CPU status flag behavior

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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 (see Status flags); 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
---- ----
|||| ||||
|||| |||+- Carry: 1 if last addition or shift resulted in a carry, or if
|||| |||     last subtraction resulted in no borrow
|||| ||+-- Zero: 1 if last operation resulted in a 0 value
|||| |+--- Interrupt: Interrupt inhibit
|||| |       (0: /IRQ and /NMI get through; 1: only /NMI gets through)
|||| +---- Decimal: 1 to make ADC and SBC use binary-coded decimal arithmetic
||||         (ignored on second-source 6502 like that in the NES)
||++------ s: No effect, used by the stack copy, see note below
|+-------- Overflow: 1 if last ADC or SBC resulted in signed overflow,
|            or D6 from last BIT
+--------- Negative: Set to bit 7 of the last operation

The B flag

There are six and only six flags in the processor status register within the CPU. Despite what some 6502 references might appear to claim on a first reading, there is no "B flag" stored within the CPU's status 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.

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