Acorn 8-Bit ADFS Filesystem Structure		  =====================================		http://mdfs.net/Docs/Comp/Disk/Format/ADFSAcorn 8-bit ADFS is often refered to as ADFS-S, ADFS-M and ADFS-L, butthese refer to the size of formatted floppy disks. The filesystem isidentical, and is the same regardless of the media the data is on.Acorn ADFS uses 256-byte logical disk sectors. Logical sectors are countedwith 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-bitfile lengths, so the largest possible file is 4G-1.However, the ADFS API uses the top three bits of the sector address tospecify the drive, leaving 21 bits to specify the logical sector, so inpractice the largest possible filesystem is 512M.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 1contains the length of each free space block.000-002 	Start sector of first free space003-005 	Start sector of second free space006-008 	Start sector of third free space...0F6		Reserved (must be set to zero)0F7-0FB 	Reserved (set to zero)		(RISC OS stores half the disk name here, interleaved		with the even characters at 1F6-1FA)0FC-0FE 	Total number of sectors on disk0FF		Checksum of sector 0100-102 	Length of first free space103-105 	Length of second free space106-108 	Length of third free space...1F6-1FA 	Reserved (set to zero)		(RISC OS stores half the disk name here, interleaved		with the odd characters at 0F7-0FB)1FB-1FC 	Disk identifier1FD		Boot option, set with *OPT 41FE		Pointer to end of free space list		ie, 3*(number of free space blocks)1FF		Checksum of sector 1The checksums are calculated by starting with 255, then adding with carrythe 255 bytes of data, adding the bytes counting downwards from byte 254to 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%AND255Note that a lot of documentation (including earlier versions of thisdocument) get this wrong, often starting from 0 instead of 255, and addingupwards instead of downwards.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 bits29-31 of any start sector are nonzero, or if bits 29-31 of any length arenonzero.Sector 2-6 - Root Directory---------------------------The five sectors following the Free Space Map contain the '$' rootdirectory. The parent of the root directory is again the root directory.Directories-----------Directories occupy five logical sectors - &500 bytes - in the followinglayout.000	Directory Header005	First directory entry01F	Second directory entry039	Third directory entry...4B1	47th directory entry4CB	Directory FooterDirectory Header----------------000		Directory Master Sequence Number in BCD001-004 	Directory identifier - "Hugo"Directory Entries-----------------000-009 	Object name and access bits		Access bits are encoded in b7 of bytes 000-009		000: 'R' - object readable		001: 'W' - object writablee		002: 'L' - object locked		003: 'D' - object is a directory		004: 'E' - object execute-only		Files are created with 'WR' access, directories		are created with 'DLR' on 8-bit systems and		'DR' on 32-bit systems.00A-00D 	Load address00E-011 	Execution address012-015 	Length016-018 	Start sector019		Sequence numberDirectory Footer----------------4CB		&004CC-4D5 	Directory name4D6-4D8 	Start sector of parent directory4D9-4EB 	Directory title - initially set to same as directory name.4EC-4F9 	Reserved (set to zero)4FA		Directory Master Sequence Number in BCD4FB-4FE 	Directory identifier - "Hugo"4FF		&00A directory is reported as 'Broken' if the Master Sequence Number and "Hugo"strings do not match - bytes 000-004 are compared with bytes 4FA-4FE.Strings in directories are terminated with &0D if shorter than tencharacters, they are not space padded as with most other filesystems.The objects in a directory are always stored in case-insensitive sortedorder.