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

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`	`├`

3.8 KiB Raw Blame History Unescape Escape