Getting started · The game · The code · Findings · Open questions
The first surprise about this tape is that it is not an MSX tape.
The file is a TSX and its blocks are ZX Spectrum ones (TZX type 0x10), with the usual structure: [flag][data][XOR]. There is not a single MSX KCS block anywhere in it.
And the loader is not the BIOS's: it is a reimplementation of LD-BYTES, the Spectrum ROM's routine, written out by hand for the MSX. It lives at 0xD6D8 and is 681 bytes. They brought the whole tape system across because it was cheaper than redoing it.
There is a second copy of that loader inside the game, at 0x07F9, plus the saving one at 0x08B2: those are what save and load a game use. The reading one is the same routine with two differences you can point at: here there is a failure exit (the ret nz at 0x0819) and the border steps through colours one at a time instead of going random.
The loader reads four blocks in a row and none of them runs where it lands:
| block | lands at | ends up at | bytes |
|---|---|---|---|
| the loading screen | 0x88B8 | the VRAM | 12,388 |
| the low block | 0x0190 | 0x0190 (never moved) | 15,807 |
| the middle block | 0x3F4F | 0x5E00 | 14,577 |
| the high block | on top of the screen | 0x9E00 | 18,552 |
The loading screen is dumped to VRAM the moment it is read, which is why the high block can load right on top of it: by the time it arrives, that piece of RAM is no longer needed.
The low block is what relocates the other two. Its sums are written into the listing itself and come out exact:
0xE677 - 0x4878 + 1 = 0x9E00 (the high block, 18,552 bytes)
0x96F0 - 0x38F1 + 1 = 0x5E00 (the middle block, 14,577 bytes)and it copies them backwards, because source and destination overlap.
Before anything else, the loader puts all four MSX pages into RAM. From there on there is no BIOS: the game does everything itself, which is why the low block carries its own layer for VRAM, keyboard, joystick and colour conversion.
That forces the game to be traced over a 64 KB image — work/juego64.bin — and split afterwards into three listings, each with the org where it really runs.
The boot code (0x0190) does something you do not see every day: it checks that the first three of the 100 bytes the loader brought to 0x012C are 0xC9, and if so applies whatever comes behind them. It is a letterbox for patching the game from the tape without touching the code.
module bytes code data unexplained
loader 681 675 6 0
screen 12388 34 12354 0
low 15807 1218 14589 0
middle 14577 9887 4690 0
high 18552 0 18552 0
BASIC loader 256 two lines of text and padding
TOTAL 62261 bytes, 62261 explained (100.00%)The BASIC loader is two lines: one setting COLOR and SCREEN 2, and one with the BLOAD. The rest of its 256 bytes is the ^Z padding the MSX writes to fill out the chunk.