I used to work for a (no defunct) company, ARDI, that made a Mac emulator. We dynamically translated 68K to x86 and did a clean room reimplementation of Mac OS 9. (Yes, crazy.) Much of the job involved debugging the OS when running 3rd party software (binaries). The first step was to collect a syscall trace. Then lots of thought experiments. It was a great way to develop extreme debugging skills.

> clean room reimplementation of Mac OS 9

Wow. Was the project successful? Where can we read more about it? Was it a perfect implementation - all the images in the right places pixel by pixel?

Source is at https://github.com/ctm/executor

I think the OP is of by two OS versions, but I didn't work at ARDI, so I may be wrong.