NMI

The 2A03 and most other 6502 family CPUs are capable of processing a non-maskable interrupt (NMI). This input is edge-sensitive, meaning that if other circuitry on the board pulls the /NMI pin from high to low voltage, this sets a flip-flop in the CPU. When the CPU checks for interrupts and find that the flip-flop is set, it pushes the processor status register and return address on the stack, reads the NMI handler's address from $FFFA-$FFFB, clears the flip-flop, and jumps to this address.

"Non-maskable" means that no state inside the CPU can prevent the NMI from being processed as an interrupt. However, most boards that use a 6502 CPU's /NMI line allow the CPU to disable the generation of /NMI signals by writing to a memory-mapped I/O device. In the case of the NES, the /NMI line is connected to the NES PPU and used to detect vertical blanking.

Operation

Two 1-bit registers inside the PPU control the generation of NMI signals. Frame timing and accesses to the PPU's PPUCTRL ($2000) and PPUSTATUS ($2002) registers change these registers as follows:

1. Start of vertical blanking: Set NMI_occurred in PPU to true.
2. End of vertical blanking, sometime in pre-render scanline: Set NMI_occurred to false.
3. Read $2002: Return old status of NMI_occurred in bit 7, then set NMI_occurred to false.
4. Write to $2000: Set NMI_output to bit 7.

The PPU pulls /NMI low if and only if both NMI_occurred and NMI_output are true. By toggling NMI_output ($2000 bit 7) during vertical blank without reading $2002, a program can cause /NMI to be pulled low multiple times, causing multiple NMIs to be generated.

Caveats

Old emulators

Some platforms, such as the Game Boy, keep a flag turned on through the whole vertical blanking interval. Some early emulators such as NESticle were developed under the assumption that $2002.7 worked the same way and thus do not turn off NMI_occurred in line 3. Thus, some defective homebrew programs developed in this era will wait for $2002.7 to become false and expect this to happen at the end of vblank. (The right way to wait for the end of vblank involves triggering a sprite 0 hit and waiting for that flag to become 0.) Some newer homebrew programs have been known to display a diagnostic message if an emulator incorrectly returns true on two consecutive reads of $2002.7 .

Race condition

If 1 and 3 happen simultaneously, $2002.7 is read as false, and NMI_occurred is set to false anyway. This means that the following code that waits for vertical blank by spinning on $2002.7 is likely to miss an occasional frame:

wait_status7:
  bit $2002
  bpl wait_status7
  rts

Code like wait_status7 is fine while your program is waiting for the PPU to warm up. But once the game is running, the most reliable way to wait for a vertical blank is to turn on NMI_output and then wait for the NMI handler to set a variable:

wait_nmi:
  lda retraces
@notYet:
  cmp retraces
  beq @notYet
  rts

nmi_handler:
  inc retraces
  rti