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Things I learned while writing an x86 emulator (2013)
If you’ve read my first post about assembly language, you might expect that this is another post on how to understand assembly language. I will write more about that at some point, but this post is not that. Instead, this post is going to talk about some of the weird things and random trivia I learned while writing an x86 and amd64 emulator. The emulator I wrote was for Time Travel Debugging.
The encoding space used by INC and DEC was quite large, so it’s understandable why the AMD64 designers decided to use these bytes for the new prefixes to expand the register set in 64 bit mode. I’ve heard that the Atom line of CPUs used a cheaper/slower way of doing bit shifts in the ALU, which results in the undefined flags having different values, although I haven’t tested this myself. What can complicate things is the fact that usermode code doesn’t have access to the CPU configuration that determines the base address of the FS or GS segments.
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