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A snake game clone for Commodore home computers
68 Commits 3 Branches 4 Tags 190 KiB
Assembly 95.6%
C++ 2.3%
Makefile 2.1%
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README.md

snake6502

Gameplay screenshot

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 - $07FF no free ram
$1000 - $1FFF yes SID tune, may overlap charset
$2000 - $23FF yes custom char, unused, allow SID overlap
$2400 - $27FF yes custom char (actual)
$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 - $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 - game tiles