Raw Disk Image File
===================

The disk image consists of:
  <track data>
  <track data>
  <track data>
  ... etc ...

The tracks can be stored in any order, but /should/ be stored in physical
track order, either interleaved or noninterleaved, ie:
 ie 0:00, 0:01 .. 0:78, 0:79, 1:00, 1:01 .. 1:78, 1:79
 or 0:00, 1:00, 0:01, 1:01 .... 0:78, 1:78, 0:79, 1:79

Each physical track can be a different density, a different logical track
to the physical track, and a different logical side to the physical side. 
This allows oddball formats such as dual 40/80 track disks and Challenger
CPM disks (track 0/1/2=FM, track 3+=MFM) to be imaged.

Track Data
==========
Each track consists of the following:

Track density
-------------
&xx                     : Physical track
&xx                     : Physical side
&xx                     : Density &00=FM, &01=MFM DD, &02=MFM HD, &03=MFM OD
                        :  Density=&00 - track length=3125 bytes (eg 400k DFS)
                        :  Density=&01 - track length=6250 bytes (eg 640k/800k ADFS)
                        :  Density=&02 - track length=12500 bytes (eg 1600k ADFS)
                        :  Density=&03 - track length=25000 bytes (eg 3800k ADFS)
&xx                     : Spare

Track header
------------
&FC                     : Index mark
&FF * gap1              : gap1

Each sector
-----------
&00 * sync1             : Sync bytes
&FE                     : ID mark
&xx                     : logical track
&xx                     : logical head
&xx                     : logical sector
&xx                     : sector size 0=128, 1=256 2=512 ... 7=16384
&xx &xx                 : Sector ID CRC

&FF * gap2              : gap2

&00 * sync2             : Sync bytes
&FB                     : Data mark
&xx * size              : <size> bytes of data
&xx &xx                 : Sector data CRC

&FF * gap3              : gap3

Track tail
----------
&FF * gap4-gap3         : gap4 without duplicate gap3
&FF * gap5              : gap5

Padding
-------
&00 * pad               : padding in disk image file to next track

Total bytes:
 Density 1
 Track header 3+gap1
 Sectors      num*(sync1+gap2+sync2+size+gap3+10)
 Track tail   gap4+gap5
 Padding      pad
              3+gap1+gap4+gap5+pad+num*(gap2+gap3+sync1+sync2+size+10)-gap3

For Acorn DFS (ADUG) this is:
              3+16+30+0+pad+10*(11+21+6+6+256+10)-21
              28+pad+10*(310)
              28+pad+3100
              3128+pad
              3125 (pad=-3!) - this is why slightly dodgy

For Acorn DFS (optimised) this is:
              3+16+30+0+pad+10*(11+16+6+6+256+10)-16
              33+pad+10*(308)
              33+pad+3080
              3113+pad
              3125 (pad=22)

For Watford DDFS this is:
              3+60+45+0+pad+16*(22+43+12+12+256+10)-43
              65+pad+16*(355)
              65+pad+5680
              5745+pad
              6250 (pad=505)

For Acorn ADFS 640k this is:
              3+42+40+0+pad+16*(22+57+12+12+256+10)-57
              28+pad+16*(369)
              28+pad+5904
              5932+pad
              6250 (pad=318)

For Acorn ADFS 800k this is:
              3+303+40+0+pad+5*(22+90+12+12+1024+10)-90
              256+pad+5*(1170)
              256+pad+5850
              6106+pad
              6250 nominal (pad=144)

              3+gap1+gap4+gap5+pad+num*(gap2+gap3+sync1+sync2+size+10)-gap3
For Acorn ADFS 1600k this is:
              3+50+30+0+pad+10*(22+90+12+12+1024+10)-90
              -7+pad+10*(1170)
              -7+pad+11700
              11693+pad
              12500 nominal (pad=807)

For DOS 720k this is:
              3+50+50+0+pad+9*(22+90+12+12+512+10)-90
              13+pad+9*(658)
              13+pad+5922
              5935+pad
              6250 (pad=315)

For DOS 1440k this is:
              3+50+50+0+pad+18*(22+90+12+12+512+10)-90
              13+pad+18*(658)
              13+pad+11844
              11857+pad
              12500 (pad=643)

For Acorn ADFS 3800k this is:
              3+40+30+0+pad+3*(22+80+12+12+8192+10)-80
              -7+pad+3*(8328)
              -7+pad+24984
              24977+pad
              25000 nominal (pad=23)