Commodore 64 / VIC-20 / PET file formats
The Commodore machines (C64, VIC-20, PET) share two native binaries: the .prg load-address-prefixed program and the block-carrying .d64 disk image. Both double as export files and import formats that round-trip back to editable source, and the .d64 carries memory blocks in both directions. All three machines also export and import a cassette .wav (see Cassette audio below). Only the load address in the first two bytes differs between them: $0801 C64, $1001 unexpanded VIC-20, $0401 PET.
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 Commodore BASIC reference and the escape codes.
Commodore 64 / VIC-20 / PET .prg
The 2-byte little-endian load address ($01 $08 = $0801) followed by the tokenized program as it sits in memory from $0801: for each line a 2-byte link to the next line (an absolute address), the 2-byte line number, the tokenized body and a 0x00 terminator, ending with a 0x0000 null link. This is the same image the emulator injects and the import/export file.
The VIC-20 and PET use the identical .prg format — the language is the same Commodore BASIC V2 (the PET adds the BASIC 4.0 disk tokens $CC–$DA) — and only the load address in the first two bytes differs: the unexpanded VIC-20 loads at $1001 ($01 $10), the PET at $0401 ($01 $04).
On import, a .prg whose load address is not the BASIC start imports as a single block at that address; a normal program with extra bytes past the end of the tokenized program imports the program plus those trailing bytes as a block.
Commodore 64 / VIC-20 / PET .d64
The C64, VIC-20 and PET all import and export .d64 disk images — a byte-exact image of a 1541 5.25" floppy, the multi-file container most Commodore disk archives use (not to be confused with raw .tap pulse recordings, which are recognised and refused with a clear message). A .d64 mirrors the real disk geometry: 35 tracks of 256-byte sectors (21 on the outer tracks down to 17 on the inner ones, 683 sectors = 174848 bytes), with the block-availability map and the directory on track 18 and each file stored as a chain of sectors linked by their first two bytes. A PRG file's data begins with its 2-byte load address. The disk format is identical across the three machines; only the BASIC program's load address differs ($0801 C64, $1001 unexpanded VIC-20, $0401 PET).
A multi-file image imports the way a multi-part Spectrum .TAP does: the largest BASIC program opens for editing, other BASIC programs are preserved with the document, and files loading anywhere other than the machine's BASIC start import as memory blocks at their own load address.
A document with memory blocksexports as a .d64 too, the same way — the direct counterpart to the Spectrum .TAP export: the BASIC program is written as the machine's BASIC-start file and each memory block becomes a further file at its own load address. With the Transfer dialog's auto-loader on, a generated auto-running loader program leads the disk (it LOADs each block from device 8, then chains into the main program), and the main program — being the largest BASIC-start file — is still what re-import opens for editing while the loader rides along as a preserved file. The exported image re-imports here with the program and every block intact, and the cassette .wav export carries the same files as a multi-file tape (the .prg export still holds the BASIC program alone).
Cassette audio
All three Commodore machines expose 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 pilot for temperamental hardware.
The encoding is the authentic KERNAL datasette format, shared across the whole Commodore lineage. Information is in the spacing between edges; three pulse lengths are used - short (S), medium (M), long (L), each one full square-wave cycle: bit 0 = S,M; bit 1 = M,S; new-data marker = L,M; end-of-data = L,S. A byte is a new-data marker then 8 data bits LSB-first then an odd-parity bit. Each block is a long pilot of short pulses, the bytes, then an end-of-data marker - and the KERNAL writes every block twice (first copy prefixed with the countdown $89..$81, second with $09..$01, each carrying an XOR checksum byte). A program is two blocks: a 192-byte header (file type, start/end address, filename) and the tokenized program bytes. The single shared encoder/decoder is parameterized by the machine's load address ($0801 C64, $1001 VIC-20, $0401 PET) and the machine's detokenizer (so the PET's BASIC 4.0 disk tokens list correctly on decode); each of the three exports and imports through it. A document with memory blocks exports as a multi-file tape — the program followed by one file per block, optionally behind a generated auto-loader — and re-imports with the blocks recovered, exactly as the .d64 does; all three machines share this too.