In real life I can't imagine a read of both of them simultaneously would do anything useful; there'd be a bus fight that would potentially damage one of the PIAs.

Yeah, you are probably right, I haven't seen a dual device read so far and since the second PIA is optional it is unlikely that the rom code does it. However, just to be accurate I will split out the reads and log an error if it occurs, eg that both A4 and A3 are high in a read operation.

The IAI Swyft does almost exactly the same thing with two PIAs, they use an address line each for /CS so it is possible to enable both at once and 'bad things' will happen.

According to the debugger two overlapping writes are both executed as are the reads, so I let the writes work with the default behavior of MAME while I made a board specific read driver to catch an eventual dual, or even zero, device read. So far nothing that 'bad', lets see what happens when add the keyboard to the PIA.

Overlapping writes are fine, since only one thing's driving the bus in that case.

sounds like something you might want to watch for in a driver debug mode

Yeah, a logerror() would be nice in case the original programmers did something wrong and are getting away with it by accident.

I use logerror if that happens, it hasn't so far. I couldn't see that the Swyft driver detects it but is isn't fatal in an emulator of course unless we have a smoke_callback() to the UI that is...

hmm, maybe if we emulated the heat generated from the system and compared that to the known cooling factor of the system and the room temperature we can generate smoke or boil emulated eggs on the Pentiums one day...

In any case, the logerror() of this multi device reads is just a curiosity which can reveal rom programming errors and of course driver programming errors, so it is kind of fun/useful I think.

I think it would be helpful for people writing new code for a platform.

Absolutely, especially in this case since the PIA:s was intended to be the students play ground with lab expansion boards and such