Recently I have been looking into Bochs to try to figure out why the multitasking code (which works on physical machines) fails under Bochs.
The discovery was made yesterday that the reason it was failing under Bochs specifically is because Bochs does not start with zero-initialised memory, and that the initialisation code for my multitasking did not clear its variables. What's more is that the compiler assumed that the BSS would be zeroed, and even refused my explicit zero initialisation. This was fixed up and the code got further before crashing.
Last night and today, I was looking into the Bochs debugger and trying to figure out why it seemed that the code compiled on my laptop was causing it to fail even after fixing the above issue. After hours of stepping and deleting code and stepping and disassembling, I could not figure how the EBP register was being overwritten by the EIP value.
Finally, I spotted it. The code I was using (modified from some tutorial code) set up the tasking jump using unnamed registers, and the GCC compiler was choosing its own registers to use. Unfortunately, one of it's choices conflicted with a register I was using. A few tweaks later and the code compiled correctly. Also setting the "clobber" registers to the call made it compile reliably.
This was a particularly difficult issue to track down as each time the code was changed and recompiled, it could have compiled differently, and some changes would "fix" the problem, even though they did not directly affect it. This made the problem intermittent at best.
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