I have fond memories of the IRQ from the mouse card. It was the only source of regular interrupts you could get on the Apple II (that I'm aware of). So in 1987, I tried to write a preemptive switcher, so you could run two code paths at the same time.<p>The development of that code was very painful. At the time, there was no external debugger. The moment you enabled the interrupt, your interrupt handler would get called, and it would try to program a context switch (something the 6502 is definitely not supposed to do). If you had any bug in there, your Apple II would be completely frozen, all you could do is reboot, and try to guess what went wrong and try again.
Such an odd choice not to have a timer interrupt on Apple II and then having to do this kind of trickery on a MouseCard. I guess this saved a few bucks...
That mouse card is tricky to emulate. I'm surprised actually that the author's shufflepuck port seems to work fine in our Apple IIe emulator, microM8, since our mouse code is a bit of a mess!