.D71 File Format

Introduction

A D71 file is a copy of a floppy disk for the Commodore 1571 disk drive, the double-sided drive made for the Commodore 128. It is the 1571 counterpart of the D64 image of the 1541: the same 256-byte sectors, the same directory and file chains, but 70 tracks, 35 on each side of the disk.

The file holds every sector in order, side 0 first. A standard image is 349,696 bytes (1,366 sectors); with one error byte per sector it is 351,062 bytes.

Structure

Side 0 is laid out exactly like a 1541 disk: tracks 1 to 35 with 21, 19, 18 and 17 sectors a track, the header and BAM in sector 0 of track 18 and the directory from 18/1. Side 1 is tracks 36 to 70 with the same four zones again (21 sectors a track on 36 to 52, 19 on 53 to 59, 18 on 60 to 65, 17 on 66 to 70). The BAM entries of side 1 are split: the free counts are the bytes $DD to $FF of 18/0 and the bitmaps are in sector 0 of track 53. See Layout, BAM and directory.

Sizes

In the explorer

Drop a .d71 file on the explorer to see:

• the tree: every track, and in it every sector named for what it is (header, BAM, directory sector, sector n of a file, free, orphan, shared) with its error byte; the header and BAM sectors show their fields and one entry a track, the directory entries show theirs;
• a picture of the disk (one square a sector, side 1 below side 0), the directory, a table of the tracks and a table of all the sectors;
• every file to open: a PRG in the .PRG viewer, a SEQ file as text from PETSCII, any other as bytes;
• the problems found, listed, with the disk still opening: a chain that loops, a sector two files use, a free count that disagrees with its bitmap, a size field that disagrees with the chain, errors in the error bytes.

The options bar sets whether error bytes follow the sectors (detected from the size) and the character set for names and text.

Not described

Known, not yet covered

• Reference: REL files: the side-sector blocks and the 1571's super side sector are not described and not followed (the file's data chain is).
• Reference: GEOS 128 disks and the 1571's native-mode extras (for example a second directory layout) are not described.
• Viewer: the double-sided flag is read only as a check, and a D71 whose BAM marks it single-sided is not told apart; 40-track D71 files (non-standard) are not read.
• Viewer: graphics characters in names and text are mostly shown as U+FFFD (the same limit as D64).

Unknown

• Reference: the 1571's file allocation order (only the interleave of 6 for files and 3 for the directory is given in the manual we read).
• Viewer: no real D71 image is on this Mac and none was found to download, so the reading is checked on built images only, against track offsets printed in the VICE manual (see the Links).

Links