Sector Dump Format

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".

In the TI world, all sectors are 256 bytes long.

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.

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	Sector count	Cylinder count	Head 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.