View Full Version : Help the PC be all it can be

Great Hierophant
March 21st, 2006, 05:34 PM
In the BIOS of the IBM PC and XT, the following were supported:

3xParallel Ports
4xSerial Ports
1xGame Port
1xMonochrome Adapter
1xColor Adapter
2xInternal 360KB drives (720KB supported in later XTs)
2xExternal 360KB drives
640KB of RAM
8087 Math Co-processor
83-Key Keyboard (101-Key LEDless Keyboard for later XTs)
5 (PC) or 8 (XT) 8-bit ISA slots
Cassette Port (PC only)

Of course, the problem was that there weren't enough resources in a PC or XT to support all these options, nevermind additional devices like a hard drive.

Each printer port came on a separate card, as did the game port and the serial port. One printer port is given by the MDA card. Only with an Expansion Chassis could you possibly completely fill out a PC using contemporaneous IBM Hardware.

Color/Graphics Adapter
Diskette Drive Adapter
Printer Adapter
Game Control Adapter
Expansion Card

Asychronous Communications Adapter
Asychronous Communications Adapter
Asychronous Communications Adapter
Asychronous Communications Adapter
Monochrome Display and Printer Adapter
384-512KB RAM Card
Printer Adapter
Expansion Card

Without considering a Fixed Disk Drive Adapter (technically not supported in PC or XT BIOS), IRQs seem to be a major problem. Only 3-5,7 are available in an 8-bit system. I believe that all the Printer Adapters would use IRQ7 and one pair of Asych Adapters should use IRQ 4 and the other pair IRQ3. It is interesting to note that if you use a Color/Graphics Adapter, the system will only support two parallel cards. However, never use COM1/COM2 at the same time as COM3/COM4 and never use any two LPTs at the same time

If you add a Fixed Disk Adapter (which would require an XT), it will take IRQ 5 and you can only add one (IBM brand) as the I/O ports of two would overlap (as would a second IBM Diskette Drive Adapter.)

March 21st, 2006, 06:27 PM
Your post does an excellent job of explaining why the third party cards like the AST 6 PAK were so popular. On a single card you could get memory, a clock/calendar, serial port, parallel port, etc.

As for the IRQ problems, on the parallel ports it is not a big deal. It was very rare to have software enable the IRQs on the parallel ports because polling was so much easier. So you can have all three parallel ports sharing the same IRQ because in practice, none of them actually use the IRQ. And if you did decide to use an IRQ, you could still enable it for one of the three cards and continue to use polling on the others.

The COM ports are a different story - software tended to use IRQs on those. In that case it became important not to use COM1/COM3 or COM2/COM4 at the same time if you had them on their standard port/IRQ combinations.

Great Hierophant
March 21st, 2006, 06:53 PM
Unfortunately IBM's early cards didn't allow you to change settings by jumpers and 8-bit PCs didn't have CMOS to save settings either.

Also, while it may be off-topic, if you used an external 720KB floppy drive in a PC or XT without 720 support in the BIOS, would you be able to use it as a reliable 360KB drive?