Asi64
1 Asi64 Overview
2 Getting Started
3 Core Assembler Syntax
3.1 General
3.2 Number Literals
3.3 Addressing Modes
3.4 Constants and Variables
3.5 Memory Alignment
4 Labels
4.1 General
4.2 Loading Label Locations
5 Racket Integration
6 Pseudo-Ops
7 Data
8 Code Diagnostics
9 Metaprogramming Helpers
10 Emulator Configuration
11 Interesting Examples
12 C64 Helpers
7.1

Asi64

Written by Ross McKinlay

The Racket based cross-platform 6502 assembler. Primarily aimed at programming the Commodore 64, with VICE emulator support.

    1 Asi64 Overview

    2 Getting Started

    3 Core Assembler Syntax

      3.1 General

      3.2 Number Literals

      3.3 Addressing Modes

      3.4 Constants and Variables

      3.5 Memory Alignment

    4 Labels

      4.1 General

      4.2 Loading Label Locations

    5 Racket Integration

    6 Pseudo-Ops

    7 Data

    8 Code Diagnostics

    9 Metaprogramming Helpers

    10 Emulator Configuration

    11 Interesting Examples

    12 C64 Helpers

1 Asi64 Overview

The overall philosophy of this project is to take an old idea (a 6502 macro assembler) and supercharge it with Racket’s macro system. The end result is a powerful combination of part assembler, part meta-programming language that lets you combine all the power of Racket with a raw but feature-rich assembler.

Since Racket embraces this manner of design, it is quite easy to change, extend, or even build new languages over the top of it.

The assembler is primarily aimed at programming the Commodore 64, and includes support for the emulator VICE. It is able to launch the emulator post-compilation, passing along both your breakpoints and labels, resulting in a fast development cycle. The full instruction set is supported, including the illegal opcodes.

An assembler is a powerful low-level tool. An overall choice was made to expose all (or almost all) of the internals of the assembler to the programmer. The special assembler syntax provided is, afterall, simply a wrapper around the underlying library, consisting of some macros, functions and read-table modifications. The programmer is able to inspect and modify the assembler internals as they please. This can make it easier to write complex macros that directly program the internals without having to jump through hoops using the provided layers of syntax.

The code can be found at the git repository found here.

2 Getting Started

You can find asi64 on Racket’s package manager. raco pkg install asi64 should get you setup and ready to go. Create a Racket file somewhere for your program, and have VICE installed and ready to go.
 #lang asi64 package: asi64

; setup emulator             
(set-emulator-program! emu "c64.prg")
(set-emulator-execute?! emu #t)
(if (eq? (system-type 'os) 'windows)
  (set-emulator-path! emu "C:\\Program Files\\WinVICE-3.0-x64\\x64.exe")
  (set-emulator-path! emu "/snap/bin/vice-jz.x64"))
 
; main program
(C64 {
      *= $1000   ; start at $1000
:loop inc $d021  ; flash border
      jmp loop-  ; loop forever
})

The first few lines tell the assembler where to create the output file, whether to start the emulator post-assembly, and the location of the emulator itself. See Emulator Configuration for more information.

An asi64 program must contain a single core form C64. Within this form is expected to be a single 6502 block, denoted by  { }.

If you compile this program, it should assemble the resulting file and start the emulator with it for you to enjoy (hopefully!)

3 Core Assembler Syntax

3.1 General

Asi64 tries to stay as close to traditional 6502 assembler as possible. However, a few changes had to be made to ensure all of Racket is available.

You can write 6502 assembler code anywhere between { }. These blocks can be mixed and nested arbitrarily with other Racket forms - see Racket Integration for more infromation.

Within the 6502 blocks, each opcode must appear on its own line. Operands can be formed of literals or Racket expressions, and labels can appear in a variety of places, see the Labels section for more information.

3.2 Number Literals

Asi64 provides hexadecimal $ and binary % literals, which can be used everywhere in an asi64 program, not just within 6502 blocks.

{
   lda @%1111_0000   ; binary
   sta $d020         ; hex   
}

Binary literals are able to have any amount of underscore characters within them, to help visually separate groups of bits.

3.3 Addressing Modes

Immediate addressing mode is traditionally #. Since # is already used for things in Racket, asi64 instead uses @. Therefore, lda #42 becomes lda @42

Indexed adrressing modes are almost the same, removing the comma. sta $2000,x becomes sta $2000 x.

Finally, indirect addressing modes are quite different, since the traditional parenthesis would have caused havoc. Instread, prefix the operand with £, and remove the comma where applicable. sta ($20),y becomes sta £ $20 y and jmp ($4000) becomes jmp £ $4000.

