NES — Reference Material

Chip-level facts about the Nintendo Entertainment System / Famicom (Ricoh 2A03 + 2C02 + cartridge mapper). Curated from nesdev.org/wiki for use across the Game Annotation Series.

Links: PPU Reference, APU Reference, Mappers Reference, TIA Reference (comparison), Game Annotation Series

The NES platform

Component	Detail
CPU	Ricoh 2A03 — 6502 core with integrated APU; 1.79 MHz NTSC
Picture	Ricoh 2C02 PPU — separate chip; 5.37 MHz (3× CPU)
Audio	APU (inside the 2A03) — 5 channels
Internal RAM	2 KiB ($0000–$07FF, mirrored to $0800–$1FFF)
Pattern/name table RAM	2 KiB on PPU bus
Cartridge	PRG ROM + CHR ROM/RAM + optional mapper logic

Comparison to the Atari 2600 family

The NES uses a different 6502 variant than the 2600. Both are 6502-derivatives; the differences matter for source reading:

Aspect	Atari 2600 (6507)	NES (2A03)
6502 core	Yes	Yes
Address lines	13 (8 KiB max)	16 (full 64 KiB)
Decimal mode (BCD)	Present	Removed (Ricoh disabled the BCD circuitry to dodge MOS’s patent)
IRQ / NMI	Neither exposed	Both available; NMI from PPU VBlank, IRQ from APU/mapper
Stack	Mirrors to RIOT RAM via partial decode	Standard $0100–$01FF
RAM	128 bytes (RIOT)	2 KiB internal + cartridge expansion
Audio	TIA (2 channels, 32 pitches)	APU (5 channels, real envelopes, samples)
Graphics	TIA (no frame buffer, beam-racing required)	PPU (own memory, frame-driven via NMI)

The 2A03’s “no decimal mode” is the famous Ricoh amputation — relevant because if you SED (set decimal flag) on the NES, arithmetic does not switch to BCD. Code ported from full 6502 systems can break in subtle ways. Most NES games never SED for this reason.

The storytelling-vs-logic axis

A throughline of the Game Annotation Series:

2600 era: TIA forces the CPU to spend most cycles racing the beam. Game logic gets ~5,000 cycles per frame. Engines have to encode meaning structurally — Adventure’s design IS the story; there’s no spare cycles for runtime narrative state.
NES era: PPU does the rendering autonomously. CPU gets ~27,000 cycles per frame for game logic. Programmers spend that on richer state machines, scripted events, level scripting, music engines. Stories can be told in code, not just in design.
Mappers extend the canvas: not just bigger ROMs but enabling things like granular CHR swapping (MMC3 → animated tiles, parallax), scanline IRQs (status bars without sprite 0 hit), expanded audio (VRC6, MMC5).

Each NES game we cover should be asked: what does this code do that Adventure couldn’t have? The answer is the design lesson.

Reading order for someone new to NES code

Start here (this README) for the big picture.
PPU Reference — the rendering system that frees the CPU.
APU Reference — the music/sound capability that enables soundtracks.
Mappers Reference — the cartridge logic that scales the platform.

NES — Reference Material

The NES platform

Comparison to the Atari 2600 family

The storytelling-vs-logic axis

Reading order for someone new to NES code

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