The famicom is an 8bit video game console by Nintendo.

The famicom notes were created during the production of the NES release of donsol and nespaint, to learn more about programming for the console, see assembly.

NES System Architecture

The NES screen resolution is 256x240.

• ROM: Read Only Memory, holds data that cannot be changed. This is where the game code or graphics is stored on the cart..
• RAM: Random Access Memory, holds data that can be read and written. When power is removed, the chip is erased. A battery can be used to keep power and data valid..
• PRG: Program memory, the code for the game.
• CHR: Character memory, the data for graphics.
• CPU: Central Processing Unit, the main processor chip.
• PPU: Picture Processing Unit, the graphics chip.
• APU: Audio Processing Unit, the sound chip inside the CPU.

6502 Processor Overview

Backgrounds

To make graphics on the screen you must write graphic data to the PPU memory, but you can't write directly to PPU memory, you have to use PPU ports $2006 and $2007. By using $2006 you declare the address of PPU memory then by using $2007 you write the desired value to that address, PPU Memory addresses are 16bit starting from $0000~$3FFF(0000-1fff = tiles & 2000-23ff = nametable 0).

So you need to write twice to $2006 to declare it's address, the first write declares high byte of address, the second write declares the low byte of address. Each time you write a value to $2007, the PPU address is automatically adjusted to the next address, so you don't need to declare the PPU address with $2006 for sequential PPU memory addresses.

  LDA #$20   ; high byte
  STA $2006
  LDA #$00   ; low byte
  STA $2006
  LDA #$04   ; sprite-id
  STA $2007

Calculate at what address to draw it: $2000 plus 32 times the vertical position of the tile (in 8-pixel units) plus the horizontal position of the tile (in 8-pixel units), write the high byte of the address to $2006: this is usually values $20 to $23, and write the low byte of the address to $2006. In other words, calculate the tile offset (TileY * 32 + TileX) and then add the base address. This will give you a pointer you can use to access any part of the map.

Palette Codes

Audio

The NES and Famicom use a set of memory mapped registers to configure the 5 different sound channels, we just write byte data to the ports to configure the ports (though we cannot read back).

To produce a sound, first we enable the channel via $4015.

LDA #%00000010    ; Enable Channel2(Pulse2)
STA $4015

Then we write to the Square 2 ports:

LDA #%00111000    ; Duty Cycle 00, Volume 8 (half volume)
STA $4004
 
LDA #<$356        ; 356 = C2
STA $4006

LDA #>$356
STA $4007

The following table holds all values of every note on every octave that the NTSC NES can produce for the Square and Triangle Wave. These will be listed as 11 bit values that can be stored into the sound registers. All values are rounded to the nearest number. Note that for the Triangle Wave, these values will make a pitch one octave below that of the Square Wave.

Controller Ports

The controllers are accessed through memory port addresses $4016 and $4017. First you have to write the value $01 then the value $00 to port $4016. This tells the controllers to latch the current button positions. Then you read from $4016 for first player or $4017 for second player. The buttons are sent one at a time, in bit 0. If bit 0 is 0, the button is not pressed. If bit 0 is 1, the button is pressed.

Button status for each controller is returned in the following order: A, B, Select, Start, Up, Down, Left, Right.

Mapping

Some cartridges have a CHR ROM, which holds a fixed set of graphics tile data available to the PPU from the moment it turns on. Other cartridges have a CHR RAM that holds data that the CPU has copied from PRG ROM through a port on the PPU.

• famicom cookbook

incoming donsol dexe nasu dito donsol famicom spacetime 6502 nespaint 6502 chr format uxn devlog