The tape

How Temptations loads from a cassette, and why it takes seven minutes.

What is inside the file

The TSX format is the MSX version of TZX, a preservation format that does not store audio but the description of the pulses that were on the tape, plus metadata. That makes it much smaller and much more exact than a WAV.

Our file has 12 blocks:

	Contents
3 blocks	Metadata: who dumped it, the load command, the TOSEC name
1 block	Game record: title, publisher, year, language
6 blocks	The first three programs, in header + data pairs
2 blocks	The two big blocks, in turbo format

Blocks of type KCS (0x4B, the MSX's own extension) carry the bytes already demodulated. So there is no audio to decode: the extraction is exact, with no margin for error.

The four programs

Name	Loads at	Runs at	Size	What it is
TEMPT	—	—	256 B	BASIC loader
TOPO	0x9470	0x9470	4,254 B	The Topo Soft logo
SLOTS	0xC350	0xC58F	700 B	RAM finder + turbo loader
(turbo 1)	0x88B8	0x88B8	12,388 B	The title screen
(turbo 2)	0x4000	0x8000	40,449 B	The game

The BASIC loader is disarmingly simple:

10 COLOR 1,1,1:SCREEN 2
20 BLOAD"cas:",R
30 BLOAD"cas:",R

It sets the display to black on black (so you do not see the mess while it loads), switches to graphics mode, and loads the next two programs, running them.

SLOTS: the interesting piece

SLOTS does two jobs, and both of them have their charm.

Finding the RAM

The game needs the MSX's full 64 KB. But on an MSX the memory is spread across slots, and every machine organises them its own way. So SLOTS walks through all of them, writing and reading back to work out where there is real RAM:

SONDEA_PAGINA:
    ld a,080h       ; marca de slot expandido
    ld c,004h       ; 4 slots primarios
    ...
    ld (hl),020h    ; escribe 0x20 y lo relee
    ld a,(hl)
    cp 020h
    jr nz,siguiente
    ld (hl),0fah    ; y luego 0xFA, para descartar ROM

And here comes the nice detail: probing the first page of memory is a chicken-and-egg problem. That page is where the BIOS lives, and the routine you need in order to switch slots is in the BIOS. If you switch it out to test the RAM, you are left without the routine.

Topo's solution: copy the routine into itself. SLOTS copies its own version of that function into high memory, rewrites on the fly the destination of its own CALL instruction so that it points at the copy, and only then can it probe page 0 without depending on the BIOS.

    ld hl,00024h        ; primero, la de la BIOS
    ld (0c3b3h),hl      ; escrita como operando del CALL de 0xC3B2
    ...
    ld hl,lc418h        ; ahora copia la nuestra a RAM
    ld de,09c40h
    ld bc,000c8h
    ldir
    ld hl,09c40h
    ld (0c3b3h),hl      ; y reescribe el CALL para usarla

Reading the tape the raw way

The second half is a turbo loader: it reads the tape much faster than the system routines do, measuring the width of the pulses by hand.

It reads bit 7 of register 14 of the sound chip, which is where the MSX hooks up the cassette input, and counts how long it takes to change level. Short pulse, one bit; long pulse, the other.

And while it is at it, this detail:

    ld a,r          ; el registro R del Z80: un valor practicamente aleatorio
    and 00fh
    out (099h),a    ; ...escrito en el registro de color del video

The Z80's R register increments on its own with every instruction. Taking it and dumping it into the backdrop colour produces the coloured stripes you see while it loads. It is not decoration that somebody added: it is a by-product of the read loop itself, for free.

Turbo block format

Each turbo block is:

[ 0x00 ] [ ......data...... ] [ checksum ]

A sync byte, the data, and a check byte worked out so that the XOR of the whole block comes out zero.

This was verified by comparing the memory the emulator had loaded after reading the tape against the bytes in the TSX file: identical, with the one-byte offset that corresponds to the sync byte.

The full sequence

Timed in the emulator, in real machine time:

Moment	What happens
0 s	The tape starts
65 s	The Topo Soft logo appears
96 s	SLOTS starts up: it finds the RAM and takes over the loading
181 s	The title screen is in; it gets drawn on the display
406 s	The game starts

Almost seven minutes. That is how it was.

Rebuilding the tape

The project can go the other way round: take the commented listings, assemble them, and put the whole tape back together with its headers, sync bytes and checksums.

python3 tools/build_tape.py build build/Temptations_rebuild.tsx

The result is byte-for-byte identical to the original TSX, and it loads in the emulator with the same timings. That is the proof that the format is understood all the way through: nothing is left uninterpreted.