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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.
| | #REDIRECT [[Status flags]] |
| It is composed of six one-bit registers (see [[Status flags]]); instructions modify one or more bits and leave others unchanged.
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| Instructions that save or restore the flags map them to bits in the architectural 'P' register as follows:
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| <pre>
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| 7654 3210
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| || | ||||
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| || | |||+- C: 1 if last addition or shift resulted in a carry, or if
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| || | ||| last subtraction resulted in no borrow
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| || | ||+-- Z: 1 if last operation resulted in a 0 value
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| || | |+--- I: Interrupt priority level
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| || | | (0: /IRQ and /NMI get through; 1: only /NMI gets through)
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| || | +---- D: 1 to make ADC and SBC use binary-coded decimal arithmetic
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| || | (ignored on second-source 6502 like that in the NES)
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| || +------ B: see note below
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| |+-------- V: 1 if last ADC or SBC resulted in signed overflow,
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| | or D6 from last BIT
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| +--------- N: Set to bit 7 of the last operation
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| </pre>
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| == The B flag ==
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| There are six and only six flags in the processor status register within the CPU.
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| 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.
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| 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.
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| {| class="tabular"
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| |-
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| ! Instruction || Bits 5 and 4 || Side effects after pushing
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| |-
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| | PHP || 11 || None
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| |-
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| | BRK || 11 || I is set to 1
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| |-
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| | /[[IRQ]] || 10 || I is set to 1
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| |-
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| | /[[NMI]] || 10 || I is set to 1
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| |}
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| Two instructions (PLP and RTI) pull a byte from the stack and set all the flags. They ignore bits 5 and 4.
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| 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.
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| The slowness of this is one reason why BRK wasn't used as a syscall mechanism.
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| 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.
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| 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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| == External links ==
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| *[http://forums.nesdev.org/viewtopic.php?p=64224#p64224 koitsu's explanation]
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