Overflow Flag

The Overflow Flag (V) is bit 6 of the status register. It is affected only by the following eight instructions:

• ADC (indicates signed sum overflowed)
• SBC (indicates signed difference underflowed)
• CLV (always clears it)
• BIT (becomes bit 6 or bit 14 of operand except in immediate addressing)
• PLP
• RTI
• SEP (might set it)
• REP (might clear it)

Here is how it behaves:

• When the accumulator is 8 bits wide, V indicates whether the sum/difference of ADC/SBC is outside the range of -128 to 127.
• When the accumulator is 16 bits wide, V indicates whether the result of ADC/SBC is outside the range of -32768 to 32767.

Adding a positive and negative integer together never sets the overflow flag because the sum has a smaller magnitude.

It may be the most misunderstood flag of the 65c816 and the 6502.^[2][3]

There is no SEV instruction to directly set the overflow flag, but the 6502 and 65c02 do have a hardware signal to set it. Unfortunately this hardware signal was removed on the 65c816 (or fortunately, as now it can no longer get set by accident.)

On the SPC700 it can be cleared with CLRV and is set whenever the half-carry flag is set.

BVC and BVS both examine the overflow flag to decide whether or not to branch.

See Also

• BVC (SPC700)
• BVS (SPC700)
• O/V
• Carry Flag
• Zero Flag

References

1. Eyes & Lichty, page 439: https://archive.org/details/0893037893ProgrammingThe65816/page/439
2. Clark, Bruce. http://www.6502.org/tutorials/vflag.html
3. Shirriff, Ken. The 6502 overflow flag explained mathematically. https://www.righto.com/2012/12/the-6502-overflow-flag-explained.html?m=1
4. lbid. https://www.righto.com/2013/01/a-small-part-of-6502-chip-explained.html?m=1
5. Labiak. https://archive.org/details/Programming_the_65816/page/n118
6. Pickens, John. NMOS 6502 Opcodes. http://www.6502.org/tutorials/6502opcodes.html#VFLAG
7. https://forums.nesdev.org/viewtopic.php?t=6331