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(In the current code, ST(I) and not R(I) is stored, so we end up with MM(I) == regcache(FP0 + (TOP + 1) % 8) :-( I'm almost ready to dust off the patch that does this.)The example of the IA-32's MMX and FP registers is a great example for this. The MMX registers, MM0--MM7, and the FP registers, ST(0)--ST(7), actually refer to the same set of eight eighty-bit registers, R0--R7. A reference to the floating-point register ST(i) becomes a reference to R((TOP + i) % 8), where TOP is a three-bit field in the FPU status register. But a reference to the MMX register MM(i) becomes a reference to the lower 64 bits of R(i) (which would be the mantissa of some ST(i)).
Clearly that isn't low level enough! GDB needs to be getting down to the level of quantum effects :-^processors, then there are all sorts of confusing questions that brings up --- e.g., "How in the world would GDB get hold of the state of the raw flip-flops on a native Linux system?")
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