Negative Flag

The Negative Flag (N) is bit 7 of the 65c816's status register. It is set or cleared to reflect the most significant bit of arithmetic/logical results or pulled/loaded/transferred values. Although there are no dedicated SEN or CLN instructions to set or clear it, it can be set with SEP and cleared with REP:

SEP #$80
REP #$80

These instructions also affect the negative flag (this bulleted list being 38 long):

• ADC
• AND
• ASL
• BIT
• CMP
• CPX
• CPY
• DEC
• DEX
• DEY
• EOR
• INC
• INX
• INY
• LDA
• LDX
• LDY
• ORA
• PLA
• PLB
• PLD
• PLP (missing from the Labiak textbook's list)
• PLX
• PLY
• ROL
• ROR
• SBC
• TAX
• TAY
• TCD
• TDC
• TSC
• TSX
• TXA
• TXY
• TYA
• TYX
• XBA

Also, LSR always clears the negative flag because it also shifts a zero into the operand's most significant bit, making it non-negative. Push instructions do not affect the negative flag.

BPL and BMI examine the negative flag to decide whether or not to branch.

See Also

• Zero Flag
• Overflow Flag
• Carry Flag
• Sign Flag

References

• Labiak, Willam. Page 109
• Eyes & Lichty, "Branching Based on the Negative Flag" on page 148