It's actually a fairly common approach to driving some old-school processors. For example I've used an Arduino to drive a Z80 processor. I toggle the clock li

It's actually a fairly common approach to driving some old-school processors. For example I've used an Arduino to drive a Z80 processor. I toggle the clock line high & low, and monitor the I/O ports on the Z80 such that when it thinks it is reading RAM it is really reading from an array on the Arduino. You can get quite far, given the relative clock-speeds of the two chips. Of course it gets a bit fiddly if you need to halt the processor and proxy reads to other I/O devices - such as using the Z80 "out (1), a" instruction to print a character to an LCD. But this is a good way to get started.