CPU interrupts

Interrupt hijacking

The MOS 6502 and by extension the 2A03/07 has a quirk that can cause one type of interrupt to partially hijack another type if they occur very close to one another.

For example, if NMI is asserted during the first four ticks of a BRK instruction, the BRK instruction will execute normally at first (an additional PC increment will occur compared to other IRQ types, and the status word will be pushed with the B flag set), but execution will branch to the NMI vector instead of the IRQ/BRK vector:

Each [] is a CPU tick. [...] is whatever tick precedes the BRK opcode fetch.

Asserting NMI during the interval marked with * causes the interrupt hijacking behavior.

     ********************
[...][BRK][BRK][BRK][BRK][BRK][BRK][BRK]

In a tick-by-tick breakdown of BRK, this looks like

 #  address R/W description
--- ------- --- -----------------------------------------------
 1    PC     R  fetch opcode, increment PC
 2    PC     R  read next instruction byte (and throw it away),
                increment PC
 3  $0100,S  W  push PCH on stack, decrement S
 4  $0100,S  W  push PCL on stack, decrement S
*** At this point, the signal status is queried to determine which interrupt vector to use ***
 5  $0100,S  W  push P on stack (with B flag set), decrement S
 6   $FFFE   R  fetch PCL
 7   $FFFF   R  fetch PCH

Similarly, an NMI can hijack an IRQ, and an IRQ can hijack a BRK (though it won't be as visible since they use the same interrupt vector). The tick-by-tick breakdown of all types of interrupts is essentially like that of BRK, save for whether the B bit is pushed as set and whether an additional PC increment occurs.

Delayed IRQ response after CLI, SEI, and PLP

(The below description was pulled from blargg's cpu_interrupts_v2.)

The RTI instruction affects IRQ inhibition immediately. If an IRQ is pending and an RTI is executed that clears the I flag, the CPU will invoke the IRQ handler immediately after RTI finishes executing.

The CLI, SEI, and PLP instructions effectively delay changes to the I flag until after the next instruction. For example, if an interrupt is pending and the I flag is currently set, executing CLI will execute the next instruction before the CPU invokes the IRQ handler. This delay only affects inhibition, not the value of the I flag itself; CLI followed by PHP will leave the I flag cleared in the saved status byte on the stack (bit 2), as expected.

Notes

• The above interrupt hijacking and IRQ response behavior is tested by the cpu_interrupts_v2 test ROM.
• For more quirky behavior related to VBlank NMI's from the PPU, see PPU frame timing.
• NMI is edge-sensitive while IRQ is level-sensitive. Both are active low.
• Internally, the 6502 IRQ and NMI implementations drive $00 (BRK) into the opcode reg, emphasizing the close connection between the different interrupt types.