Status flags

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The flags register, also called processor status or just P, is one of the six architectural registers on the 6502 family CPU. It is composed of six one-bit registers; instructions modify one or more bits and leave others unchanged.

Instructions that save or restore the flags map them to bits in the architectural 'P' register as follows:

7  bit  0
---- ----
NVss DIZC
|||| ||||
|||| |||+- Carry
|||| ||+-- Zero
|||| |+--- Interrupt Disable
|||| +---- Decimal
||++------ No CPU effect, see: the B flag
|+-------- Overflow
+--------- Negative
  • The PHP (Push Processor Status) and PLP (Pull Processor Status) instructions can be used to retrieve or set this register directly via the stack.
  • Interrupts, including the NMI and also the pseudo-interrupt BRK instruction, implicitly push the status register to the stack.
  • Interrupts returning with RTI will implicitly pull the saved status register from the stack.

C: Carry

  • After ADC, this is the carry result of the addition.
  • After SBC or CMP, this flag will be set if no borrow was the result, or alternatively a "greater than or equal" result.
  • After a shift instruction (ASL, LSR, ROL, ROR), this contains the bit that was shifted out.
  • Increment and decrement instructions do not affect the carry flag.
  • Can be set or cleared directly with SEC, CLC.

Z: Zero

  • After most instructions that have a value result, if that value is zero, this flag will be set.

I: Interrupt Disable

  • When set, all interrupts except the NMI are inhibited.
  • Can be set or cleared directly with SEI, CLI.
  • Automatically set by the CPU when an IRQ is triggered, and restored to its previous state by RTI.
  • If the /IRQ line is low (IRQ pending) when this flag is cleared, an interrupt will immediately be triggered.

D: Decimal

  • On the NES, this flag has no effect.
  • On the original 6502, this flag causes some arithmetic instructions to use binary-coded decimal representation to make base 10 calculations easier.
  • Can be set or cleared directly with SED, CLD.

V: Overflow

  • ADC, SBC, and CMP will set this flag if the signed result would be invalid[1], necessary for making signed comparisons[2].
  • BIT will load bit 6 of the addressed value directly into the V flag.
  • Can be cleared directly with CLV. There is no corresponding set instruction.

N: Negative

  • After most instructions that have a value result, this flag will contain bit 7 of that result.
  • BIT will load bit 7 of the addressed value directly into the N flag.

The B flag

While there are only six flags in the processor status register within the CPU, when transferred to the stack, there are two additional bits. These do not represent a register that can hold a value but can be used to distinguish how the flags were pushed.

Some 6502 references call this the "B flag", though it does not represent an actual CPU register.

Two interrupts (/IRQ and /NMI) and two instructions (PHP and BRK) push the flags to the stack. In the byte pushed, bit 5 is always set to 1, and bit 4 is 1 if from an instruction (PHP or BRK) or 0 if from an interrupt line being pulled low (/IRQ or /NMI). This is the only time and place where the B flag actually exists: not in the status register itself, but in bit 4 of the copy that is written to the stack.

Instruction Bits 5 and 4 Side effects after pushing
PHP 11 None
BRK 11 I is set to 1
/IRQ 10 I is set to 1
/NMI 10 I is set to 1

Two instructions (PLP and RTI) pull a byte from the stack and set all the flags. They ignore bits 5 and 4.

The only way for an IRQ handler to distinguish /IRQ from BRK is to read the flags byte from the stack and test bit 4. The slowness of this is one reason why BRK wasn't used as a syscall mechanism. Instead, it was more often used to trigger a patching mechanism that hung off the /IRQ vector: a single byte in PROM, UVEPROM, flash, etc. would be forced to 0, and the IRQ handler would pick something to do instead based on the program counter.

Unlike bits 5 and 4, bit 3 actually exists in P, even though it doesn't affect the ALU operation on the 2A03 or 2A07 CPU the way it does in MOS Technology's own chips.

External links

References

  1. Article: The Overflow (V) Flag Explained
  2. Article: Beyond 8-bit Unsigned Comparisons, Signed Comparisons