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One of the more painful things I had to debug with machine code. Tricks
I used can be seen in ex10.subx:
- printing argv[1] in various places
- printing a single 'X' in various places to count how many times we get
to different instructions
- exiting with the current value of EAX in various places
I repeatedly went down the wrong trail in several ways:
- forgetting that the problem lay in native runs, and accidentally switching
to subx runs during debugging.
- forgetting to pass commandline args, because ex10 doesn't check its argv
- writing the wrong comment for an instruction, and then miscalculating
the set of registers that need to be saved.
- forgetting that syscalls clobber EAX.
Debugging native runs is hard, because you have to write non-trivial code
to instrument the binary, and instrumentation can itself be buggy.
When we finally tracked it down, I recognized the problem immediately.
I'd meant to confirm the behavior of opcode 8a against bare metal, and
then forgot.
In any case, opcode 8a was inconsistent with 88. Sloppy.
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