Acorn CP/M Filesystem Structure
                      ===============================
              http://mdfs.net/Docs/Comp/Disk/Format/AcornCPM

CP/M uses 128-byte records, which Acorn CP/M collects together into 512-byte
logical disk sectors. These are mapped to 256-byte single-density physical
disk sectors or 512-byte double-density physical disk sectors.

Single density Acorn CP/M floppy disks are formatted to 2 sides, 80 tracks,
10 sectors, 256 bytes. Double density disks are formatted to 10 sectors of
512 bytes from track 3 onwards, with the system tracks formatted to 10
sectors of 256 bytes. Images of double-density disks have 256 bytes of
padding after each 256 bytes of data in the first three tracks.

Tracks are sequential, up and down; that is side 0 tracks 0 up to 79 are
followed by side 1 tracks 79 down to 0. There are three reserved tracks
(&1E00 bytes) used to hold the operating system on bootable floppy system
disks before the filesystem.

Hard drives use the track and sector numbers are an index into a hard drive
image, with a 256-byte header at the start of the image.

Acorn CP/M uses 2048-byte or 4096-byte logical disk blocks, counted with
16-bit numbers from the start of the filesystem, which are mapped to
collections of 128-byte CP/M records. Block 0 is &1E00 bytes from the start
of a single-density disk, &3C00 bytes from the start of a double-density
disk, or &100 bytes from the start of a hard drive image.

Acorn CP/M disks are defined with the following CP/M Disk Parameter Blocks:

.DPB_Acorn400K   ; DPB for AcornCPM 400K disk
EQUW &0014       ; SPT=20   Records Per Track
EQUB &04         ; BSH=4    Block Shift
EQUB &0F         ; BLM=15   Block Mask
EQUB &01         ; EXM=1    Extent Mask
EQUW &00C3       ; DSM=195  Disk Sector Max
EQUW &007F       ; DRM=127  Maximum directory entry number
EQUB &C0         ; AL0=&C0  Directory occupies
EQUB &00         ; AL1=&00     first two blocks
EQUW &0020       ; CKS=32   Size of directory checksum vector
EQUW &0003       ; OFF=3    Reserved tracks before logical start of disk
; Total disk size is 128*(BLM+1)*(DSM+1)=392K
; Physical disk size is 128*SPT*OFF+128*(BLM+1)*(DSM+1)=399.5K

.DPB_Slogger800K ; DPB for Challenger/Slogger 800K disk
EQUW &0028       ; SPT=40   Records Per Track
EQUB &05         ; BSH=5    Block Shift
EQUB &1F         ; BLM=31   Block Mask
EQUB &03         ; EXM=3    Extent Mask
EQUW &00C3       ; DSM=195  Disk Sector Max
EQUW &00FF       ; DRM=255  Maximum directory entry number
EQUB &C0         ; AL0=&C0  Directory occupies
EQUB &00         ; AL1=&00    first two blocks
EQUW &0040       ; CKS=64   Size of directory checksum vector
EQUW &0003       ; OFF=3    Reserved tracks before logical start of disk
; Total disk size is 128*(BLM+1)*(DSM+1)=784K
; Physical disk size is 128*SPT*OFF+128*(BLM+1)*(DSM+1)=799K

.DPB_HardDrive   ; DPB for 8M Hard Drive
DEFW &0100       ; SPT=256  Records Per Track
DEFB &05         ; BSH=5    Block Shift
DEFB &1F         ; BLM=31   Block Mask
DEFB &01         ; EXM=1    Extent Mask
DEFW &07FF       ; DSM=2047 Disk Sector Max
DEFW &03FF       ; DRM=1023 Maximum directory entry number
DEFB &FF         ; AL0=&FF  Directory occupies
DEFB &00         ; AL1=&00    first eight blocks
DEFW &0000       ; CKS=0    Size of directory checksum vector
DEFW &0000       ; OFF=0    Reserved tracks before logical start of disk
; Total disk size is 128*(BLM+1)*(DSM+1)=8192K
; Physical disk size is 128*SPT*OFF+128*(BLM+1)*(DSM+1)=8192K


Floppy Disk Filesystem Layout
-----------------------------
  SD    DD
00000 00000 DFS disk title, eg "Acorn CP", "Slogger "
00008 00008 DFS dummy file entry, eg "CPMDISC",&A4
00010 00010 BIOS code to be loaded to &EA00-&EAEF
00100 00200 DFS disk title, eg "/M  ", "CP/M"
00104 00204 DFS disk cycle, eg &00
00105 00205 DFS entry index: &08
00106 00206 DFS disk size: &03,&20
00108 00208 DFS file addresses: EQUW &D400:EQUW &0000:EQUW &1E00:EQUW &0230
00110 00210 BIOS code to be loaded to &EAF0-&F1DF
00800 01000 CCP code to be loaded to &D400
01000 02000 BDOS code to be loaded to &DC00
01E00 03C00 Directory and file data

