6502 Assembly is the language used to program the Famicom, BBC Micro and Commodore 64 computers.

This page focuses on the assembly language for the 6502 processor, targetting the Famicom.

Lexicon

Directives are commands you send to the assembler to do things like locating code in memory. They start with . and are indented. This sample directive tells the assembler to put the code starting at memory location $8000, which is inside the game ROM area. Labels are aligned to the far left and have a : at the end. The label is just something you use to organize your code and make it easier to read. The assembler translates the label into an address.

Opcodes are the instructions that the processor will run, and are indented like the directives. In this sample, JMP is the opcode that tells the processor to jump to the MyFunction label. Operands are additional information for the opcode. Opcodes have between one and three operands. In this example the #$FF is the operand:

Comments are to help you understand in English what the code is doing. When you write code and come back later, the comments will save you. You do not need a comment on every line, but should have enough to explain what is happening. Comments start with a ; and are completely ignored by the assembler. They can be put anywhere horizontally, but are usually spaced beyond the long lines.

  .org $8000
MyFunction:                    ; A comment
  LDA #$FF
  JMP MyFunction

Styleguide

Major comments are prefixed with two semi-colons, and minor comments are found at the end of a line on the 32nd column if available. Variables and subroutines are lowercase, constants and vectors are uppercase, and routines are capitalized.

;; Variables

  .enum $0000                  ; Zero Page variables
pos_x                   .dsb 1
pos_y                   .dsb 1
  .ende

;; Constants

SPRITE_Y            .equ $0200
SPRITE_X            .equ $0203

RESET:
  NOP

Forever:
  JMP Forever

NMI:
  RTI

;; Routines

Check_Collision: 
  LDA pos_y
  CMP #$88                     ; Floor is at 32y
  BCC @done
  LDA #$88
  STA pos_y
@done:
  RTS

;; Tables

Table_Name:
  .db $40,$46,$4c,$52,$58,$5e,$63,$68

;; Vectors

  .pad $FFFA
  .dw NMI
  .dw RESET
  .dw 0
  .incbin "src/sprite.chr"

The lin6 linter is used to enfore this style on the various assembly projects found on this site.

Registers

The 6502 handles data in its registers, each of which holds one byte(8 bits) of data. There are a total of three general use and two special purpose registers:

Note: When you use X it adds the value of X to the memory address and uses the 16-bit value at that address to do the write. Whereas when you use Y it adds the value of Y to the address stored in the memory address it's reading from instead. 6502 is little-endian, so $0200 is stored as $00 $02 in memory.

• A: The accumulator handles all arithmetic and logic. The real heart of the system..
• X&Y: General purpose registers with limited abilities..
• SP: The stack pointer is decremented every time a byte is pushed onto the stack, and incremented when a byte is popped off the stack..
• PC: The program counter is how the processor knows at what point in the program it currently is. It’s like the current line number of an executing script. In the JavaScript simulator the code is assembled starting at memory location $0600, so PC always starts there..
• PF: The Processor flag contains 7 bits, each flag live in a single bit. The flags are set by the processor to give information about the previous instruction.

Addressing

The 6502 has 9 major(13 in total) addressing modes, or ways of accessing memory.

Common Opcodes

Compare

The compare instructions set or clear three of the status flags (Carry, Zero, and Negative) that can be tested with branch instructions, without altering the contents of the operand. There are three types of compare instructions:

The CMP instruction supports eight different addressing modes, the same ones supported by the ADC and SBC instructions. Since the X and Y registers function primarily as counters and indexes, the CPX and CPY instructions do not require this elaborate addressing capability and operate with just three addressing modes (immediate, absolute, and zero page).

The compare instructions subtract (without carry) an immediate value or the contents of a memory location from the addressed register, but do not save the result in the register. The only indications of the results are the states of the three status flags: Negative (N), Zero (Z), and Carry (C). The combination of these three flags indicate whether the register contents are less than, equal to (the same as), or greater than the operand "data" (the immediate value or contents of the addressed memory location. The table below summarizes the result indicators for the compare instructions.

The compare instructions serve only one purpose; they provide information that can be tested by a subsequent branch instruction. For example, to branch if the contents of a register are less than an immediate or memory value, you would follow the compare instruction with a Branch on Carry Clear (BCC) instruction, as shown by the following:

Comparing Memory to the Accumulator

  CMP  $20    ; Accumulator less than location $20?
  BCC  THERE
HERE:
  ; No, continue execution here.
THERE:
  ; Execute this if Accumulator is less than location $20.

Use of Branch Instructions with Compare

Math

Modulo

Returns in register A.

Mod:
  LDA $00  ; memory addr A
  SEC
Modulus:  
  SBC $01  ; memory addr B
  BCS Modulus
  ADC $01
  RTS

Division

Rounds up, returns in register A.

Div:
  LDA $00 ; memory addr A
  LDX #0
  SEC
Divide:   
  INX
  SBC $01 ; memory addr B
  BCS Divide
  TXA
  RTS

• famicom cookbook

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

Lin6 is a linter for 6502 assembly code.

The linter is a utility to format assembly code to be used with the asm6 assembler. Lin6 was used in the creation of Donsol, Spacetime and NesPaint.

• source, ANSI C.