NES Programming

This book is an open work designed for people interested in learning to program for the NES (Nintendo Entertainment System).

Nintendo Entertainment System

Hardware specifications

An annotated MOS 6502 die.

• CPU - Ricoh 2A03 CPU which is a MOS 6502 without a decimal mode.
  • Clocked 1.789773Mhz for NTSC (System 21.47727Mhz / 12) and 1.773447Mhz for PAL (System 26.601171Mhz / 15)
• Memory - 2 KB
• Video
  • Ricoh 2C02 PPU (Picture Processing Unit)
  • Resolution - 256 x 240 pixels (Only 256 x 224 pixels are visible on NTSC)
  • Colors - Fixed 52 (44 colors, 5 grays and 3 duplicated)
  • Palette
    • Background: 4 Palettes x 3 Colors + 1 Back Color
    • Sprites: 4 Palettes x 3 Colors
  • Tiles - 8 x 8 pixels tiled background
  • Sprites - Up to 64 8x8-pixel or 8x16 pixel sprites but only 8 can be displayed per scan-line
• Sound
  • 2 Pulse wave channels
  • 1 Triangle wave channel
  • 1 Noise channel
  • 1 DPCM channel

NES 2A03 CPU memory map

Address Range (Hexadecimal)	Size	Notes (Page size is 256 bytes)
$0000–$00FF	256 bytes	Zero Page — Special Zero Page addressing modes give faster memory read/write access
$0100–$01FF	256 bytes	Stack memory
$0200–$07FF	1536 bytes	RAM
$0800–$0FFF	2048 bytes	Mirror of $0000–$07FF	$0800–$08FF Zero Page $0900–$09FF Stack $0A00–$0FFF RAM
$1000–$17FF	2048 bytes	Mirror of $0000–$07FF	$1000–$10FF Zero Page $1100–$11FF Stack $1200–$17FF RAM
$1800–$1FFF	2048 bytes	Mirror of $0000–$07FF	$1800–$18FF Zero Page $1900–$19FF Stack $1A00–$1FFF RAM
$2000–$2007	8 bytes	Input/Output registers
$2008–$3FFF	8184 bytes	Mirror of $2000–$2007 (multiple times)
$4000–$401F	32 bytes	Input/Output registers
$4020–$5FFF	8160 bytes	Expansion ROM — Used with Nintendo's MMC5 to expand the capabilities of VRAM.
$6000–$7FFF	8192 bytes	SRAM — Save Ram used to save data between game plays.
$8000–$FFFF	32768 bytes	PRG-ROM
$FFFA–$FFFB	2 bytes	Address of Non Maskable Interrupt (NMI) handler routine
$FFFC–$FFFD	2 bytes	Address of Power on reset handler routine
$FFFE–$FFFF	2 bytes	Address of Break (BRK instruction) handler routine

Tutorials

• Introduction: A few words on assembly
• Initializing the NES

Appendices

• The NES memory map

Hardware I/O ports

The NES has I/O ports located at addresses $2000-$2007 (hexadecimal) and $4000-$4017.

PPU

• $2000 - write only - PPU Control Register 1
• $2001 - write only - PPU Control Register 2
• $2002 - read only - PPU Status Register

Video control

• Sprites
  • $2003 - write only - Sprite Memory Address
  • $2004 - read / write - Sprite Memory Data
  • $2005 - write only - Background Scroll

PPU (continued)

• $2006 - write only - PPU Memory Address - indexing into PPU memory location
• $2007 - read / write - PPU Memory Data - data to read from a PPU memory location or write to a PPU memory location

Sound

• Square wave 1
  • $4000 - Register 1 - Bits:
    • 0-3: volume / envelope decay rate
    • 4: envelope decay disable
    • 5: length counter clock disable / envelope decay looping enable
    • 6-7: duty cycle type (unused on noise channel)
  • $4001 - Register 2 - Bits:
    • 0-2: right shift amount
    • 3: decrease / increase (1/0) wavelength
    • 4-6: sweep update rate
    • 7: sweep enable
  • $4002 - Register 3 - Bits:
    • 0-7 Eight LSB of wavelength
  • $4003 - Register 4 - Bits:
    • 0-2: 3 MSB of wavelength (unused on noise channel)
    • 3-7: length counter load register
• Square wave 2
  • $4004 - Register 1 - Same as Square wave 1
  • $4005 - Register 2 - Same as Square wave 1
  • $4006 - Register 3 - Same as Square wave 1
  • $4007 - Register 4 - Same as Square wave 1
• Triangle
  • $4008 - Register 1 - Bits:
    • 0-6 linear counter load register
    • 7 length counter clock disable / linear counter start
  • $4009 - Unused Register
  • $400A - Register 3 - Same as Square wave 1
  • $400B - Register 4 - Same as Square wave 1
• Noise
  • $400C - Register 1 - Same as Square wave 1
  • $400D - Unused Register
  • $400E - Register 3 - Bits:
    • 0-3 playback sample rate
    • 4-6 unused
    • 7 random number type generation
  • $400F - Register 4 - Same as Square wave 1

DMC

• $4010 - Register 1 - Bits:
  • 0-3: sample rate
  • 4-7: unused?
• $4011 - Register 2 - Bits:
  • 0-7: volume
• $4012 - Register 3 - Bits:
  • 0-7: voice table start address(N*0x40+0xc000)
• $4013 - Register 4 - Bits:
  • 0-7: voice table byte size(N*16+1)

Sprites (continued)

• $4014 - write only - DMA Access to the Sprite Memory

Sound (continued)

• $4015 - read / write - enable/disable individual sound channels

Joystick

• $4016 - read / write - Joystick 1
• $4017 - read / write - Joystick 2

External links

• Super NES Programming
• More info can be found on IRC, on the EFnet server, in channel #nesdev
• NesDev, contains detailed technical documents, tools, and examples.
• P65 Assembler, an assembler written in Perl for the 6502 family of processors with support for programming the NES.