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This is an assembler that takes custom, user-defined instruction sets and uses them to assemble source files.
It can be useful, for example, if you're trying to test the bytecode of a new virtual machine, or if you're eager to write programs for that new microprocessor architecture you just implemented in an FPGA chip! Latest Release Releases


πŸ–₯️ Try it right now on your browser!

πŸ•ΉοΈ Check out an example project which targets the NES!

πŸ“œ Check out the changelog for new features!

⌨️ Install the VSCode syntax highlight extension!

❀️ Support me!


πŸ“š Check out the Documentation for advanced features and a how-to guide!


You can install directly from by running cargo install customasm. Then the customasm application should automatically become available in your command-line environment.

You can also download pre-built executables from the Releases section.

You can compile from source yourself by first cloning the repository and then simply running cargo build. There's also a battery of tests available at cargo test.

Upgrade to v0.11

πŸ“– Check out instructions for migration from older versions to v0.11!


Given the following file:

    load r1, {value} => 0x11 @ value`8
    load r2, {value} => 0x12 @ value`8
    load r3, {value} => 0x13 @ value`8
    add  r1, r2      => 0x21
    sub  r3, {value} => 0x33 @ value`8
    jnz  {address}   => 0x40 @ address`16
    ret              => 0x50

    load r1, 0
    load r2, 3
    load r3, 4
        add r1, r2
        sub r3, 1
        jnz .loop

...the assembler will use the #ruledef directive to convert the instructions into binary code:

 outp | addr | data

  0:0 |    0 |          ; multiply3x4:
  0:0 |    0 | 11 00    ; load r1, 0
  2:0 |    2 | 12 03    ; load r2, 3
  4:0 |    4 | 13 04    ; load r3, 4
  6:0 |    6 |          ; .loop:
  6:0 |    6 | 21       ; add r1, r2
  7:0 |    7 | 33 01    ; sub r3, 1
  9:0 |    9 | 40 00 06 ; jnz .loop
  c:0 |    c | 50       ; ret

Command-Line Usage

Usage: customasm [options] <asm-file-1> ... <asm-file-N>

    -f, --format FORMAT The format of the output file. Possible formats:
                        binary, annotated, annotatedbin, binstr, hexstr,
                        bindump, hexdump, mif, intelhex, deccomma, hexcomma,
                        decc, hexc, logisim8, logisim16
    -o, --output [FILE] The name of the output file.
    -s, --symbol [FILE] The name of the output symbol file.
    -t, --iter [NUM]    The max number of passes the assembler will attempt
                        (default: 10).
    -p, --print         Print output to stdout instead of writing to a file.
    -q, --quiet         Suppress progress reports.
    -v, --version       Display version information.
    -h, --help          Display this information.