IBM System/32 available

Guy Sotomayor ggs at
Mon Nov 9 22:25:49 CST 2015

On 11/9/15 7:31 PM, Eric Smith wrote:
>>> 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.

Oh, trust me I know about the edge triggered interrupt problem.  I was 
trying to support
a "simple" multi-com board for Xenix/286 on the PC-AT and just couldn't 
make it work
properly with edge triggered interrupts.

One of the things that the MCA in the PS/2's did was to do away with 
edge triggered
interrupts...they were all level.  A number of us demanded that of the 
new bus.  Of course
we didn't get the most important thing right (configurability) but we 
were constrained by
being able to take existing designs and just "tweak" them in order to 
make them MCA
> 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.

The 8259 (along with the damned 8254) are still alive and well in 
current Intel PCHs.  The
current (Skylake) generation of CPUs and PCHs still have them.  :-(

Actually as of Windows 8.x the 8254 is *still* used.  :-(

TTFN - Guy

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