snake6502

snake6502 is a snake-like game clone for Commodore home computers, written for fun because «I always wanted to code something for a computer of my retrocomputers collection – actually, this is the main reason I collect them: to write programs».

Download the binary .prg.

Current development status here.

Compile

You need the GNU compiler collection and the dasm macro assembler, then:

$ make

You can also define the following environment variables:

$ DEBUG=1 make build with debugging artifacts

$ VERBOSE=1 make output useful info during compilation

Developer docs

Package

The whole program is assembled into a snake.pack binary blob with the following structure.

Absolute	Offset	Description
$1000	$0000	load address
$2800	$1800	entry point (start address)

Memory map

Address	PRG	Description
$0000 - $0001	no	hardware
$0002 - $00FF	no	zero page pointers
$0100 - $01FF	no	stack page
$0200 - $07FF	no	free ram
$1000 - $1FFF	yes	SID tune
$2000 - $27FF	yes	custom char
$2800 - $xxxx	yes	Program segment (only needed part used)
$xxxx - $CCFF	no	free ram
$CD00 - $CDFF	no	data segment (not-initialized vars)
$CE00 - $CEFF	no	list X
$CF00 - $CFFF	no	list Y
$D000 - $DFFF	no	I/O
$E000 - $FFFF	no	Kernal

Compression

snake.pack is compressed into snake.pack.lz using liblzg, to save space in order to fit the game in a *PROM.

Decompression

cart.asm is located at $8000 (standard org address for C64 cartridges), and contains the decompression routine and the snake.pack.lz. It decompresses snake.pack.lz back to $1000, and jumps to its entry point at $2800.

Miscellanea

Custom charset

Index	Description
$00 - $1F	A-Z (space first)
$20 - $3F	A-Z, reversed (space first)
$40 - $4F	hex digits
$50 - $5F	hex digits, reversed
$60 -	game tiles