|I tri-state gate on a interrupt line is NOT what you'd want to use if the
|interrupt were sharable.  Two systems that decide to DRIVE the interrupt
|line at the same time would still cause problems.  You need something like
|open-collector lines instead.

IBM in their infinite wisdom decided to have the interrupts
on the PC be active high.  Thus, the simplest way of making
a driver that can share the line is to use a 3-state buffer
chip whose input is always high; the enable on the buffer
then switches the output between floating and high.

|My understanding is that no one implemented shared interrupt drivers because
|the hardware was essentially broken...

In another piece of their infinite wisdom, IBM decided to
make interrupts edge-triggered rather than level-controlled.
Sharing a level-controlled interrupt is easy: simply have the
ISR check the state of any device which could cause the int-
terrupt.  Even if a device earlier in the testing sequence
starts asserting IRQ while a later device is being checked,
the device will keep IRQ asserted until it's acknowleged;
this in turn will force the CPU to re-execute the ISR from the
start, and detect the interrupting condition which it earlier
had missed.

Edge-triggered interrupts are much harder to share reliably:
after any interrupting device is dealt with it's essential to
re-poll *ALL* the devices that could assert the interrupt line.
Returning from an interrupt when a device on the line has not
yet been serviced will effectively put that IRQ line out of
commission.  Annoying...