John also wrote Gryzor which is another top CPC game.
- The game is designed to run in 64KB of memory. Therefore they made decisions to make this possible. Even so, the game is one of the best on CPC
- With 64KB one level at a time is loaded. With 128KB all 4 levels are loaded.
- The game doesn't use hardware scrolling and uses a reduced screen and heavily uses the invisible areas of both screens to store code and data.
Characters and Enemies
- Characters and Enemies are stored as multiple sprite parts and are composed during drawing. Each part has a width, height, and signed x and y offset relative to the character's base x,y position. The base x,y position's origin is at the bottom of the frame and in the middle on the x axis. There is no clear separation between heads, bodies and legs. Some character frames are made of 4 or more parts. This can be seen in the walk and punch animation frames.
- Characters and Enemies are sorted by their base y position each frame so that the draw correctly when walking in front of each other and to give a 3d look.
- Sprite pixels are stored upside down in memory. Therefore left to right, bottom to top.
- Sprite pixels are stored as mode 0 with pen 0 as transparent.
- Sprite pixels are stored facing right only and flipped by code in realtime (e.g. the facing left sprite is a mirror of the facing right sprite.).
- Sprite pixels are stored as follows:
1 byte like normal (same as mode 0 screen bytes), followed by 1 byte with pixels swapped, this repeats.
It is believed this storage is done so that drawing a sprite facing left, and drawing a sprite facing right will take the same cpu time and this means a more stable frame rate regardless of the direction each sprite is facing.
- There is a list of sprites per level. The start address of this is stored at &e00 from the start of the level data. This takes the form of width, height and pixel data location. With 128KB there is 1 extra ram page per level for level specific sprites. Sprites common to each level are stored in main RAM. There are around 223 sprites (including common sprites). During the game the level page is mapped into RAM at &4000.
- Each level has it's own music.
- Each level takes &1000 bytes. There are 4 stored in 1 16KB page. The data is copied to &3000->&3fff before then being copied to other locations in RAM.
- Each level's data is prefixed with "JCB Overlay xxxx" where x is the level number. e.g. "JCB Overlay 0000".
- Map is 128 tiles wide and 18 tiles tall. But not all map space is used. The remainder is garbage. In reality, Level 1,2,3 are 80 tiles wide. Level 4 is 112 tiles wide.
- Each level has it's own palette of 16 colours. The palette for level 1 is at &87b3, level 2 is &87c3, level 3 is &87d3, level 4 is at &87e3. The palette is stored backwards with pen 15 first, then 14 all the way down to 0.
- "Macro tile map" uses 64 bytes. It is stored at offset +0x01a0 from the start of the level's data. Each byte is a macro tile id. Each macro tile is 4 tiles wide and 9 tiles tall. The data is copied to &BF90 and this is where the runtime uses it from.
- The tile ids per macro block are stored at offset +0x09e0 from the start of the level's data. The data is encoded as 7 bits per tile all put together. It is decoded to &31a0 overwriting the level data. This is copied to &9600-&9720,&9e00-&9f20,&a600-&a720,&ae00-&af20. The runtime uses the data from &9600. The ids are stored pre-multiplied by 2.
- 2048 bytes for tile graphics. These are defined per-level. These are stored at +&1e0 from the level data. They are copied from &31e0 in 4 parts. There are 128 possible tiles. Runtime ranges: &e600-&e800, &ee00-&f000, &f600-&f800, &fe00-&ffff.
- Tile graphics are 4 pixels wide and 8 pixels tall. Each tile's graphics uses 16 bytes. Tiles are stored uncompressed, left-right and top to bottom.