Within the directory and file area logical disk blocks are mapped to pairs
of physical disk sectors using the following mapping:

                      Single-density disk
          Logical CP/M   Logical disk   Physical disk
          record         sector         sector
          (128 bytes)    (512 bytes)    (256 bytes)
          0,1,2,3        0,0,0,0        0,0,1,1
          4,5,6,7        1,1,1,1        4,4,5,5
          8,9,10,11      2,2,2,2        8,8,9,9
          12,13,14,15    3,3,3,3        2,2,3,3
          16,17,18,19    4,4,4,4        6,6,7,7

                      Double-density disk
          Logical CP/M   Logical disk   Physical disk
          record         sector         sector
          (128 bytes)    (512 bytes)    (512 bytes)
          0,1,2,3        0,0,0,0        0,0,0,0
          4,5,6,7        1,1,1,1        2,2,2,2
          8,9,10,11      2,2,2,2        4,4,4,4
          12,13,14,15    3,3,3,3        6,6,6,6
          16,17,18,19    4,4,4,4        8,8,8,8
          20,21,22,23    5,5,5,5        1,1,1,1
          24,25,26,27    6,6,6,6        3,3,3,3
          28,29,30,31    7,7,7,7        5,5,5,5
          32,33,34,35    8,8,8,8        7,7,7,7
          36,37,38,39    9,9,9,9        9,9,9,9

This is probably made clearer with the following code:

DEFPROCAccessCPMBlock(BlockNum%,DiskOp%)
FOR part%=0 TO 3                       :REM Do each block of 512 bytes
  sector256%=2*(BlockNum%*4+part%)     :REM Physical sector offset
  Sector%=sector256% MOD 10            :REM Physical sector
  Sector%=(Sector%*2) MOD 10           :REM Deblock the physical sector
  Track%=(sector256% DIV 10)+Reserved% :REM Physical track
  Cylinder%=Track% DIV 80
  Track%=Track% MOD 80
  IF (Cylinder% AND 1):Track%=79-Track%:REM odd sides count downwards
  IF floppydisk THEN PROCAccessFDC(Cylinder%,Track%,Sector%)
  IF imagefile THEN PROCAccessFile(Sector%+Track%*10+Cylinder%*800)
NEXT part%
ENDPROC


CP/M Directory
-------------
Acorn CP/M disks start with a directory in the blocks specified in the DPB.
This is blocks 0 and 1 on floppy disks and blocks 0 to 7 on hard drives.
This gives a directory of 128 32-byte entries (4096 bytes) on single density
disks, 256 32-byte entries (8192 bytes) on double density disks and 1024
32-byte entries (32768 bytes) on hard drives.

The directory follows the standard CP/M layout. Each entry has an allocation
table of 16 8-bit block numbers on disks with fewer than 256 total disk
blocks (ie, floppy disks) or 8 16-bit block numbers on disk with 256 or more
total disk blocks (ie hard drives).


Directory Entry
---------------
00    : User number:
          00-0F   User Number
          10-1F   Password entry
          20      Disk label
          21      Date stamps
          E5      Erased File (usually only b7 significant)
01-08 : Filename
09-0B : Extension and attributes
0C    : Extent
0D    : Final byte count - if not &00, the number of bytes in the final record
0E    : unused
0F    : Record Count - total number of 128-byte records in this extent
10-1F : File allocation vector, 16x8-bit block numbers

The file length can be calculated with:

    p%?15*128-128*(p%?13<>0)+p%?13+16384*(p%?12 AND exm%)-16384*(p%?15>127)

where p% points to the directory entry, and exm% is the extent mask.


Attributes
----------
File attributes are stored in b7 of the filename and extension. b7 of bytes
01 to 04 are reserved for user functions, b7 of bytes 05 to 08 are reserved
for system functions. b7 of bytes 09 to 0B indicate the following:

09 b7 : Read-Only file
0A b7 : System file
0B b7 : Archive file


cpmtools Disk Definition
------------------------
The following is the disk definition for AcornCPM disks. I don't know if
this takes account of tracks 80-159 counting backwards on side 1 of the
disk.

diskdef acornsd
  seclen 512
  tracks 160
  sectrk 5
  blocksize 2048
  maxdir 128
  skewtab 0,2,4,1,3
  boottrk 3
  os 2.2
end

diskdef acorndd
  seclen 512
  tracks 160
  sectrk 10
  blocksize 4096
  maxdir 256
  skewtab 0,2,4,6,8,1,3,5,7
  boottrk 3
  os 2.2
end

diskdef acornhd
  seclen 512
  tracks 64
  sectrk 256
  blocksize 4096
  maxdir 1024
  boottrk 0
  os 2.2
end

Version History
---------------
1.00 12-Nov-2002 First draft.
1.01 15-Nov-2010 Added hard drive disk definitions.