NESdev Wiki, “NMI”

Cite as: NESdev Wiki, “NMI”
Original: https://www.nesdev.org/wiki/NMI
sha256: ad04d1b5bbff7ed1c51b46fc6a2d34cd635d908228397a96bc82a3f3bfef9aaf
Version: oldid 23420
Archived: https://web.archive.org/web/20260630231257/https://www.nesdev.org/wiki/NMI
Accessed: 2026-10-28
Licence: none stated on the page
Credit: NESdev Wiki editors (nesdev.org); see the page history.
Notes: Read from the Wayback capture of 2026-06-30. Used for NMI occurring at the start of vertical blank each frame while PPUCTRL bit 7 is set.

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While bit 7 of PPUCTRL ($2000) is set, the PPU raises an NMI at the start of vertical blank (scanline 241, dot 1), so a game's NMI handler runs at the display's frame rate, one NMI a frame, except in three cases. Changing the bit from 0 to 1 while the vertical-blank flag is set raises one at once; toggling it during vertical blank without reading PPUSTATUS can raise several; and a read of PPUSTATUS just as vertical blank starts suppresses that frame's NMI. The two pages differ on that read's dot: the PPU registers page gives the dot before the flag is set, and maybe the next one or two; the NMI page, the same dot.	`OperationOperationOperationRace condition`	Wiki