If the £ character is not on your keyboard, you can easily change it to something else by modifiying "reader.rkt".

3.4 Constants and Variables

Although it is possible to use Racket’s define and friends as normal in 6502 blocks, it can get quite verbose. A shorthand for define is provided in the format of  name = value .

{
    a = $42
    b = %0000_1111
    c = (list 1 $2 %11)
}

These can appear in any 6502 block. It can be quite useful to have a large block near the top of the file that holds global constants and variables.

3.5 Memory Alignment

Asi64 provides two commands for controlling the current assembly location. The first is an absolute address in the format *= $2000 . The second aligns the assembler to the nearest address divisible by the operand /= $1000 .

{
   *= $2000   ; start assembling at $2000
   ; some code here ...   
 
   /= $1000   ; align to the nearest $1000 boundary
 
   ; some data table here ...            
}

Asi64 will warn you if it assembles over the top of a memory location it has previously written to.

4 Labels

4.1 General

Labels in asi64 must always start with :

When using a label as a target, drop the leading : and suffix with one of the following.

The latter two options allow you to have many labels with the same name.

The assembler will uniquely number any labels with the same name before passing them to the emulator, to ensure they always appear in the disassembly.

Labels can appear in most places, including before raw data and operands.

The following example shows various absolute labels including an operand label that enables simple self-modifying code

{
:a  lda :b @0
    sta $d021
    inc b:
    jmp a:
}

The following example shows multiple labels with the same name, and using relative labels with + and -

{
      ; some zero-page address
      state = $42
:loop
      ; wait for the raster to hit the bottom of the screen
      lda $d012
      cmp @$ff
      bne loop-
      ldx state
      bne next+  ; not zero
      ; code here for 0...
      jmp loop-
:next  cpx @1
       bne next+  ; not 1
       ; code here for 1 ...
       jmp loop-
:next  cpx @2
       ; .. and so on
 
}

4.2 Loading Label Locations

A common assembler feature is to load a label location as an immediate value, for setting interrupt routines, and self modifiyng code. You can extract the low and high bytes from a label using < and > as you can in most assemblers.

{
    lda @<int:    ; low part of address of interrupt handler code
    ldx @>int:    ; high part of address of interrupt handler code   
    sta $0314    ; store in interrupt vector
    stx $0315
 
:int ; interrupt handler here
}

Finally, a macro label-loc will return the 16bit value of the label directly (following the suffix rules) so that you can use it as part of an expression.

{
:a  lda @0
    sta $d021
    inc (+ (label-loc a-) 1) ; self modifing code
    jmp a-
}
This currently only works if the label was defined before the macro use in the source.

The above example is for illustration only, it could be written easier using an operand label like so, as per the example in the previous section.

{
:a  lda :b @0
    sta $d021
    inc b:
    jmp a:
}

5 Racket Integration

Even inside { } 6502 blocks, you still have all of Racket. As long as the program ends up outputting 6502 blocks, or lists of them, you can write whatever you like. Anything that is not a 6502 block will simply be ignored when assembling.

(define (mov src dst)   {
        lda @src
        sta dst
})
 
 
(C64 {
        *=$1000
        (define x $42)
        lda @(+ x 1)
        sta $d021
        (mov (+ x x) (- $d021 1))
})

6502 blocks can also be nested inside of each other, enabling you to mix arbitary Racket and 6502 forms wherever you like in the manner you would expect.

{
        *=$1000
        lda @$20
        ; unroll some code to clear a bit of the video memory
        (for ([i (in-range 0 10)])
          (let ([address (+ $0400 i)])
            {sta address}))
}

6 Pseudo-Ops

In the previous example, the function mov determines the immediate addressing mode itself, since ultimately it is part of the opcode rather than the operand. This greatly reduces the reusablity of the code since you’d have to have another mov to move from a location in memory. The macro (define-op ex) will re-write your function so that the addressing mode is determined at the call site.

(define-op (mov src dst) {
    lda src
    sta dst
})
 
 
(C64 {
    *=$1000
    (mov @42 $20)
    (mov $2000 $20)
    (mov $20 $2000)
})

The macro also introduces a few values for the programmer to use that provide metadata about the parameters. Currently you can use
  • param-name-16bit? True if the operand is a 16 bit immediate value or 16-bit memory address

  • param-name-immediate? True if the parameter is immediate.

