F-1 Spirit is not a 16 KB cartridge like the 1984 Konami titles. It is 131,072 bytes, sixteen 8 KB pages that the Z80 cannot see at once: the cartridge carries its own circuit, the Konami mapper with SCC, and the program keeps asking it to place the page it needs in each window.
There are four windows, and every page has a fixed one, the one its number gives it:
| pages | Z80 window |
|---|---|
| 0 | 0x4000 (always mapped) |
| 1, 4, 7, 10, 13 | 0x6000 |
| 2, 5, 8, 11, 14 | 0x8000 |
| 3, 6, 9, 12, 15 | 0xA000 |
Page 0 is the only fixed one: it holds the header, the interrupt, the bank switching routines, the joystick reading and everything the other pages need within reach at all times. The rest take turns.
That each page has only one possible window is not a convention of this disassembly: it is what the hardware does, and the project checks it on every build. make sanity walks every place where the program asks for a page to be mapped and verifies the destination matches the window it belongs to.
4000 41 42 "AB", a cartridge signature
4002 E4 40 INIT = 0x40E4
4004 00 00 00 00 00 00 STATEMENT, DEVICE and TEXT at zero
400A 00 x6 padding
4010 43 44 07 52 FF "CD" 07 "R" FF, Konami's signature
4015 C3 2E BE jp 0xBE2EThe BIOS sees the "AB" at 0x4000, maps the cartridge and jumps to INIT. From there the program never returns: INIT installs a jump to 0x4018 in the H.KEYI hook and sits in an endless two-byte loop. The whole game runs inside the interrupt, just like the 1984 Konami cartridges.
RAM runs from 0xE000 to 0xF0FF —INIT clears all of it— with the stack at 0xF0F0, and F0F1, F0F2 and F0F3 hold the mapper shadow: which page is in each window. It is needed because the interrupt can fire at any moment and has to be able to leave the mapping exactly as it found it.
The interrupt does the sound and calls the game logic. The problem is that the logic can take longer than one frame, and the bank switching routines end in ei, so the interrupt fires again while the logic is still running. Hence three mechanisms:
That last one is a fixed clock: the race advances one step every three interrupts no matter how long the code takes.
SCREEN 2, with the eight registers written by 0x4317 from the table at 0x4328:
| register | value | what it says |
|---|---|---|
| R0 | 0x02 | graphics mode 2 |
| R1 | 0xE2 | 16 K, screen and interrupt 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 | 0xE4 | ink 14 on background 4 |
In SCREEN 2 the screen is split into three eight-row thirds and each third has its own 256 patterns. The cartridge's tile loader loads all three at once: a list record carries three bits —one per third— and the same drawing is decompressed as many times as thirds are asked for.
The name table is not written directly. The game keeps two 1 KB buffers in RAM, one per player (0xE400 and 0xEC00), which are 32 × 32 circular maps: the track is painted there and then dumped to VRAM. Being circular is what lets the scroll move nothing at all: only the row it starts reading from changes.
Two chips at once: the machine's PSG and the cartridge's own SCC, which adds five more channels with programmable waveforms. The player lives in page 13 and runs once per interrupt.
Writing to the SCC is not free: you have to map page 0x3F at 0x8000, write, and put page 2 back. That is why the player does not write the fifteen values of the five channels on every interrupt: it compares them with a copy it keeps in RAM and only sends the chip the ones that changed.