The cartridge
16,384 bytes with the standard header: AB at 0x4000, INIT at 0x404F, and STATEMENT, DEVICE and TEXT at zero. The BIOS maps it into page 1 (0x4000-0x7FFF) and jumps to INIT when it has finished booting.
The memory map
| 0x4000-0x7FFF | the cartridge |
| 0xE000-0xE3FF | the game's RAM, which INIT clears |
| 0xE400 | the stack |
Of the cartridge, 7,981 bytes are code (4,038 instructions) and 8,403 are data, in 143 named ranges. Nothing is left over.
What the VDP is told
The eight registers live at 0x44F1 and 0x44D7 copies them to 0xE038 before sending them:
| register | value | what it means |
|---|---|---|
| R0 | 0x02 | SCREEN 2 |
| R1 | 0xE2 | 16K, screen on, interrupts on, 16×16 sprites |
| R2 | 0x0E | name table at 0x3800 |
| R3 | 0x7F | colour table at 0x0000 |
| R4 | 0x07 | pattern table at 0x2000 |
| R5 | 0x76 | sprite attributes at 0x3B00 |
| R6 | 0x03 | sprite patterns at 0x1800 |
| R7 | 0xE1 | black border |
How the drawings are stored
Three ways, and the listing uses all three to walk the data and find where each block ends:
Run-length, unpacked by RLE_A_VRAM (0x4D24). A count byte: zero ends the stream, a count with bit 7 set copies that many bytes as they are, and one without it repeats the next byte that many times. Entered at 0x4D24 the stream carries its own two-byte VRAM address in front; entered at 0x4D28 the address comes in DE.
Straight copies, 0x4502, with HL, DE and BC.
Mirrored copies, 0x6DC1: the same, but each byte goes in with its eight bits reversed, so one drawing serves for a thing and for the same thing facing the other way. It counts with B, and one of the calls loads ld bc,0x0018, which leaves B at zero: that copy is 256 bytes, not 24.
The font
48 glyphs of eight bytes at 0x4A37. The tile code is the ASCII, so they run from 0x30 (0) to 0x5F, and 0x4635 repeats them across the three thirds of the screen so text can be written anywhere.
After the Z there are five slots that are not letters: the word with that the menu needs for «1PLAYER with JOYSTICK», and the two arrows of the menu cursor.
The lists that draw
Two formats, both of them lists of tile codes:
PINTA_LISTA(0x45E7) reads a VRAM address and then tile codes until an 0xFF; an 0xFE in the middle starts again somewhere else. It is what paints the labels.- The label engine,
MOTOR_DE_ROTULOS(0x59BC), takes its three arguments from the six bytes that follow its own call —a list, a set of tiles and a VRAM address— and returns behind them. The list is a run of counts: with bit 7, that many cells of one single tile; without it, that many tiles copied one by one; 0x80 starts a new address and 0 ends.