CPU status flag behavior: Difference between revisions

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(Complete rewrite to make it clearer)
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The status register doesn't implement bits 4 and 5. The only way to test those bits would be to push the status on the stack, but any time the status is pushed on the stack, those bits are set to fixed values, as described below.
The '''flags''' register, also called '''processor status''' or just '''P''', is one of the six architectural registers on the 6502 family CPU.


When an IRQ or NMI occurs, the current status with bit 4 clear and bit 5 set is pushed on the stack, then the I flag is set.
There are six bits in P:
<pre>
7654 3210
||  ||||
||  |||+- C: 1 if last addition or shift resulted in a carry, or if
||  |||  last subtraction resulted in
||  ||+-- 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: 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 read as 0
+--------- N: Set to bit 7 of the last operation
</pre>


PHP and BRK push the current status with bits 4 and 5 set on the stack; BRK then sets the I flag.
== The B flag ==
There is no "B flag". Bits 5 and 4 of flags do not exist.


RTI and PLP set the status to the byte popped off the stack.
Two signals and two instructions can push flags to the stack. When they do so, they fill the unused bits with the following:
{| class="tabular"
|-
! 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 flags, but they ignore bits 5 and 4.

Revision as of 17:08, 14 July 2010

The flags register, also called processor status or just P, is one of the six architectural registers on the 6502 family CPU.

There are six bits in P: 

7654 3210
||   ||||
||   |||+- C: 1 if last addition or shift resulted in a carry, or if
||   |||   last subtraction resulted in 
||   ||+-- 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: 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 read as 0
+--------- N: Set to bit 7 of the last operation

The B flag

There is no "B flag". Bits 5 and 4 of flags do not exist.

Two signals and two instructions can push flags to the stack. When they do so, they fill the unused bits with the following:

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 flags, but they ignore bits 5 and 4.

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