After a lot of preparatory work, we are now in position to finally tighten
the types of the 6502 registers defined in the CPURegs struct of sim65.
All registers were previously defined as bare 'unsigned', leading to subtle
bugs where the bits beyond the 8 or 16 "true" bits in the register could
become non-zero. Tightening the types of the registers to uint8_t and
uint16_t as appropriate gets rid of these subtle bugs once and for all,
assisted by the semantics of C when assigning an unsigned value to an
unsigned type with less bits: the high-order bits are simply discarded,
which is precisely what we'd want to happen.
This change cleans up a lot of spurious failures of sim65 against the
65x02 test-set. For the 6502 and 65C02, we're now *functionally*
compliant. For timing (i.e., clock cycle counts for each instruction),
some work remains.
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