r64
- a c64/6502 assembler in R
- Package home is on github
- Full documentation is available here: coolbutuseless.github.io/package/r64
Rationale
- Who doesn’t want a compiler for a 1 MHz 8-bit computer with 16 colours and a max resolution of 320x200?
- A 6502 assembler in R will allow for preparing/calculating data in R and then incorporating directly into the assembly code. e.g.
- creating character sets
- computing animation paths
Features
General features
- Basic syntax only. Similar to TASS64 syntax.
- Settable program counter
- e.g.
* = $0801
- e.g.
- Defined variables
- e.g.
border = $d020
- e.g.
- Low/High byte extraction from symbol (similar to TASS)
- e.g.
lda #<routine
will store the low byte of the address of symbolroutine
in theA
register
- e.g.
For integration with R
.rtext
directive to include anR
string as text data.rbyte
directive to include anR
integer vector as bytes{...}
to delimit code to be evaluated at run time to manipulate labels and variables e.g.lda {border + 1}
Installation
r64
depends on a lot of tidyverse packages as well as the minilexer package (for splitting the c64 asm code into tokens).
devtools::install_github('coolbutuseless/minilexer') # for lexing the 6502 assembly into tokens
devtools::install_github('coolbutuseless/r64')
A simple 6502 program
The following c64/6502 ASM code will clear the screen and then write Hello #rstats!
at the top
asm <- '
*=$0801
.byte $0c, $08, $0a, $00, $9e, $20 ; 10 SYS 2080
.byte $32, $30, $38, $30, $00, $00
.byte $00
*=$0820
lda #$93 ; Clear the screen
jsr $ffd2
ldx #$00 ; initialise the offset pointer into our message
loop lda message,x ; load a character and write it to screen
and #$3f ; Manually place chars on screen
sta $0400,x
inx
cpx #$0e
bne loop
rts
message
.text "Hello #rstats!"
'
Compile the program and run it using VICE
prg_df <- r64::compile(asm)
prg_filename <- tempfile()
r64::save_prg(prg_df, prg_filename)
# system(paste("/usr/local/opt/vice/x64.app/Contents/MacOS/x64", prg_filename), wait=FALSE)
Output from animation using a custom character set
Breakdown of assembly process
The compiler makes a few passes through the data to resolve symbol values.
The r64::compile()
function is just a wrapper which calls the following 4 functions
line_tokens <- r64::create_line_tokens(asm)
- For each line in the input break it into tokens.
- Filter any rows that contain no instructions
prg_df <- r64::create_prg_df(line_tokens)
- Create a data.frame from
line_tokens
- This is the key data structure for the compilation process
- The compilation process is just a matter of manipulating this data.frame and merging with information about the instructions
- Create a data.frame from
prg_df <- r64::process_symbols(prg_df)
- Resolve labels to their actual addresses
- Replace any defined variables with their values
prg_df <- r64::process_zero_padding(prg_df)
- If there are gaps between blocks of code, insert zero bytes
An example of the final form of the prg_df
data.frame is show below. The actual contents of the c64 prg
file is just the sequence of values in the hexbytes
column.
prg_df %>%
select(addr, label, line, opcommand, op, ophex, nbytes, hexbytes) %>%
knitr::kable()
addr | label | line | opcommand | op | ophex | nbytes | hexbytes |
---|---|---|---|---|---|---|---|
2049 | NA | * = $0801 | NA | NA | NA | 0 | |
2049 | NA | .byte $0c $08 $0a $00 $9e $20 | NA | NA | NA | 6 | 0c, 08, 0a, 00, 9e, 20 |
2055 | NA | .byte $32 $30 $38 $30 $00 $00 | NA | NA | NA | 6 | 32, 30, 38, 30, 00, 00 |
2061 | NA | .byte $00 | NA | NA | NA | 1 | 0 |
2062 | NA | (zero padding) | NA | NA | NA | 18 | |
2080 | NA | * = $0820 | NA | NA | NA | 0 | |
2080 | NA | lda #$93 | lda #$93 | lda | a9 | 2 | a9, 93 |
2082 | NA | jsr $ffd2 | jsr $ffd2 | jsr | 20 | 3 | 20, d2, ff |
2085 | NA | ldx #$00 | ldx #$00 | ldx | a2 | 2 | a2, 00 |
2087 | loop | loop lda message x | lda message x | lda | bd | 3 | bd, 35, 08 |
2090 | NA | and #$3f | and #$3f | and | 29 | 2 | 29, 3f |
2092 | NA | sta $0400 x | sta $0400 x | sta | 9d | 3 | 9d, 00, 04 |
2095 | NA | inx | inx | inx | e8 | 1 | e8 |
2096 | NA | cpx #$0e | cpx #$0e | cpx | e0 | 2 | e0, 0e |
2098 | NA | bne loop | bne loop | bne | d0 | 2 | d0, f3 |
2100 | NA | rts | rts | rts | 60 | 1 | 60 |
2101 | message | message | NA | NA | NA | 0 | |
2101 | NA | .text “Hello #rstats!” | NA | NA | NA | 14 | c8, 45, 4c, 4c, 4f, 20, 23, 52, 53, 54, 41, 54, 53, 21 |
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