ApriDisk file format

This version of the file format document has been updated with corrections from Jonathan Marsters (the author of Apridisk).

Header

The header is 128 bytes long. It contains the C string

	"ACT Apricot disk image\032\004"

padded out to 128 bytes with zeros. LibDsk 1.1.3+ requires the presence of this header.

Sector records

The records for each sector now follow. In existing examples they are sorted by sector, head, cylinder, but this is not necessary. LibDsk does not sort ApriDisk files it generates, though ApriDisk files created by dsktrans will be in the expected order.

A sector header is at least 16 bytes long. It is formed:

	DD	item_type	;4 bytes, little-endian.
				;0xE31D0000 => deleted item
				;0xE31D0001 => sector
				;0xE31D0002 => comment
	DW	compression	;2 bytes, little-endian.
				;0x9E90 => not compressed
				;0x3E5A => compressed
	DW	header_size	;2 bytes, little-endian.
	DD	data_size	;4 bytes, little-endian. 
	DB	head		;Head ID, 0 or 1
	DB	sector		;Sector ID, 1 based
	DW	cylinder	;2 bytes, little-endian.
				;Cylinder ID, 0 based.

If the header_size is greater than 0x10, the remainder of the header then follows.

The data_size then gives the number of bytes to read. If the sector is not compressed, the bytes read will be the sector.

The Deleted Item tag allows in-place rewriting of Apridisk files; if a compressed sector has to be replaced with an uncompressed one, then the compressed sector is marked as deleted and the uncompressed one appended to the file. Currently LibDsk does not rewrite in place; it loads the file into memory and writes a new file on close. This means the DOS version can't handle 720k Apridisk images.

Compression

Compressed sectors use an RLE scheme:

	DW	count		;2 bytes, little-endian.
	DB	byte		;The byte to repeat count times.

Compression only appears to be used in sectors all bytes of which are identical. LibDsk only generates a compressed sector if all bytes are the same.

Comment

After all the sectors, there may be a comment record. This is stored in the same way as a sector:

	DD	item_type	;4 bytes, little-endian.
				;0xE31D0000 => deleted item
				;0xE31D0001 => sector
				;0xE31D0002 => comment
	DW	compression	;2 bytes, little-endian.
				;0x9E90 => not compressed
				;0x3E5A => compressed
	DW	header_size	;2 bytes, little-endian.
	DD	data_size	;4 bytes, little-endian. 
	DB	0,0,0,0		;Comment ID, always 0.

The comment itself is stored as an ASCII string, complete with terminating 0 byte. Newlines in it are stored as '\r' characters.