CPU status flag behavior: Difference between revisions
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Despite what some 6502 references might appear to claim on a first reading, there is no "B flag" in the CPU. | Despite what some 6502 references might appear to claim on a first reading, there is no "B flag" in the CPU. | ||
Two interrupts (/[[IRQ]] and /[[NMI]]) and two instructions (PHP and BRK) push the flags to the stack. | 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). | ||
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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. | 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 from the stack and test bit 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. | 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. | 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. | ||
Revision as of 16:33, 4 May 2013
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:
7654 3210 || |||| || |||+- C: 1 if last addition or shift resulted in a carry, or if || ||| last subtraction resulted in no borrow || ||+-- Z: 1 if last operation resulted in a 0 value || |+--- I: Interrupt priority level || | (0: /IRQ and /NMI get through; 1: only /NMI gets through) || +---- D: 1 to make ADC and SBC use binary-coded decimal arithmetic || (ignored on second-source 6502 like that in the NES) |+-------- V: 1 if last ADC or SBC resulted in signed overflow, | or D6 from last BIT +--------- N: Set to bit 7 of the last operation
The B flag
There are six and only six flags in the processor status register. Despite what some 6502 references might appear to claim on a first reading, there is no "B flag" in the CPU.
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).
| 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.
