When I was in Uni in the early 90s I used a ICE dev system to write code for the 6809. ICE stands for "in circuit emulator". So you could create a prototype board and connect the ICE to where the CPU was meant to go and then step through any code it needed to run, seeing all the registers on the ICE.

The dev system was a dual M68000 computer running Unix. I don't know what the machine was called.