I was intrigued with some interrupt response time figures. Theoretically I should have figures slightly over 1us (and indeed I have) but sometimes the response time scaled up from 2.0 to 3.5 us.

Then I realized that the longest response times were all on rising edges!

What happens is that I am using DDR register to emulate open collector outputs and for that reason the fall times are within expected, but the rise time will depend upon the R-C constant formed by the pullup resistors on the MSX computer along with the parasitic capacitance (predominantly from the wiring).

I should have seen that before!