... and if anyone is wondering, here's how to deal with such things (ie. got code for a strange undocumented CPU): https://www.robertxiao.ca/hacking/dsctf-2019-cpu-adventure-unknown-cpu-reversing/ … — congrats to the solver team! :)
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Too many people consider this sort of things to be impossible. In fact, some statistical analysis + guesswork + staring at the code a lot and trying out many hypotheses can go a long way, and instruction sets can be recovered from much smaller samples than this.
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This task was born because I did just that (reversing a binary for a custom arcane CPU from a single code sample without any documentation) multiple times, and wanted to demonstrate that it can be done, and it's not as hard as it seems.
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Unfortunately, I cannot really publish what I've done. So — big thanks to PPP for their writeup! It's really a nice instruction that I've always missed :)
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Oh, and if you're wondering about some weirdness in my little CPU, please note that I didn't really invent any of it — it's an amalgamation of real production CPUs that I've come across, and every single feature is stolen, most of them from the last CPU I've blackbox-reversed.
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