The code

The state machine

The interrupt (0x402C) calls 0x40A6 every frame. It reads 0xE000, the main state, and with it a jump through the table at 0x40C2 reaches one of the handlers; the high byte, 0xE001, is the sub-state. Bit 6 of 0xE002 marks the demo, and 0x40A6 pushes as the return address either the game handler (0x430A) or the demo one (0x44C9) before jumping to the state handler.

Two drawing interpreters

Nothing on screen is a stored bitmap. Two interpreters build the VRAM:

• 0x405E copies byte by byte into the name table, with a destination word, a jump to a new destination (0xFE) and an end (0xFF).
• 0x440D decompresses by runs into the pattern and colour tables: a count byte whose top bit tells a literal copy from a fill of one byte, and a 0x00 that ends the block. The website's figures come from running this one in Python.

Two players in one place

The event runs on the block at 0xE050. When the turn changes, 0x413E swaps that block with the one at 0xE080 in a single 32-byte routine, so every other routine always works on 0xE050 without caring who is playing.

The sound

0x7C3D is the per-frame player: three eleven-byte channel records at 0xE010, each reading its melody with a small command interpreter and writing the PSG through the BIOS. It is the framework Konami reused across its MSX cartridges, with its tone table and effects at the end of the ROM.