Acorn Atom file formats

The Acorn Atom exports two native binaries: the .atm interchange binary and the block-carrying .dsk disk image. Both double as export files and import formats that round-trip back to editable source, and the .dsk carries memory blocks in both directions. The machine also exports and imports a cassette .wav (see Cassette audio below).

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 Atom BASIC reference and its escape codes.

Acorn Atom .atm

The de-facto interchange format used by Atom emulators (Atomulator, AtoMMC): a 22-byte header followed by the raw memory image.

 0..15  filename, ASCII, NUL-padded to 16 bytes
16..17  load address  (little-endian)
18..19  exec address  (little-endian)
20..21  data length   (little-endian)
22..    data bytes

For a BASIC program the data is exactly the #2900 program image the tokenizer produces (line records ending in 0D FF), with load = exec = #2900. Import accepts either an .atm or a bare image (a bare image always begins with the 0D line marker). An .atm that loads anywhere other than #2900 is a machine-code or data file: it imports as a memory block, its exec address is kept with the block, and Run starts it there with LINK — the way *RUN would on real hardware.

Acorn Atom .dsk

The .atm carries a single file, so a document with memory blocks exports instead as a .dsk disk image that carries the BASIC program and its blocks together. The .dsk is an Acorn DFS-family single-sided disc: a two-sector catalogue lists each file with its own load and exec address, exactly what tells a #2900 BASIC program apart from machine code at some other address. The program is written with load = exec = #2900 and each block at its own address (its exec kept as the block's entry). Import opens the largest #2900 program for editing, preserves any other BASIC programs with the document, and brings the remaining files back as blocks. Exporting to .atm (or cassette .wav) instead warns first that the blocks would be dropped.

Cassette audio

The Acorn Atom 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 and offers a "robust" mode that lengthens the leader for temperamental hardware. The cassette .wav carries the BASIC program only.

The encoding is the Acorn cassette filing system over Kansas City Standard FSK, but at 300 baud: 0 = four 1200 Hz cycles, 1 = eight 2400 Hz cycles, each byte framed 8N1 (start 0, 8 data bits LSB-first, stop 1) with a 2400 Hz carrier leading in and between blocks. The program is split into ≤256-byte blocks, each four * (0x2A) sync bytes then a header (filename + 0x0D, flag, block number, data length−1, exec address, load address - the addresses big-endian), the data, and a single checksum byte (a plain sum mod 256 over the header and data). The flag's bit 7 is set on every block except the last. The decoder classifies half-cycles relative to the carrier and uses the checksum to find block boundaries and reject noise.