These let you write general operations that are intelligent about their operands.
{
; adds to a 16 bit number, little-endian fashion.
; detects 8 bit immediate numbers and optimises
; as appropriate. for absolute, assumes 16bit to 16bit
(define-op (add16 source dest)
   (if source-16bit? {
     clc
     ; 16 bit
     lda dest
     adc (lo-byte source)
     sta dest
     lda (hi-byte source)
     adc (+ dest 1)
     sta (+ dest 1)}
 
    {clc
     lda dest
     adc (lo-byte source)
     sta dest
     bcc (+ (here)  3)
     inc (+ dest 1)}))
 
}

This example could be better, it doesn’t deal with 8 bit to 16 bit memory locations.

A few other handy functions are being used here - (lo-byte) (high-byte) and (here). The latter will yield the current instruction location and is handy for infinte loops, skipping instructions and self-modifing code, without having to use labels.

Pseudo-ops can sometimes be nested in each other. (working on this!)

7 Data

Inserting blocks of arbitrary binary data is accomplished with the (data ...) macro. It accepts any number of bytes and writes then directly. If an argument is a Racket list, it will be recusrively unwrapped and its ultimate contents written to memory.

{
  (define (deg->rad rad)
    (* (/ pi 180) rad))
 
:sine
  (data
    (for/list ([x (in-range 0 365 5)])
      (bitwise-and (exact-round (* (sin (deg->rad x)) 100)) 255)))
 
}

8 Code Diagnostics

Asi64 knows about the 6502 instruction set and is able to tell you useful information about your code, such as how many cycles each instruction takes, which processor flags they might affect, and how much space the block occupies.

Simply wrap any code you are interested in seeing metrics about between ?= and =?. When you assemble you will also see the information about your code blocks in the output window. This doesn’t affect the program that is produced in any way.

{
?= ; begin diagnostics
:sprite-y-char-top
     lda $d001
     sec
     sbc @$32
     lsr
     lsr
     lsr
     tax
     lda screen-rows-lo: x
     sta screen-lo
     lda screen-rows-hi: x
     sta screen-hi
     rts
=? ; end
}

this produces :

diagnostics started at $30ec
opcode a-mode cycles flags
lda    abs    4      (Z N)
sec    none   2      (C)
sbc    i      2      (C Z V N)
lsr    none   2      (C Z N)
lsr    none   2      (C Z N)
lsr    none   2      (C Z N)
tax    none   2      (Z N)
lda    absx   4/5    (Z N)
sta    zp     3      ()
lda    absx   4/5    (Z N)
sta    zp     3      ()
rts    none   6      ()
diagnostics finished at $3101
total code size $15 (21)  min/max cycles (36/38)

Note that since you can put numbers and data anywhere, asi64 can only show you information about code assembled directly with the assembler syntax. If you put a (data ...) block in the middle of the code, the diagnostics will simply ignore it, even if the bytes equate to valid opcode(s).

9 Metaprogramming Helpers

When you want to write code that writes other code, asi64 has a macro that helps you load the correct value for the combination of opcode and addressing mode you require.

{
   lda @(infer sta £ $ff y)
}

This code will load the accumulator with the immediate value of $91, the byte that represents the sta opcode in its indirect, y-offset addressing mode.

Of course, the actual address $ff here makes no difference, it is simply used to infer the addressing mode. If you had wrote $ffff it would have produced an error since the sta indirect addressing modes do not work with 16 bit addresses.

Unlike the rest of the assembler, you cannot use labels, expressions and other features within the infer macro, since it would make no sense to do so.

10 Emulator Configuration

Emulator conifguration is achieved by setting fields of a struct that is exposed directly to the programmer.

(struct emulator
  (path           ; location of the emu executable
   program        ; assembled file target path
   breakpoints?   ; pass along breakpoints
   labels?        ; pass along labels
   execute?)      ; execute the emu post assembly
  #:mutable)

An instance of this struct emu is exposed and the programmer can set the fields in the typical Racket style.

(set-emulator-execute?! emu #f) ; stop the emulator executing

It can be useful to prevent the emulator executing after every assemble when working on long pieces of code or using the diagnostics facility to cycle count.

Note that the program field is not strictly associated with the emulator. It is included here since it seems unlikely you would want to use asi64 without an emulator. In the future, if asi64 is expanded to support other emulators, machines and same-family chips (NES, Lynx, SNES, Apple][+ ... ) this minimal design will likely change to something slightly more abstract.

An additional convenience function is provided that sets all the fields in one call.

(define (configure-emu emu-path program-path execute-emu? enable-breakpoints?)
  (set-emulator-program! emu program-path)
  (set-emulator-execute?! emu execute-emu?)
  (set-emulator-breakpoints?! emu enable-breakpoints?)
  (set-emulator-path! emu emu-path))
  (set-emulator-execute? emu #f)

11 Interesting Examples

Coming Soon!

12 C64 Helpers

Coming Soon!