Sector Dump Format

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A disk image in Sector Dump Format is defined as a concatenation of all sector contents in logical sequence. These disk image files are typically suffixed with ".dsk" or ".DSK". Note that the Track Dump Format images are also suffixed with ".dsk" or ".DSK", so this cannot be decided by the file suffix alone.


0 1 2 3 4 5 6 7 ... n-1 [Bad sectors map]


It should be noted that the SDF does not represent the physical sequence of sectors per track. The sectors on a real floppy disk are arranged by an interleave per track, with a skew from track to track. See more about the real arrangement in the article on the Interleave.

Standard format

In the standard format, which is used in many emulators like v9t9, MAME, or js99er, the file only consists of the sectors, and no bad sector map is available.

In the TI world, all sectors are 256 bytes long. Files can easily be checked for this format by verifying that the total file size is a multiple of 256. Also, single density format defines tracks with 9 sectors per track, while double density contains 18 sectors per track. There is one exception: The original TI DD controllers (or the HX5102 Hexbus floppy) use 16 sectors/track for double density.

Assuming that we have a 9 or 18 sectors/track layout, SDF image sizes should be expected to be a multiple of 9·256 = 2304 bytes.

Since the format does not encode any metadata, there are situations where the original disk format cannot be determined easily. In particular, a double-sided/single density (DSSD) format has the same sector count as a single-sided/double density (SSDD) format. The same issue appears for 80 track images with single density versus 40 track images with double density.

In these cases, looking into sector 0 may help to disambiguate the format analysis. This will only work if the disk image is formatted, of course.

File sizes for formats:

Format Description Recording Cylinder count Head count Sector count File size Occurance
SSSD Single-sided, single density FM 40 1 360 92160 typical
DSSD Double-sided, single density FM 40 2 720 184320 typical
DSSD80 DSSD with 80 tracks FM 80 2 1440 368640 rare, but increasing
SSDD Single-sided, double density MFM 40 1 720 184320 very rare
DSDD Double-sided, double density MFM 40 2 1440 368640 typical
DSDD80 DSDD with 80 tracks MFM 40 2 2880 737280 rare
DSHD80 Double sided, high density MFM 80 2 5760 1474560 very rare
SSDD16 Single-sided, double density (TI) MFM 40 1 640 163840 not seen yet
DSDD16 Double-sided, double density (TI) MFM 40 2 1280 327680 not seen yet
DSDD77 DSDD (TI) with 77 tracks MFM 77 2 2464 630784 not seen yet

Extended format

There is one extension to the SDF which adds 768 bytes at the end, containing a map of bad sectors. File sizes are a multiple of 9·256 plus 768; that is, if the file size is not a multiple of 9·256, but is 768 bytes longer, the file is very likely a sector dump with bad sectors map.

This was introduced by a tool to read real TI floppy disks on a PC. Emulators typically ignore these additional bytes. For that reason, this format is really rare to find.

MAME Hard disk format (legacy)

In very early releases, the MESS emulator (now part of MAME) supported hard disk image files in a sector dump format. However, for quite some time, MESS (and MAME) require images in CHD format.