Assembler Simulator

I was reading about old assembly code today and I thought "I would really like to see how drastic these crazy speed tweaks are!" I don't have access to an assembler or a machine slow enough to let me notice, so I decided it would be a good idea for someone to build a program that simulates running assembler code on various old CPUs. It would be fine if it was in some high-level language; I just want to be able to control the clock speed and watch memory locations. There should be readouts for the registers and a display corresponding to a graphics memory location. Of course, all of this would be simulated - but it would make perusing old documentation so much more interesting. I just might make this.