IBM System/32 available

Eric Smith spacewar at
Mon Nov 9 21:31:23 CST 2015

>> Do you have any idea why it was thought a good idea to use
>> edge-triggered interrupts?
> I wasn’t really involved in the HW side of things but I think it was because it was
> “easier”.  I think at least one device just generated a pulse for an interrupt rather
> than having some sort of latch.

I can easily believe that to be true of the System/23. To the best of
my knowledge, no add-on boards IBM marketed for the PC worked that
way.  For the PC, edge-triggered interrupts were a pain in the ass,
especially if you wanted to have multiple devices (e.g., COM ports)
sharing an interrupt. Oh well.

It took a long time, but the PC legacy interrupts are finally all but
eliminated from modern PCs, replaced by the much saner APIC. While the
PCI bus still used discrete interrupt lines, PCIe now uses bus
transactions, which further eliminates legacy restrictions. Of course,
for compatibility, the north bridge (sometimes integrated into the CPU
chip) still has to provide 8259A interrupt controller functionality
and by default map at least some of the modern PCIe interrupt hardware
to it, to support legacy software, but most recent PC OSes use the
APIC natively even on uniprocessor systems.

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