docs/ADFS.txt

		  Acorn 8-Bit ADFS Filesystem Structure
		  =====================================
		   mdfs.net/Docs/Comp/Disk/Format/ADFS

Acorn 8-bit ADFS is often refered to as ADFS-S, ADFS-M and ADFS-L, but
these refer to the size of formatted floppy disks. The filesystem is
identical, and is the same regardless of the media the data is on.

Acorn ADFS uses 256-byte logical disk sectors. Logical sectors are counted
with 24-bit numbers starting from &000000 at the start of the filesystem,
so the largest possible ADFS disk system is 4G. Acorn ADFS uses 32-bit
file lengths, so the largest possible file is 4G-1.

However, the ADFS API uses the top three bits of the sector address to
specify the drive, leaving 21 bits to specify the logical sector, so in
practice the largest possible filesystem is 512M.

Floppy disks are formatted to 2 sides, 80 tracks, 16 sectors, 256 bytes
for ADFS-L, 1 side for ADFS-M, and 1 side 40 tracks for ADFS-S. Tracks
are sequential, that is logical_sector=sector+track*16+side*1280. Logical
sector zero is zero bytes from the start of the physical disk (ie, sector
&000000). Hard drives have 256-byte sectors. Logical sector zero is 0
bytes from the start of the physical disk (ie sector &000000).


Sector 0,1 - Free Space Map
---------------------------
The first two sectors contain the free space map and disk identifier.
Sector 0 contains the start sectors of the free space blocks, sector 1
contains the length of each free space block.

000-002 	Start sector of first free space
003-005 	Start sector of second free space
006-008 	Start sector of third free space
...
0F6-0F8         Sector of Level 3 fileserver partition 'sec1', or zero
0F9-0FB 	Reserved (set to zero)
		(RISC OS stores half the disk name at 0F7-0FB, interleaved
		with the even characters at 1F6-1FA)
0FC-0FE 	Total number of sectors on disk
0FF		Checksum of sector 0

100-102 	Length of first free space
103-105 	Length of second free space
106-108 	Length of third free space
...
1F6-1F8         Sector of Level 3 fileserver partition 'sec2', or zero
1F9-1FA 	Reserved (set to zero)
		(RISC OS stores half the disk name at 1F6-1FA, interleaved
		with the odd characters at 0F7-0FB)
1FB-1FC 	Disk identifier
1FD		Boot option, set with *OPT 4
1FE		Pointer to end of free space list
		ie, 3*(number of free space blocks)
1FF		Checksum of sector 1

The checksums are calculated by starting with 255, then adding with carry
the 255 bytes of data, adding the bytes counting downwards from byte 254
to byte 0. The following BASIC code will do this.

	DEFFNadfs_sum(mem%):LOCAL sum%:sum%=255
	FOR A%=254 TO 0 STEP -1
	IF sum%>255:sum%=(sum%+1)AND255
	sum%=sum%+mem%?A%:NEXT:=sum%AND255

Note that a lot of documentation (including earlier versions of this
document) gets this wrong, often starting from 0 instead of 255, and adding
upwards instead of downwards.

The Disk Identifier is set to a random 16-bit number on initialisation.

A 'Bad map' error is generated if the checksums are wrong, or if bits
29-31 of any start sector are nonzero, or if bits 29-31 of any length are
nonzero.


Sector 2-6 - Root Directory
---------------------------
The five sectors following the Free Space Map contain the '$' root
directory. The parent of the root directory is again the root directory.


Directories
-----------
Directories occupy five logical sectors - &500 bytes - in the following
layout.

000	Directory Header
005	First directory entry
01F	Second directory entry
039	Third directory entry
...
4B1	47th directory entry
4CB	Directory Footer


Directory Header
----------------
  0  1  2  3  4
+--+--+--+--+--+
|Sq| H  u  g  o|
+--+--+--+--+--+
000		Directory Master Sequence Number in BCD
001-004 	Directory identifier - "Hugo"


Directory Entries
-----------------
  0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F 10 11 12 13 14 15 16 17 18 19 
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| Object name and attributes  | Load Addr | Exec Addr |  Length   | Sector |Sq|
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
000-009