This project isn't mine. Original code here: https://github.com/skilldrick/6502js

This is a mere version modified by me, which implements a better front-end interface for programming in 6502 Assembly, and a few minor details here and there. All credit goes to skilldrick (https://github.com/skilldrick).

To use this, you must first extract ace-builds-master.zip, then open index.html as you would normally. You can also find example codes labelled testN.txt.

The changes I made to the project were:

• Added better code editor (https://ace.c9.io/).
• Created specific syntax highlighter for the 6502 Assembly language.
• Added emulation speed controls.
• Added ASCII character support to the canvas (see Notes).
• Added more colour support.
• Added "define" command to the assembler.
• Added binary literals.

• Improved overall layout of the page.
• Added some CSS style and colouring.

• Code window is no longer resizeable.

You can check out the simulator here: https://marcuspeixe.github.io/6502js/

It runs entirely on the client-side.

Memory location $FE contains a new random byte on every instruction.

Every time a key is pressed, its ASCII code is stored in $FF, and a IRQ is triggered.

If the "Timed IRQs" option is set to a non-zero value, every N milliseconds a IRQ is triggered and location $FF is set to 0.

To assign functions as interrupt handlers, name their label as res, irq and nmi (RESETs, IRQs and Non Maskable Interrupts, respectively).

To start generating new code at a specific address in memory, you can use the syntax * = $xxxx, where $xxxx is that address.

Memory locations $0200 to $05FF map to the screen pixels. Different values will draw different colour/character pixels. The pixels are:

$00: Black
$01: Red
$02: Green
$03: Yellow
$04: Blue
$05: Purple
$06: Cyan
$07: White

$00 - $07: Dark colours
$08 - $0F: Saturated colours
$10 - $17: Bright colours
$18 - $1F: Bright and saturated colours

$20 - $7F: ASCII table

$80 - $BF: Colours (each channel has 2 bit depth)
$C0 - $FF: Greyscale 

To extract the low or high byte of a label's address, suffix it with < or > respectively. Example:

test_label:
  lda #<test_label
  sta $00
  lda #>test_label
  sta $01

(Please note it only works with immediate addressing mode)

You can also use the label name without any suffixes in absolute addressing modes.

You can use the keyword define to assign names to certain values. These names can be used throughout your code.

dcb value, value, ... value is used to define constant bytes in memory (its best to use a label right before to provide easy access to them)

Number literal notations are either:

• 123 for decimal
• $1234 for hex
• %10101010 for binary

; Sample assembly code:

res:              ; Program entry point
  lda #$41
  ldx #$00
loop:
  sta $0200, x
  eor #$20        ; Fill a quarter of the screen
  inx             ; with alternating 'A' and 'a'
  bne loop        ; characters
  brk

irq:              ; Interrupt service routine
	nop
	lda #>irq       ; Loop forever
	pha
	lda #<irq
	pha
  rts

* = $0700         ; Set Program Counter

dcb 10            ; Define Constant Bytes
dcb $10
dcb %00000010

dcb $12, $34