The MCU that I use for this series of boards is a STM32F405VGT6. It has ample room for controlling bits, and it can also run a software emulator for most of the 8-bit CPUs out there. The STM can support DMA and interrupts.
Supported CPUs so far:
Z80, Z8001, 8088, PC-XT, Generic
All these provide test features:
1) Bit control and probe to test for short, continuity, and other basic malfunction
2) Bus transaction to test peripheral (helps tame periferals to peripherals!)
3) Memory editing, IO manipulations
4) (Z80 for now) Emulation, stepping and register manipulation
Jacques Pelletier
Jacob Hahn
Grant Giesbrecht
Michael Cullen
Eric Lind