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Zero Flag: Difference between revisions
From SnesLab
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The [[BEQ]] and [[BNE]] instructions examine the zero flag to decide whether or not to branch. | The [[BEQ]] and [[BNE]] instructions examine the zero flag to decide whether or not to branch. Push instructions do not affect the zero flag. | ||
The [[SPC700]] and [[Super FX]] chips also have a zero flag. | The [[SPC700]] and [[Super FX]] chips also have a zero flag. | ||
Revision as of 05:11, 16 August 2024
The Zero Flag (Z) is bit 1 of the 65c816's status register. It indicates whether the last value computed, transferred, or pulled is zero: set if it is and clear if not. Although there are no dedicated SEZ or CLZ instructions to set or clear it, it can be directly set with SEP and cleared with REP:
SEP #$02 REP #$02
It is also affected by these 41 instructions:
- ADC
- AND
- ASL
- BIT
- CMP
- CPX
- CPY
- DEC
- DEX
- DEY
- EOR
- INC
- INX
- INY
- LDA
- LDX
- LDY
- LSR
- ORA
- PLA
- PLB
- PLD
- PLX
- PLY
- ROL
- ROR
- SBC
- TAX
- TAY
- TCD
- TCS
- TDC
- TRB
- TSB
- TSC
- TSX
- TXA
- TXY
- TYA
- TYX
- XBA
The BEQ and BNE instructions examine the zero flag to decide whether or not to branch. Push instructions do not affect the zero flag.
The SPC700 and Super FX chips also have a zero flag.
See Also
References
- Labiak, page 108
- Eyes & Lichty, "Branching Based on the Zero Flag" on page 146