio Tape Cassette cable, to store your programs on ordinary Audio cassette tapes.... on an ordinary tape recorder.
The bus, was the ISA, 8 bit bus, and the IRQ ( Interupt ReQuest ) table typically had reserved functions to drive the motherboard peripherals, and any of these could not be used by any cards inserted. Since, in the beginning, the " clones " could arrange the IRQ's and the motherboard controller chips in any order they wanted, and in many instances, they DELIBERATELY changed functions on the motherboards to avoid conflicts and legal implications with " IBM " et al, there were many proprietary "ISA " problems in compatability - you really had to read the manuals and experiment, since there was no real co-operation or rules that were universally followed. The first units " generally " used the same chipset as the ' IBM " the 8088, and generally followed the IRQ designations ( with a few changes from time to time ).
IBM realized that with thousands of aftermarket manufacturers building " ISA " cards - cards that no one even dreamed of, during the design of the " PC " itself, that the 8 IRQs offered were totally inadequate. Clone manufacturers were free to develope newer versions of the " PC", and added many features - the 80186 chip, for example, while IBM persisted with the 8088.
Finally IBM changed to the 80286 chip, and with this, used a bit of a patch to get around the IRQ limit - they wired a SECOND entire chip into IRQ 2. Thus the Extended, ISA slot on the new 286 had an additional extension on the end, and this extension had a 16 bit ISA addressing capability. The IRQs were increased, --however the numbering was a bit lop-sided, since the entire second IRQ chip was at IRQ 2 ! The original IRQ on the motherboard, that was now used by the new chip, was re-wired into the second chip. Here it gets confusing.... the second chip has, like the first chip IRQs 0,1,2,3,4,5,6 & 7, however, they are wired to continue from the original chips' IRQ's # 7, and are re-labelled,
0 = 8
1 = 9
2 = 10
3 = 11
4 = 12
5 = 13
6 = 14
7 = 15

The original IRQ #2 is now wired into the second chips' IRQ 1, which is re-labelled IRQ 9 ( could it possibley get more confusing ??? )

Bottom line, - on the 16 BIT, ISA slot, you cannot access IRQ 2.
IRQ 8 is not wired on the ISA 16 bit bus, and is the Real-time clock
IRQ 13 is not wired on the 16 bit bus, and is the Math Co-processor
Most motherboard BIOS settings exclude the use of various IRQ's by the ISA cards, such as 6 for the floppy, or 14 and 15 for the IDE harddrive controllers, etc.

So .....
your question is in reference to an 8-BIT ISA bus, and NOT to the 16 bit ISA bus that uses IRQ 2 as an output of the IRQs on the second, added, chip.

SO, back to the 8 bit, original, " PC ", ISA, 8088, bus...
Even here, as I pointed out earlier, there were many discrepancies in day-to-day computer assemblies - a technician could spend weeks trying to figure out why a specific ISA, 8 BIT card would not work in one slot, in one computer, since any manufacturer could change anything they wanted, and documentation was rather sketchy. If you used a " genuine" " IBM " PC, then you could count of more or less " standard" card slot configurations, with documentations available, and technical support.... Otherwise you were guessing --- there were MANY aftermarket software programs and hardware ISA cards that did nothing except monitor the ISA buses in an attempt to resolve the many conflicts.... On top of this problem there were hundreds of NON-Standard ISA cards that would only work in specific machines... -- it was quite a mess...

So lets look at the original specification for the IBM, 8-bit, ISA, IRQ table, as it was first implimented....

THERE were 8 IRQ's
0 was the system timer - and was not accessible by the
ISA slots.
1 was the KEYBOARD controller, and again was not wired to the
slots.


2, 3, 4, 5, 6, and 7 were wired into the slots, and could be used by any of the added ISA 8 bit cards...

IBM generally used the following scheme though, as did many card manufacturers... Keep in mind, that if you had an original " AS SHIPPED" IBM 8-BIT, ISA PC, there were NO cards in any slots, no floppy, no Harddrive, No CD, no sound, 16 K of ram, no mouse, etc.

Therefore, you needed to add ISA cards to add a floppy controller, a harddrive controller, a serial port ( all printers in the beginning were SERIAL, 9 pin PLUG, printers, which were like big, modified typewriters ! ), a parallel port ( the 25 pin " printer" plug ).

2 was OPEN and free for anything.

3 was designated for Serial Port ( mouse port or comm port 2)
4 was designated serial port 1
5 was for the HARD DISK card
6 was for the Floppy controller card
7 was reserved for the Parallel port ( 25 pin port ) controller.

This left nothing BUT ... IRQ 2 available for use by thousands of cards manufactured for the PC....
The only way to put in a new card on a PC was to take OUT some card that you could do without at the moment....

IRQ 0 and IRQ 1 were NOT available.

Hope that this clears up some of the confusion....


robin

Answer 2

2