Altair 8800 file formats

The Altair had no single program container, so it exports the three things the hardware actually offered: the .bin CSAVE image, a cassette .wav carrying the same bytes as audio, and a plain-ASCII paper tape as .txt. The .bin is also an import format and round-trips back to editable source; a paper tape is text, so it opens through the ordinary text file path. No Altair export carries memory blocks — CSAVE wrote the program area and nothing else — though an import can bring one back (below). The Transfer dialog names the blocks an export would leave behind before it writes the file.

For the shared editor .txt, .zip project bundle, escape notation and the cross-machine machine-code overview, see the file formats overview. See also the Altair 8K BASIC reference and its escape codes.

Altair .bin

The byte stream CSAVE wrote and CLOAD read back, which is the closest thing the machine has to a native program file. .bin because there is no documented extension to claim, and a raw byte stream is what an Altair simulator feeds to a virtual cassette board.

There is almost nothing to it: three 0xD3 marker bytes, one character of program name, then the tokenized program exactly as it sits in memory, ending in its own 0x0000 end-of-program link. No leader, no sync byte, no load address, no length field, no checksum and no trailer. Saving an empty program writes the four header bytes and the link and stops — which is what the manual's "reading continues until 3 consecutive zeros" describes.

The name is a single character. BASIC takes the first character of whatever string you give it, so CSAVE"HELLO" and CSAVE"H" write the same tape, and CLOAD has to be given that one letter back. A tape holding a file named B in front of the wanted A is skipped rather than loaded.

Import accepts the header or no header: a saved image and a bare program image both open, and a program can never be mistaken for a header because its first two bytes are a line link. Bytes found after the end-of-program marker are what would have been written straight after the program, so they come back as a memory block at the address they followed the program at.

The three 0xD3 bytes are where Microsoft's tokenized-BASIC marker starts. The TRS-80 puts the same three behind a sync byte (see TRS-80 file formats) and MSX writes ten of them; the Altair needs none in front, because the cassette board's idle tone already gives the receiver something to lock onto.

Altair paper tape .txt

How a program left a machine with no cassette board at all: LIST with the Teletype's paper-tape punch running, and BASIC re-reads the listing line by line on the way back in. The export is the listing as bytes rather than as editor text — {0xNN} escapes are punched as the bytes they name, and lines end with carriage return and line feed as a Teletype needs, not with the editor's bare line feed.

A real punch also interleaved null bytes after each line ending, which is what BASIC's NULL command sets, giving the carriage time to return. They are left out here: BASIC ignores them on the way back in, and without them the file stays a plain readable listing.

Cassette audio

The Altair exposes a .wav export (and "play through speakers") and a cassette-audio import — listening on the mic / line-in, or decoding a .wav recording, back into editable source. The encoder emits mono 44.1 kHz. The cassette .wav carries exactly the bytes of the .bin above.

The encoding is the MITS 88-ACR board's own, and it is not the Kansas City Standard other machines here use: it is 2400 Hz for a 1, 1850 Hz for a 0, at 300 baud. Those are the frequencies MITS settled on in March 1976, having widened the split from the original pair because tapes would not interchange between machines. Above the tones it is ordinary asynchronous serial, because the board is a serial card with a modulator bolted on: each byte is a start bit, eight data bits least-significant first, and a stop bit, with the line idling on the 2400 Hz tone before, between and after — which is what gives the receiver its leader.

Because a bit lasts a fixed 1/300 s whatever tone carries it, bit boundaries do not fall on whole cycles, and the decoder times each half-cycle and samples the bit cells at their centres rather than counting cycles. That is also what lets it read a recording made at a different sample rate, or one played back by a recorder running slightly off speed.

On a real Altair, type CSAVE"A" and start the recorder to save, and CLOAD"A" and start playback to load; BASIC returns to OK when the program is in, and RUN starts it. The emulated machine has no cassette board fitted — as most Altairs did not — so those two commands wait at its console for hardware that never answers. Import and export here work on the file and the audio directly, without going through the emulator, so neither needs the board.