LoadDskF/SaveDskF

Revision as of 01:14, 25 August 2021

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.

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