BBC Micro / Master file formats

BBC BASIC (Micro & Master) exports two native binaries: the headerless .bbc tokenized program, and the block-carrying .ssd disc image. Both double as export files and import formats that round-trip back to editable source, and the .ssd 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 BBC BASIC reference and its escape codes.

BBC Micro / Master .bbc

The exact byte layout BBC BASIC keeps from PAGE and that SAVE writes to disc, so it doubles as the export file and the payload the emulator pokes in at PAGE. For each line: 0x0D, the line number big-endian, a length byte (= body length + 4), then the tokenized body; the program ends with 0x0D 0xFF. The output is byte-for-byte what the genuine ROM tokeniser produces (regression-tested). The BBC Master uses the same format.

Machine code can ride inside the .bbc itself as inline assembly — a [ … ] assembler block in the BASIC listing (DIM a buffer, [OPT…] assemble into it at run time). That is ordinary BASIC source, so it tokenizes to the same ROM bytes and round-trips through the .bbc with no separate file. Machine code that lives at its own fixed address instead — a separate code/data block — is carried by the .ssd disc below.

BBC Micro / Master .ssd

An Acorn DFS single-sided disc image (80 tracks × 10 × 256 = 200K), the standard BBC multi-file container. Unlike the headerless .bbc, a .ssd holds several files, each with its own load and exec address in the two-sector catalogue — the attributes MOS uses to tell a BASIC program (CHAIN, load = exec = PAGE) from machine code (*RUN, load/exec = the code's address). A document that has memory blocks exports as a .ssd: the BASIC program is the file at PAGE and each block is a further file at its own load address (its exec address remembered for machine code). With the Transfer dialog's auto-loader on (the default when blocks exist), a generated !BOOT (*OPT 4,3) leads the disc — it *LOADs each block and CHAINs the program (or *RUNs the code when there is no BASIC) — so the disc runs by itself on real hardware (SHIFT+BREAK). Import re-opens the BASIC program for editing and brings every other file back as a block. Running a document with blocks in this IDE mounts the same .ssd and boots it, so the emulator distinguishes BASIC from machine code exactly as MOS does. A pure-BASIC document keeps exporting the plain .bbc.

Many real game discs, though, load files at addresses the block model can't hold — below PAGE, overlapping each other, or overlapping the program area, because the disc's own loader stages them in at different times rather than keeping them all resident. Importing such a disc keeps the whole image and boots it verbatim (exactly as SHIFT+BREAK does on real hardware) so its loader runs and every file lands at its true address; the recovered loader listing is shown in the editor for reference. Editing that listing (or adding a block) turns the document back into a normal, editable program that runs through the tokenizer.

Cassette audio

The BBC 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 cassette filing system (CFS) over Kansas City Standard FSK at 1200 baud: 0 = one 1200 Hz cycle, 1 = two 2400 Hz cycles, each byte framed 8N1 (start 0, 8 data bits LSB-first, stop 1), with a 2400 Hz carrier tone leading in and between blocks. The program is split into ≤256-byte CFS blocks, each with a * (0x2A) sync byte, a header (filename, load/exec addresses, block number/length/flag, spare) protected by a CRC-16/CCITT, the data, and a data CRC-16. The last block sets bit 7 of the flag. The decoder classifies half-cycles relative to the carrier and uses both CRC-16s to find block boundaries and reject noise. The encoding is shared by both BBC dialects.