Development suite for the MOS Technology 6502 CPU, written in tré.
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This is Bender, a development suite for the MOS Technology 6502 CPU, written in the Lisp dialect tré. Bender can:

  • assemble programs with inlined Lisp code
  • disassemble binaries (very incomplete)
  • generate VICE monitor command files
  • generate TAP and WAV files
  • generate C code for 6502 emulation and disassembly

For tré see

Programs built with Bender


Run '' to generate a tré SBCL image called 'bender'.


The assembler is line-oriented. A line may contain a label and an instruction or a directive. Also literal values are possible.


Instructions are notated as usual. The A register must not be used as an operand. Comments start with a semicolon.

    lda #0
    ror a   ; Syntax error. 'a' is implied.
    ror     ; The right way to notate it.


Bytes, words and strings do not need red tape.

    1       ; A decimal byte.
    $1      ; A hexadecimal byte.
    $01     ; A hexadecimal byte as well.
    %101    ; A binary byte.
    "This is an ASCIIZ string." 0


Label definitions must end with a colon.


Local labels

Labels may be defined more than once. But that will give you errors unless you tell Bender the direction in which to look for:

l1: lda f0
    bne n1
    jsr s0
n1: lda f1
    bne n1      ; ERROR: Bender cannot tell which "n1" you desire.
    jsr s1
n1: jsr s2
    jmp l1      ; ERROR: Bender cannot tell which "l1" you desire.

A leading '+' makes Bender search for later labels, a leading '-' makes it search for earlier code:

l1: lda f0
    bne n1
    jsr s0
n1: lda f1
    bne +n1     ; Use next "n1".
    jsr s1
n1: jsr s2
    jmp -l1     ; Use previous "l1".

w:  jmp -w      ; Wait…

Applying Lisp expressions

Lisp expressions can be inserted starting with the '@' character. They replace the more familiar inline arithmetic expressions other assemblers use and may span multiple lines.

    ; NOT used in Bender;
    ldx #table_end-tablestart

    ; Instead use a Lisp expression:
    ldx #@(- table_end table_start)

Expressions can be used at toplevel as well.

; Make a null–terminated PETSCIIZ string at compile-time.
text: @(ascii2petscii "Hello world!") 0

Toplevel expressions are expected to return

  • numbers or lists of numbers,
  • strings,
  • lists of parsed expressions (also see ASM epressions) or
  • NIL.

ASM expressions

Function ASM generates parsed expressions for you. They are explained in the next session. It expects one or more assembly source strings.

   (defmacro inc16 (x)
     `(asm ,(format nil "
                      inc ~A
                      bne +n
                      inc @(++ ~A)
                    x x))
      nil) ; No parsed expression returned!

    @(inc16 "ptr")

Admittedly, this looks ugly but it also is a good basis for Lispy extensions, and thus, work in progress.

Parsed expressions

A parsed expression can be a number, a string or an expression of the following forms:

(LABEL . name)
(DIRECTIVE name parameters)
(INSTRUCTION mnemonic addressing-mode operand)
(EXPRESSION . Lisp expression)

IDENTIFIER gets the value of a label. An operand can be a number, IDENTIFIER or EXPRESSION.


The assembler could be controlled with Lisp expressions but Bender also comes with a couple of directives for additional comfort.


Set the program counter to the specified address.

    org $1001

Would do the same like:

@(& (= *pc* #x1001) nil)

FILL <num_bytes>

Fills in num_bytes zeroes.

    fill 256    ; Fill page with zeroes.

With Lisp expressions this would be:

@(maptimes [identity 0] 256)


Assembles the following lines unless its arguments is NIL until an 'end' directive is reached. The lines are always being parsed. 'if' can be nested.

if @*with-feature-x?*
    …some code…
end if

You must be aware that zero is not NIL, so this example might not do what you would expect:

if 0
    jmp oh_no   ; Still assembled!
end if

If you want to disable code, do it like this:

if @nil
    jmp oh_no   ; Not assembled.
end if


Causes the assembler to mute output until an "end" directive is found. Great for layouting zero pages for example:

    ; Some example zero page.
    org 0
s:      0 0
d:      0 0
scr:    0
count:  0
    end data


This directive collects code to put it into SEGMENTs in the last pass. See also function SEGMENT.

    "This will go into any SEGMENT later."

    "This as well."


Ends an "if", "data" or "block" directive. The name of the directive that should be ended should be passed as an argument.

    ; The four possible forms of `end`.
    end if
    end data
    end block
    end ; To be deprecated, soon.

Assembly source lisp functions

Function (SEGMENT &KEY size (may-be-shorter? NIL))

This is a function called from assembly sources. It fills the specified number of bytes with recently unassigned BLOCKs, starting with the biggest BLOCK that fits in.

    @(segment :size #x318)           ; Before NMI vector.
    0 0                              ; NMI vector.
    @(segment :size (- #x400 $31a))  ; After NMI vector.

If the optional last argument is not NIL, the segment is not filled up with zeroes to match its desired size.

    ; e.g. last segment in code:
    @(segment :size #xffff :may-be-shorter? t)  ; Eat up all unassigned BLOCKs.

Assembling programs

ASSEMBLE-FILES output-file &REST input-files

Takes a list of input files and generates a binary output file.

The following example generates file 'example.prg' as well as an assembler dump of all passes to 'example.prg.lst':

(assemble-files "example.prg"

GET-LABELS – Importing/exporting labels across assemblies

GET-LABELS returns an associative list of the labels found in the last call to ASSEMBLE-FILES. If assigned to global variable IMPORTED-LABELS, they are used as a fallback in the next call to ASSEMBLE-FILES:

(assemble-files "core.prg" "core.asm")

(with-temporary *imported-labels* (get-labels)
  (assemble-files "loader.prg" "loader.asm")

MAKE-VICE-COMMANDS output-file &OPTIONAL additional-commands

Generate a VICE monitor file and appends optional additional commands with the data collected from the last call of ASSEMBLE-FILES.

(make-vice-commands "example.txt" "break .stop")