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Memory/Accumulator Select: Difference between revisions
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The '''Memory/Accumulator Select''' (M) flag | 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 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. | * 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: | It can be affected by: | ||
* [[REP]] ( | * [[REP]] (clears it if bit 5 of operand is set) | ||
* [[SEP]] ( | * [[SEP]] (sets it if bit 5 of operand is set) | ||
* [[PLP]] | * [[PLP]] (pops it off the [[stack]]) | ||
* [[RTI]] | * [[RTI]] | ||
It affects the behavior of (incomplete list): | It affects the behavior of (possibly incomplete list): | ||
* [[LDA]] | * [[LDA]] | ||
* [[STA]] | * [[STA]] | ||
* [[STZ]] | |||
* [[ADC]] | |||
* [[SBC]] | |||
* [[BIT]] | |||
* [[CMP]] | |||
* [[PHA]] | * [[PHA]] | ||
* [[PLA]] | * [[PLA]] | ||
| Line 21: | Line 26: | ||
* [[ROR]] | * [[ROR]] | ||
* [[ROL]] | * [[ROL]] | ||
* [[ORA]] | |||
* [[AND]] | |||
* [[EOR]] | |||
* [[INC]] | |||
* [[DEC]] | |||
* [[TSB]] | |||
* [[TRB]] | |||
But it does not affect [[XBA]]. | But it does not affect [[XBA]], [[TDC]], [[TCD]], [[TCS]], or [[TSC]]. | ||
There are no BMS or BMC instructions that examine this flag. | There are no BMS or BMC instructions that examine this flag. | ||
Latest revision as of 21:40, 16 August 2024
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
- Eyes & Lichty, page 422, Table 18.2. 65x Flags.