Comment by gblargg

Good point. I was thinking why he didn't just do the V20 etc. on the FPGA, but then a software emulator would be more flexible. Having a real hardware interface that has the right timing is something software would have to work very hard to do. Using an FPGA also forces the designer to more or less really understand the original hardware design, not just the software-visible effects as an emulator author can get away with.