Great Hierophant
Veteran Member
As you may know, the IBM PC XT came with three different BIOSes. The first motherboard came with an 8KB BIOS virtually identical to the IBM PC's third BIOS. The XT's first motherboard removed the cassette socket and control electronics and added 3 additional ISA slots (one unbuffered) and allowed the the system to address 640KB of RAM on the motherboard with the addition of a chip and a jumper. There should not be any compatibility issues between a third BIOS PC and a first BIOS XT.
But after the AT came out IBM started to experiment with the 8/16 bit architecture of the XT. The second and third BIOSes are 32KB in size, just like the ATs. Obviously the ROM addressing on the XT was tweaked a bit to support this, but this wouldn't affect compatibility. Second, support was added for 720KB floppy drives. Again, hardly necessary to increase the ROM size, especially when there is space available from the extraneous cassette routines. No injury to compatibility there either. Finally, these XTs supported the LED-less 101-Key Keyboards.
While the interface hadn't changed, IBM did have to change how the keys were addressed in the ROM, especially as there were 18 more keys available. There were some programs that had difficulty with the new keyboard. Which ones did and could you fix them by using the 83-key keyboard?
But after the AT came out IBM started to experiment with the 8/16 bit architecture of the XT. The second and third BIOSes are 32KB in size, just like the ATs. Obviously the ROM addressing on the XT was tweaked a bit to support this, but this wouldn't affect compatibility. Second, support was added for 720KB floppy drives. Again, hardly necessary to increase the ROM size, especially when there is space available from the extraneous cassette routines. No injury to compatibility there either. Finally, these XTs supported the LED-less 101-Key Keyboards.
While the interface hadn't changed, IBM did have to change how the keys were addressed in the ROM, especially as there were 18 more keys available. There were some programs that had difficulty with the new keyboard. Which ones did and could you fix them by using the 83-key keyboard?