6502 cross-assembler with original syntax extensions
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xasm is a 6502 cross-assembler with original syntax extensions. By default it generates binaries for Atari 8-bit computers.


6502 assembly language is full of LDA, STA, LDA, STA sequences. With xasm you can use MVA as a shortcut for an LDA/STA pair or even MWA for 16-bit transfers. You can avoid defining labels when you need short jumps, thanks to conditional skip and repeat pseudo-instructions. You can put two instructions that share their argument in one line. These are just some of the features that help you program in a more concise way. Let's look at typical 6502 code (which is also valid in xasm):

    lda #<dest
    sta ptr
    lda #>dest
    sta ptr+1
    ldx #192
    ldy #39
    lda pattern,y
    sta (ptr),y
    bpl do_byte
    lda #40
    adc ptr
    sta ptr
    bcc skip
    inc ptr+1
    bne do_line

Using xasm's features this code can be rewritten to:

    mwa     #dest ptr
    ldx     #192
    ldy     #39
    mva:rpl pattern,y (ptr),y-
    lda #40
    add:sta ptr
    scc:inc ptr+1
    dex:bne do_line

xasm syntax is based on 1990's Quick Assembler. Write accumulator shifts as in asl @. Whitespace is important: it is required before the instruction and disallowed in the operands, because it separates a comment from the operand, e.g.

    lda #0   this is a comment, no need for a semicolon

This may look weird at first, but it enables nice features such as instruction pairs and two-argument pseudo-instructions.


xasm is a command-line tool. Therefore you additionally need a programmer's text editor.

I use SciTE. To install xasm syntax highlighting and single-keystroke compilation, copy xasm.properties to the SciTE directory.

For single source file programs, press Ctrl+F7 to compile. You can double-click error messages to go to the incorrect line. Press F5 to run the program in the emulator.

For larger projects, I use GNU Make. Press F7 to build (and possibly run) a project as described in the Makefile. You can find my Makefiles in my repositories on GitHub.

If you prefer VIM, see a link below.


xasm 3.1.0 for Windows, OS X, Ubuntu and Fedora.


  • Atari800 - portable emulator of Atari 8-bit computers
  • Atari XL/XE Source Archive - source code of Atari demos, utilities and games
  • cc65 - C cross-compiler targeting 6502-based systems
  • MADS - another 6502/65816 cross-assembler, partially supporting xasm's syntax
  • vim-xasm - VIM syntax highlighting for xasm
  • WUDSN IDE - Eclipse plugin, front-end to several 6502 cross-assemblers including xasm