101 lines
3.8 KiB
Markdown
101 lines
3.8 KiB
Markdown
# snake6502
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*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».
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Current development status [here](https://git.giomba.it/giomba/snake6502).
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## Download
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* [.d64](dist/snake6502.d64) for floppy disks
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* [.bin](dist/snake6502.bin) for 8KiB cartridges
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## Compile
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You need the GNU compiler collection and the [dasm](https://dasm-assembler.github.io/) macro assembler, then:
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```
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$ git submodule init
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$ git submodule update
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$ make
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```
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Interesting targets:
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* ```make bin/snake6502.bin``` produces .bin, ready to be burnt on an 8K EEPROM for making a cartridge
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* ```make bin/snake6502.d64``` produces .d64, ready to be used for floppy disks
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You can also define the following environment variables:
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```$ DEBUG=1 make``` build with debugging artifacts
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## Tape
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Copy ```loader.prg``` and ```packlz``` from disk to tape.
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On a physical machine, you can use [disk2tape](https://git.giomba.it/giomba/cbmutil).
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## Developer docs
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### Package
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The whole program is assembled into a ```snake.pack``` binary blob with the following structure.
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Absolute | Offset | Description
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------------|-------------|------------
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```$1000``` | ```$0000``` | load address
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```$2800``` | ```$1800``` | entry point (start address)
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### Memory map
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Address | PRG | Description
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----------------------|-------|------------
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```$0000 - $0001``` | no | hardware
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```$0002 - $00FF``` | no | zero page pointers
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```$0100 - $01FF``` | no | stack page
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```$0200 - $03FF``` | no | operating system variables
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```$0400 - $07FF``` | no | video memory
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```$1000 - $1FFF``` | yes | SID tune, may overlap charset
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```$2000 - $23FF``` | yes | custom char, unused, allow SID overlap
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```$2400 - $27FF``` | yes | custom char (actual 0x80+)
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```$2800 - $xxxx``` | yes | Program segment (only needed part used)
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```$xxxx - $CCFF``` | no | *free ram*
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```$CD00 - $CFFF``` | no | data segment (not-initialized vars)
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```$D000 - $DFFF``` | no | I/O
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```$E000 - $FFFF``` | no | Kernal
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### Compression
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```snake.pack``` is compressed into ```snake.pack.lz``` using [liblzg](https://github.com/mbitsnbites/liblzg), to save space, mainly in order to fit the game in a *PROM.
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Decompression occurs with one of the following loaders.
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### Loader
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#### Cartridge
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```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```.
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#### Disk
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```loader.asm``` loads at ```$801```, like any other BASIC automatic runner, and contains the decompression routine.
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```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```.
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### Miscellanea
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#### Custom charset
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Index | Description
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----------------|-------------
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```$00 - $7F``` | unused (space for SID)
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```$80 - $9F``` | A-Z (space first)
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```$A0 - $BF``` | A-Z, reversed (space first)
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```$C0 - $CF``` | hex digits
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```$D0 - $DF``` | hex digits, reversed
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```$E0 - $EF``` | game tiles
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```$F0 - $FF``` | semigraphic tiles
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##### Semigrahic Tiles
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Char | Output
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----------|--------
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```$F0``` | ```◜```
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```$F1``` | ```◝```
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```$F2``` | ```◟```
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```$F3``` | ```◞```
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```$F4``` | ```-```
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```$F5``` | ```|```
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```$F6``` | ```◢```
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```$F7``` | ```◣```
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```$F8``` | ```◥```
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```$F9``` | ```◤```
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```$FA``` | ```┴```
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```$FB``` | ```┬```
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```$FC``` | ```┤```
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```$FD``` | ```├```
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