TRS-80 file formats

The TRS-80 (Model I Level II BASIC) exports two native binaries: the .cas cassette block 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 TRS-80 Level II BASIC reference and its escape codes.

TRS-80 .cas

The Model I Level II BASIC cassette (CSAVE) block at the byte level: a leader of 0x00 sync bytes, the 0xA5 sync byte that ends the leader, the three-byte 0xD3 0xD3 0xD3 BASIC-file marker, a one-character filename, then the tokenized program exactly as it sits from 0x42E8 (which already ends with its own 0x0000 link, doubling as the end marker). The .cas is both the export file and what an emulator's virtual cassette deck reads back.

A real tape often concatenates several files — a small BASIC loader followed by the actual game is the classic layout — and import scans them all: the largest BASIC program opens for editing, other BASIC programs are preserved with the document, machine code trailing a program on the tape is kept as a memory block at the address CLOAD would have deposited it, and SYSTEM files import as blocks (below). A machine-language SYSTEM tape uses the same leader and sync followed by a 0x55 header, a six-character name, then address records (0x3C marker, length, load address, data, checksum) terminated by a 0x78 entry-point record; each record imports as a block and the entry address is kept with the block that contains it.

TRS-80 .dsk

The .cas cassette carries the BASIC program only, so a document with memory blocks exports instead as a .dsk disk image that carries the program and its blocks in one file. The .dsk is a JV1 disc — the simplest, most widely-read TRS-80 disk format: a flat run of 256-byte sectors (Model I single density, 35 tracks × 10 sectors) with a TRSDOS-style directory on track 17. The BASIC program is stored as a NAME/BAS file and each memory block as a NAME/CMD load module that records the block's load and entry addresses. Import opens the largest BASIC program for editing, preserves any other BASIC programs with the document, and brings each /CMD file back as a block at its load address. Exporting to .cas (or cassette .wav) instead warns first that the blocks would be dropped.

Cassette audio

The TRS-80 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 same content as the .cas).

The encoding is the Model I 500-baud cassette scheme. Every bit cell opens with a clock pulse; a 1 bit additionally fires a data pulse at the middle of the cell, a 0 does not - so the spacing between pulses carries the data (a 1 is two half-cell gaps, a 0 is one full-cell gap). Bytes are MSB-first. A block is a long leader of 0x00 bytes (all clock pulses, letting the reader lock on), the 0xA5 sync byte, the 0xD3 0xD3 0xD3 BASIC marker, a one-character filename and the tokenized program - i.e. the .cas image rendered to audio.