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».

Current development status here.

Download

• .d64 for floppy disks
• .bin for 8KiB cartridges

Compile

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

$ git submodule init
$ git submodule update
$ make

Interesting targets:

• make bin/snake6502.bin produces .bin, ready to be burnt on an 8K EEPROM for making a cartridge
• make bin/snake6502.d64 produces .d64, ready to be used for floppy disks

You can also define the following environment variables:

$ DEBUG=1 make build with debugging artifacts

$ VERBOSE=1 make output useful info during compilation

Tape

Copy loader.prg and packlz from disk to tape. On a physical machine, you can use disk2tape.

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 - $03FF	no	operating system variables
$0400 - $07FF	no	video memory
$1000 - $1FFF	yes	SID tune, may overlap charset
$2000 - $23FF	yes	custom char, unused, allow SID overlap
$2400 - $27FF	yes	custom char (actual 0x80+)
$2800 - $xxxx	yes	Program segment (only needed part used)
$xxxx - $CCFF	no	free ram
$CD00 - $CFFF	no	data segment (not-initialized vars)
$D000 - $DFFF	no	I/O
$E000 - $FFFF	no	Kernal

Compression

snake.pack is compressed into snake.pack.lz using liblzg, to save space, mainly in order to fit the game in a *PROM. Decompression occurs with one of the following loaders.

Loader

Cartridge

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.

Disk

loader.asm loads at $801, like any other BASIC automatic runner, and contains the decompression routine. snake.pack.lz is loaded at $8000 from disk drive, then decompressed back to $1000, and finally the loader jumps to the program entry point at $2800.

Miscellanea

Custom charset

Index	Description
$00 - $7F	unused (space for SID)
$80 - $9F	A-Z (space first)
$A0 - $BF	A-Z, reversed (space first)
$C0 - $CF	hex digits
$D0 - $DF	hex digits, reversed
$E0 - $EF	game tiles
$F0 - $FF	semigraphic tiles

Semigrahic Tiles

Char	Output
$F0	◜
$F1	◝
$F2	◟
$F3	◞
$F4	-
$F5	```
$F6	◢
$F7	◣
$F8	◥
$F9	◤
$FA	┴
$FB	┬
$FC	┤
$FD	├