Non-maskable interrupt

As explained in Classic Game Programming on the NES: Make your own retro video game

A non-maskable interrupt is a processor interrupt that cannot be disabled by the ordinary interrupt-mask mechanism.

A non-maskable interrupt (NMI) is a processor interrupt that ordinary interrupt masking cannot disable. On the NES, the PPU generates an NMI when it finishes drawing a screen and enters vBlank. The 6502 stops its current work and begins the NMI routine whose address is stored in the vector table. section 1.5.4 section 5.13

vBlank is the short interval while the display raster returns to the top of the screen. It is the normal time to update PPU video memory, because changing VRAM while the PPU is drawing can produce incorrect output and takes longer. An NMI therefore provides a regular point for screen updates, music, and sound effects. section 1.5.4 section 5.13

A safe NMI routine follows a small sequence:

• Save A, X, and Y on the stack immediately, because the interrupt may arrive during code that still needs those register values. section 5.13
• Check a zero-page flag such as nmi_ready to decide whether rendering work should run, skip VRAM writes, or disable rendering for larger video-memory changes. section 5.13
• Perform scheduled PPU work, such as transferring the sprite table, copying palette data, and setting rendering state. section 5.13
• Restore Y, X, and A from the stack, then use rti to return to the interrupted code without corrupting its state. section 5.13

Frame-driven sprite and palette update

An NMI can begin by pushing A, X, and Y. If nmi_ready permits rendering, it sets the sprite-RAM address, starts a sprite DMA transfer, writes the 32-byte palette through the PPU VRAM registers, enables rendering, and clears nmi_ready. It then pulls Y, X, and A and returns with rti. section 5.13

Figure 1.6 NES PPU chip: What is a vBlank?

Use the vBlank diagram to see why the PPU pauses after reaching the bottom of the display and why the NMI aligns updates with that pause. section 1.5.4

How Classic Game Programming on the NES: Make your own retro video game covers it

How deep it goes

Works through an NMI routine step by step, including stack saves, render gating, PPU transfers, and rti restoration.

Examples

Demonstrates a labeled nmi handler that saves registers, checks nmi_ready, transfers sprites and palettes, and returns with rti.

What it assumes

Uses 6502 registers, stack instructions, vector tables, and PPU VRAM while explicitly explaining register preservation.

What you can do after

Build it from scratch as an NES NMI handler that safely schedules rendering work during vBlank.

How it connects

Connects non-maskable interrupts to vBlank, PPU, VRAM, vector tables, the stack, registers, and DMA.

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Related concepts

• interruptkind of · An interrupt is an event or signal that breaks ongoing activity and redirects execution or attention so a condition can be handled.
• PPU control registerconfigured by · The PPU control register is an NES hardware register whose bits configure graphics operation and interrupt generation.
• nmi handleruses · An NMI handler is a program routine that executes in response to a non-maskable processor interrupt.
• exceptionkind of · An exception is a runtime signal or object representing an abnormal condition that interrupts ordinary program flow and may be handled or propagated.

This explainer was generated by Manning's knowledge graph from the text of Classic Game Programming on the NES: Make your own retro video game. The section links point to the parts of the book it draws on.