DI File Format Description

Programs

Used by disk drive emulators on STs.

Conventions

BEWORD - 16bit unsigned short (big endian)
BYTE - 8bit unsigned char

Header

Type	Name	Description
BYTE	btMagic1	'D'
BYTE	btMagic2	'I'
BYTE	btVerLo (?)	0x20
BYTE	btVerHi (?)	0x02
CHAR[]	szCreator	ascii string of creating program
BYTE	?	?
BYTE	btTracks	tracks
BEWORD	wSecPerTrack	sectors per track
BEWORD	?	flags?
BEWORD	wSecSize	sector size
BYTE	?	?

Sector table

Then there is bytes table which is long (tracks * sec per track). In every byte is stored checksum of corresponding sector.

Body

Then there are continuous sectors. If the sector is empty, it's not stored in file. Upon reading, if the checksum byte is 0, corresponding sector is not read.
Algorithm is simple - adding the bytes together with the carry. (0xFF + 0x01 = 0x01)
First three sectors of DD disk have only 0x80 bytes stored.