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WDC 65C816 register file and processor-status flags.
The 65C816 has variable-width data paths: the accumulator A is 8- or 16-bit per the
M status flag, and the index registers X/Y are 8- or 16-bit per the X status
flag. In 8-bit index mode the high byte of X/Y is forced to zero. The hidden E
(emulation) latch forces M/X to the 8-bit width and confines the stack to page $01.
See docs/cpu.md (“Registers and state”, “Emulation vs native mode”).