.G64 File Format

Introduction

A G64 file is a copy of a Commodore 1541 disk at the level of what the drive head reads: not the 256-byte sectors of a D64, but each track as the GCR-coded bit stream of its sync marks, sector headers, gaps and data blocks. It was made for the VICE emulator so that copy-protected disks, whose protection lives in the gaps, the sync lengths, the half tracks or deliberately damaged blocks, can be kept as they are.

A G64 is larger than a D64: about 330,000 bytes for a 35-track disk (the manual gives a minimum of about 333,744 bytes for the full 84 half-track table).

Structure

A 12-byte header, a table with the file offset of every half-track, a table with every half-track’s speed zone, then the track blocks anywhere in the file, each a 2-byte length and the bit stream. See Header and track tables and GCR tracks and sectors.

In the explorer

Drop a .g64 file on the explorer to see:

• the tree: the header and its fields, the two tables with an entry per half-track, and every track block;
• a table of the tracks (offset, size, speed zone, sectors decoded) and of every sector found by decoding the bit streams, with its header and data checksums;
• when all 35 tracks of a 1541 disk decode, the disk as a .D64 that opens in the D64 viewer; sectors that did not decode are zero with an error byte;
• the problems found: an offset beyond the file, a track longer than the declared maximum, blocks that overlap, a track with the wrong number of sector headers.

Not described

Known, not yet covered

• Reference: the per-byte speed-zone blocks that a speed table entry above 3 points to, and the SPS “EXT” extra-information block (the manual mentions it; its fields are not described here).
• Viewer: half tracks are shown as blocks but not decoded; speed zones are shown but not used (all tracks are decoded at the bit level without regard to the speed); GCR-1571 files are decoded track by track and not converted to a D71; the data of a sector is not shown as bytes inside the G64 (the converted D64 does).
• Viewer: non-standard or copy-protected tracks (long syncs, unusual headers, custom blocks) are listed as “other” blocks by the decoder but not described.

Unknown

• Reference: the header and data block layouts are the standard description of the 1541 format, written from it; they were not re-fetched from a second source on the day, and the 1571’s second-side header differences are not checked.
• Viewer: no real G64 file is on this Mac and none was found to download, so the decoder is tested on files made by an encoder written for the test, not on a drive’s real dump.

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