We've just updated MediaWiki and its underlying software. If anything doesn't look or work quite right, please mention it to us. --RanAS
Overflow Flag: Difference between revisions
From SnesLab
(how it behaves) |
(more description) |
||
| Line 1: | Line 1: | ||
The '''Overflow Flag''' (V) is affected only by the following eight instructions: | The '''Overflow Flag''' (V) is affected only by the following eight instructions: | ||
* [[ADC]] | * [[ADC]] (indicates signed sum overflowed) | ||
* [[SBC]] | * [[SBC]] (indicates signed difference underflowed) | ||
* [[CLV]] (always clears it) | * [[CLV]] (always clears it) | ||
* [[BIT]] | * [[BIT]] | ||
| Line 9: | Line 9: | ||
* [[SEP]] (might set it) | * [[SEP]] (might set it) | ||
* [[REP]] (might clear 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 8 bits wide, V indicates whether the sum/difference of [[ADC]]/[[SBC]] is outside the range of -128 to 127. | ||
Revision as of 16:01, 24 July 2024
The Overflow Flag (V) is affected only by the following eight instructions:
- ADC (indicates signed sum overflowed)
- SBC (indicates signed difference underflowed)
- CLV (always clears it)
- BIT
- 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.
It may be the most misunderstood flag of the 65c816 and the 6502.[2] It is bit 6 of the status register.
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.
On the SPC700 it can be cleared with CLRV and is set whenever the half-carry flag is set.
See Also
References
- Eyes & Lichty, page 439: https://archive.org/details/0893037893ProgrammingThe65816/page/439
- Clark, Bruce. http://www.6502.org/tutorials/vflag.html
- Shirriff, Ken. https://www.righto.com/2012/12/the-6502-overflow-flag-explained.html?m=1
- lbid. https://www.righto.com/2013/01/a-small-part-of-6502-chip-explained.html?m=1
- Labiak. https://archive.org/details/Programming_the_65816/page/n118
- Pickens, John. NMOS 6502 Opcodes. http://www.6502.org/tutorials/6502opcodes.html#VFLAG