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Which flag bits are set by the PLP instruction on 6502?

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randomkiwi

Post subject: Which flag bits are set by the PLP instruction on 6502?

Posted: Tue Dec 26, 2023 1:02 pm

Joined: Thu Oct 26, 2023 5:39 pm
Posts: 5

I was going through the instruction set of 6502 at https://www.masswerk.at/6502/6502_instruction_set.html#PLA . Now below the PLP (Pull Processor Status from Stack) instruction, it is stated that

Quote:

The status register will be pulled with the break flag and bit 5 ignored.

As far PLA instruction is concerned, it is clear that status bit N and Z will be affected by the operation, but I am unable to understand which bits will be affected by the PLP instruction.

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gfoot

Post subject: Re: Which flag bits are set by the PLP instruction on 6502?

Posted: Tue Dec 26, 2023 1:26 pm

Joined: Fri Jul 09, 2021 10:12 pm
Posts: 741

The status register only really has 6 bits - C, D, I, N, V, Z - and they are all affected by pulling it from the stack, each being set to the value of one of the bits in the value pulled. The values of bits 4 and 5 in the byte from the stack are discarded.

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BigEd

Post subject: Re: Which flag bits are set by the PLP instruction on 6502?

Posted: Tue Dec 26, 2023 1:31 pm

Joined: Thu Dec 11, 2008 1:28 pm
Posts: 10986
Location: England

PLP brings in six bits as literals from the byte on the stack: NV—DIZC
The other two bits of the byte have no effect.

(By way of illustration, you wouldn’t normally see both Z and N set, but you could set them both with PLP)

Edit oops crossed in post!

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BigDumbDinosaur

Post subject: Re: Which flag bits are set by the PLP instruction on 6502?

Posted: Tue Dec 26, 2023 5:46 pm

Joined: Thu May 28, 2009 9:46 pm
Posts: 8514
Location: Midwestern USA

randomkiwi wrote:

I was going through the instruction set of 6502 at https://www.masswerk.at/6502/6502_instruction_set.html#PLA . Now below the PLP (Pull Processor Status from Stack) instruction, it is stated that

Quote:

The status register will be pulled with the break flag and bit 5 ignored.

As far PLA instruction is concerned, it is clear that status bit N and Z will be affected by the operation, but I am unable to understand which bits will be affected by the PLP instruction.

As the others noted, bits 4 and 5 are not “real” bits in the 6502/65C02—and in the 65C816 when the latter is operated in emulation mode. Bit 4 only exists in the stack copy of the status register (SR) that is pushed during a response to an IRQ or as part of executing a BRK (software interrupt) instruction. If an IRQ is being processed, bit 4 will be clear, otherwise bit 4 will be set. This behavior enables the IRQ/BRK service routine to distinguish between the two interrupt types.

When SR is pulled from the stack with a PLP instruction, bits 4 and 5 will not be affected by whatever is on the stack. The sequence PHP - PLA will result in bits 4 and 5 always being set in the accumulator copy of SR.

For detailed information on the 65xx instruction set, get yourself a copy of “Programming the 65816, Including the 6502, 65C02, and 65802” by Eyes & Lichty. I usually caution people about getting this kind of information from arbitrary websites, as such sites may propagate errors posted on other sites. The Eyes & Lichty manual may be considered authoritative for all permutations of the 6502.

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x86? We ain't got no x86. We don't NEED no stinking x86!

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