LoadDskF/SaveDskF
LoadDskF and SaveDskF are floppy disk imaging utilities by IBM. They have an associated file format, sometimes called DSK or SKF. The format was notably used for OS/2 service packs.
It most likely contains an image of a FAT12 filesystem.
Some files are compressed. The compression scheme is some form of LZW.
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Format details
The LoadDskF documentation says "all diskette sectors are written [to the image file] up to and including the highest occupied data sector". So, uncompressed files can be converted to raw FAT/IMA format by deleting the header, and appending padding if needed.
File structure
The following information is from a forum post.
+0 DW magic [all words are in Intel little-endian order]
0x5AAA => Compressed
0x59AA => No compression
0x58AA => Older format (no compression), with sector data
beginning at fixed offset 0x200 rather than immediately
after the comment. Created by SAVEDSKF with the
undocumented /C option.
(In fact, LOADDSKF accepts any magic number in the range
58AAh-5AAAh, but these are the ones SAVEDSKF generates).
+2 DW media type
The first byte of the FAT.
+4 DW sector size, bytes
+6 DB cluster mask
The number of sectors per cluster, minus 1.
+7 DB cluster shift
= log2(cluster size / sector size)
+8 DW reserved sectors
As in the BIOS parameter block
+10 DB FAT copies
As in the BIOS parameter block
+11 DW Root directory entries
As in the BIOS parameter block
+13 DW Sector number of first cluster (ie, count of sectors used by boot
sector, FATs and root directory)
+15 DW Number of clusters in image. This is probably less than the number of
clusters the disc can hold, since empty clusters at the end are not
saved.
+17 DB sectors/FAT
As in the BIOS parameter block
+18 DW Sector number of root directory (ie, count of sectors used by
boot sector and FATs)
+20 DD checksum
Sum of all bytes in the file.
+24 DW cylinders
Number of cylinders, 40 or 80
+26 DW heads
Number of heads, 1 or 2
+28 DW sectors/track
Number of sectors per track
+30 DB 0,0,0,0
Do not appear to be used.
+34 DW number of sectors in image
+36 DW offset to comment
+38 DW offset to the first sector. If this is 0, assume it is 0x200.
After the header, an ASCII comment may follow.
The sectors then follow, stored in cylinder/head/sector order. If the magic
number is 5AAAh, the data will be compressed using a 12-bit LZW scheme.
Identification
Old format files start with bytes 0xaa 0x58.
New format uncompressed files start with 0xaa 0x59.
New format compressed files start with 0xaa 0x5a.
Software
- DIUNPACK Rel. 3.03 (1996-01-30) (OS/2 software)
- loaddf.zip - LoadDskF 1.16r (1993-01-14) and SaveDskF 1.14r (1992-05-26) - DOS and OS/2-compatible binaries; probably requires a floppy disk + drive.
- DSKXTRCT - Exracts files (uncompressed disk images only) (OS/2 software)
- dskdcmps - Decompresses compressed disk images (source code)
- Deark - Can convert to raw, decompress (based on dskdcmps), etc.
- MAKEDSKF - Utility for creating disk images (OS/2 software)
- 7-Zip's support for FAT is robust enough that it can often handle uncompressed LoadDskF files.
Sample files
- http://cd.textfiles.com/hobbesos2/2_1/SERVPAK/ - Old format
- https://archive.org/details/IBMOS22.11XR06200EnglishDiskettes → "IBM OS2 2.11 - XR06200 - English - Diskettes.zip" → *.DSK - Examples of new format, uncompressed
- https://archive.org/details/OS2_Install_Disks → OS2_W3B-D1CD_2000-03-19.zip → DISK1.DSK - Example of compressed format
