A D82 is every 256-byte sector of a Commodore 8250 disk, track by track, with no header. Tracks are numbered from 1 and sectors from 0. The numbers are from the VICE manual (chapter 17, section D82), read through a summarising fetch tool, so they are “our words”.
Sectors per track
| Tracks | Sectors per track | Sectors in zone |
|---|---|---|
| 1–39 | 29 | 1,131 |
| 40–53 | 27 | 378 |
| 54–64 | 25 | 275 |
| 65–77 | 23 | 299 |
| 78–116 | 29 | 1,131 |
| 117–130 | 27 | 378 |
| 131–141 | 25 | 275 |
| 142–154 | 23 | 299 |
4,166 sectors. The manual’s zone table leaves track 54 out of its ranges but lists it with 25 sectors in its offset table; we take 25. Offsets printed in the manual, which the
tests check: track 1 at 0x000000, 2 at 0x001D00, 40 at 0x046B00, 78 at 0x082300, 154 at 0x102F00.
Image sizes
1,066,496 bytes (4,166 sectors); with one error byte per sector 1,070,662 bytes. The error-byte size is our sum; the text we read did not state it.
Header, track 39 sector 0
The same as the D80 header: first BAM sector 38/0 at $00, DOS format $43 at $02, disk name at $06,
disk ID at $18, DOS version “2C” at $1B.
BAM, track 38 sectors 0, 3, 6 and 9
Each BAM sector has the layout of the D80 BAM (link, version, first track covered at $04, last track plus one at $05, then
5-byte entries from $06: a free count and a 4-byte bitmap):
| Sector | Covers tracks | Links to |
|---|---|---|
| 38/0 | 1–50 | 38/3 |
| 38/3 | 51–100 | 38/6 |
| 38/6 | 101–150 | 38/9 |
| 38/9 | 151–154 (last plus one: 155, $9B) | 39/1, the first directory sector |
The BAM interleave is 3. The viewer checks every count against its bitmap and that no bit is set for a sector the track lacks.
Directory and file entries
As on the D80: the first directory sector is 39/1, sectors 1 to 28 of track 39 give up to 224 entries, the entry layout is that of the D71 page. Files and the directory use an interleave of 1.
File chains
As on the other Commodore disks: the first two bytes of a sector link to the next; track 0 ends the chain and the second byte is the index of the last used byte; other sectors hold 254 bytes.
Error bytes
When present, one byte per sector in sector order after the 4,166 sectors; $01 is no error; the codes are those of the
D64 page (that the 8250 reports the same ones is our assumption).
Not described
Known, not yet covered
- REL side sectors.
Unknown
- Whether the 8250’s error codes are exactly the 1541 set.