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Memory/Accumulator Select

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The Memory/Accumulator Select (M) flag is bit 5 of the 65c816's processor status register. It indicates whether the accumulator is 8 or 16 bits wide:

  • When clear, the accumulator is 16 bits wide. It can only be clear in native mode.
  • When set, the accumulator is 8 bits wide, but the high byte (B) is still retained. This is the case after reset because the processor is in emulation mode.

It can be affected by:

  • REP (clears it if bit 5 of operand is set)
  • SEP (sets it if bit 5 of operand is set)
  • PLP (pops it off the stack)
  • RTI

It affects the behavior of (possibly incomplete list):

But it does not affect XBA, TDC, TCD, TCS, or TSC.

There are no BMS or BMC instructions that examine this flag.

See Also

Reference