Tape loading by itself¶

A real tape deck knows nothing about the program: someone presses Play, waits, and presses Stop. Xpeccy+ can do that by itself, and load in seconds instead of minutes. For that it has to tell, from what the machine is doing, when a loader is reading the tape. This page says how it tells, and what each switch does with it. The switches are in the Tape Player and on the Machine: Tape sheet; all of them are on by default.

How a loader is recognized¶

A program reads the tape through port #FE: one bit of it is the level of the signal from the tape. The same port also gives the keyboard, so a read alone says little. What follows the read says more, so Xpeccy+ looks at the code right after it:

code that tests the tape’s bit is a loader;
code that tests only the key bits is a keyboard scan;
a read by the ROM’s own loading routine belongs to flash loading.

A loader waiting for the next pulse also reads in a tight loop: the same instruction again and again, with exactly one register moving - its counter, which counts by one when it is B. Code that plays a sound or draws from the port stores what it reads, and more registers move; a loop where nothing moves, or only the A register, waits for a key.

Auto play / stop¶

Starting. A stopped tape starts after ten loader reads in a row, each soon after the one before. A keyboard scan never starts it, nor any read from the ROM: a disk system that checks the BREAK key through the ROM must not start the tape. A tape started afresh gives the loader half a second of silence first, as a deck does.

Stopping. The tape stops at its end and at a stop mark in a TZX image. It also stops when, for about a frame, every read is unlike a loader’s: the program has finished loading and is scanning the keys. One odd read is not enough - a key scan in the middle of a load, from an interrupt, would otherwise cut the tape mid-pulse. A loader busy between blocks - unpacking, decrypting - reads nothing at all, and that does not stop the tape either.

A key scan during a pause between blocks does not stop the tape while there is more to play: a deck plays the whole pause, and a loader may be timing it.

Pause, not stop. When auto play stops the tape and starts it again with nothing in between, it goes on from the same point of the same pulse, like a deck’s pause button. Started fresh, it would cut the signal into pieces each time.

Stop by hand wins. After Stop is pressed, nothing moves the tape by itself until Play, a rewind, a recording or another tape.

Flash loading¶

Most tapes load at least their first blocks through the ROM’s loading routine. With Flash loading, a block that this routine asks for goes straight into memory, and the processor leaves the routine exactly as the ROM would have: the same registers and flags, a wrong flag byte skipped, a short block failing as on a real tape. Programs often copy the routine into RAM and call the copy; such a copy is recognized by its bytes, and loads the same way.

Only that routine is caught. A loader of a program’s own is never decoded, it is played - so anything that loads at all still loads, if not in a flash. Two cases are played on purpose:

a block longer than the routine was asked for: some loaders have the ROM read only the start of a block and read the rest themselves;
the pause after a block, if it is under 0.2 seconds: a loader that goes on reading needs the tone that follows.

After a longer pause, the next block that is not a standard one waits until the program’s own loader first reads the tape, then plays. A loader that shows a picture or a prompt first would otherwise find its pilot tone half gone.

A SAVE through the ROM writes its block straight onto the tape the same way.

Fast loading¶

Fast loading does not change how anything loads: while a loader reads the tape, the machine simply runs as fast as the computer can, without sound. The loader runs instruction by instruction, as at normal speed, so turbo loaders and protected tapes load this way too.

When the tape stops, the machine is put back to where the loader finished. The moment the loader stopped reading, the whole machine is saved; if it reads again, the save is dropped. When fast loading ends, that save is put back. Without it, the game would already have run for a second or two at full speed - its title music half played, its demo started - by the time it is shown at normal speed.

Part of fast loading is always on: for the edge loops of the ROM and of the well-known turbo loaders, the turns of the loop up to the next pulse are counted instead of run, and the whole machine - picture and all - is moved on by the same time. The result is the same machine, only sooner.

Edge detection¶

Edge detection goes further: a loader waiting for the next pulse gets it at once. Only the tape is moved on; the rest of the machine - the picture, the interrupts - stays where it was. The loader’s own delay loops between pulses are skipped the same way. Loading gets several times faster, but it is no longer what the machine would have done: a loader that also plays music, counts interrupts or watches the picture while loading can see the difference. If a tape loads with normal speed and fails with Edge detection, turn it off.

Rewind at end¶

A game that loads its levels as it goes reads the tape again later. Rewind at end acts when the tape is wanted again, never at the end itself:

at Play: back to the start;
at a load through the ROM: to the first block with the flag and length that load asks for - not simply the start, as a game may react to a block it did not ask for;
when a program’s own loader reads a tape that has run out: back to the start, and it plays. Such a loader cannot say which block it wants, so the start is the only safe guess; a game that reloads a level after each game over finds it there every time.

A load through the ROM on a stopped tape also winds forward, past blocks that do not fit, to a block that fits exactly - as winding a deck to side B. A longer block is not skipped: a loader may read only its start.

When the tape is one of a set (Game (Side A).tzx, Game (Side B).tzx… - see The other sides beside an image), a loader reading past its end, or a ROM load asking for a block it does not have, gets the next tape of the set instead of a rewind, and the picture says which side is in. Play still rewinds the tape that is in. Next tape of a set off in Options leaves the set out: the tape rewinds as a lone one does.

With Rewind at end off, the tape stays at its end, as a deck would leave it: a game that says “turn the tape over” expects that.