The cartridge

16 KB in page 1 (0x4000–0x7FFF), with no bank switching. The "AB" header declares only INIT (0x4010); STATEMENT, DEVICE and TEXT are zero.

What INIT does

INIT sets interrupt mode 1, writes a jp into the H.KEYI hook (0xFD9A) pointing at 0x4051, puts the stack at 0xE800, clears the work RAM 0xE000–0xE7FF, sets up the VDP and the first screen, and then falls into a jr to itself at 0x404F. From that point on the main program does nothing: the interrupt, once per frame, runs the whole game.

The screen

SCREEN 2. The name table is at 0x3800 and the sprite attribute table at 0x3B00. The cartridge does not store whole screens: it stores compressed scripts that a decompressor (0x446D, with its out (c),a core relocated to 0xE310) unpacks into VRAM.

The RAM map

Everything the game touches lives from 0xE000 up, under the stack:

• 0xE000 main state (0..8), the index into the jump table at 0x40AA
• 0xE001 sub-state, split by the djnz chains inside each handler
• 0xE004 a frame delay the handlers load and wait out
• 0xE010–0xE03A the three sound channels, fourteen bytes each
• 0xE055–0xE058 the score and best in BCD
• 0xE060 the stage number (1..0x0C); 0xE061 its terrain parameter
• 0xE074 the road's curvature; 0xE085 the speed
• 0xE120.. the car and rival sprite records