Spelunking Apple DOS 3.3 DSK files

If you happen to have had an Apple][, you might be interested in retrieving your data from old DOS 3.3 floppies.  Or maybe you're just interested in retrocomputing and downloaded some floppies at Apple2Online.  In my case, I had an ulterior motive.  I've been learning about FreeBSD device drivers and was wondering how difficult it would be to add support for Apple][ DOS 3.3 to FreeBSD.  I used Virtual][ as a desktop emulator to verify my DSK files.

In short; use the md device to mount a DSK image, which is simply a binary dump of a floppy disk.  This is exactly how FreeBSD hackers build .img files or mount .iso images.  easy.

Prior to writing code, the first task was to investigate the DSK image.  I used this image, for nostalgic reasons, however any valid DSK file should work.  I referenced this document for technical details of the VTOC, which I won't explain here.  It is, important, however, that on a DOS 3.3 disk, the VTOC is located on track 17, sector 0.

A DOS 3.3 disk has these characteristics:

  • 35 tracks per disk
  • 16 sectors per track
  • 256 bytes per sector

So a simple formula, in base-10, to find the byte offset of the first byte of track T, sector S is:

Offset = T*16*256 + S*256

For a typical disk with 35 tracks, the total storage space works out to 35*16*256 = 143360 bytes / disk.  Not surprisingly, thats the byte size of a DSK image.   The first byte of the VTOC by the above formula, is therefore 17*16*256 = 69632.

The arrangement of the VTOC, from the manuals is this:

Byte        Description
00          Not Used
01          Track number of first catalog sector
02          Sector number of first catalog sector
03          DOS release number
04-05       Not Used
06          Volume number
07 - 26     Not Used
27          Maximum number of track-sector pairs which will fit in a track-sector list
28 - 2F     Not Used
30          Last allocated track
31          Track allocation direction
32 - 33     Not Used
34          Tracks / diskette
35          Sectors / track
36 - 37     Bytes / sector
38 - FF     Track-Sector allocation bitmaps

In my case the first couple bytes of the VTOC are:

04 11 0F 03 00 00 FE 00
  • Disk catalog is at track 0x11, sector 0x0F.  Not surprisingly track 0x11 is track 17 in decimal.
  • DOS 3.3 disk
  • Volume number 0xFE

Once we've read the VTOC, we can find the disk catalog, which contains the list of every file on the disk.  In my case the byte offset for the catalog would be

69632 + 10*256 = 72192 (0x11A00)

A catalog sector looks like

Byte        Description
00          Not Used
01          Track number of next catalog sector
02          Sector number of next catalog sector
03 - 0A     Not used
0B - FF     File descriptor entries (34 bytes each)

There is room for 7 file descriptor entries in a catalog sector, each having this format

Byte        Description
00          Track of the first track-sector list entry
01          Sector of the first track-sector list entry
02          File type
03 - 20     File name
21 - 22     File length, in units of sectors

The start of my catalog is:

00 11 09 00 00 00 00 00 00 00 00 0F 0C 84 C7 CF D4 C8 C9 C3 AE D3 C5 D4 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 05
  • The next catalog sector is at track 0x11, sector 0x09
  • The first track sector list for the first file is at track 0x0F, sector 0x0C
  • The file type is 0x84 (a locked binary file)
  • The file name bytes are "C7 CF D4 C8 C9 C3 AE D3 C5 D4".
  • The file is 5 sectors in size

The A0 character bytes are ASCII extended character codes for spaces.  The file names are ASCII with the high bit set, so the bytes "C7 CF D4 C8 C9 C3 AE D3 C5 D4", as decimal with the high bit unset are "71 79 84 72 73 67 46 80 69 81" which is "G O T H I C . S E T".   Therefore the file name is "GOTHIC.SET".

The track-sector list for "GOTHIC.SET" is track 0x0F, sector 0x0C.  The offset, in decimal, is therefore:

15*16*256+12*256 = 61440 + 3072 = 64512

The format of a track-sector list is:

Byte        Description
00          Not Used
01          Track number of next track-sector list, or 00
02          Sector number of next track-sector list
03 - 04     Not used
05 - 06     Sector offset
07 - 0B     Note used
0C - FF     Track and sector pairs for data sectors, or 00

The bytes at this offset are:

00 00 00 00 00 00 00 00 00 00 00 00 0F 0B 0F 0A 0F 09 0F 08
  • There is no next track-sector list
  • The track and sector pairs are for data for this file are all on track 0x0F, and they are sectors {0x0B, 0x0A, 0x09, 0x08}.

So, to read this file, I would read the entire data of those four sectors.

3 Responses to "Spelunking Apple DOS 3.3 DSK files"

  • Troy Sutton
    October 15, 2013 - 4:03 pm Reply

    In your catalog listing, you say that the file if 5 sectors in size, but in the end you only read 4 sectors.

    Where is the missing sector?

    • tom@khubla.com
      October 17, 2013 - 6:58 pm Reply

      I had the same question. Perhaps the missing sector is the track-sector list for the file?

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