NES Programming

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Nintendo Entertainment System

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

Hardware specifications[edit]

  • 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[edit]

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[edit]

Appendices[edit]

Hardware I/O ports[edit]

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

PPU[edit]

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

Video control[edit]

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

PPU (continued)[edit]

  • $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[edit]

  • 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[edit]

  • $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)[edit]

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

Sound (continued)[edit]

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

Joystick[edit]

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

External links[edit]

  • 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.