Floppy Disks

There is a good set of tools for manipulating most floppy disks under Linux. If you do not already have them, you need the tools and documentation from http://fdutils.linux.lu/. They are, unfortunately, a bit cryptic to use so here are a few pointers:

• If you are using the first floppy drive, you probably don't need to specify a drive. If you are using the second drive, specify drive=/dev/fd1 and add one to the drvsel parameter. So, to look at the ID bytes of a (random) sector on the first head on the second drive you need something like
          fdrawcmd drive=/dev/fd1 readid 1 need_seek track=0
  and for the second head you need
          fdrawcmd drive=/dev/fd1 readid 5 need_seek track=0
   
• Data transfer rates must be correct. You need rate=0 for high density (like 1.2M on 5.25inch and 1.44M on 3.5 inch) , rate=1 for double density (like 360k, 720k) 5.25 inch and rate=2 for double density (like 720k) 3.5 inch. See below for 8 inch drives.
   
• Don't forget that you also have to worry about the track density for some disks: see the documentation at http://fdutils.linux.lu.
   
• If you are trying things out, don't forget to reset the controller and drive after failed attempts at fdrawcmd:
          floppycontrol --resetnow 2
     fdrawcmd drive=/dev/fd1 recalibrate 1
     fdrawcmd drive=/dev/fd1 recalibrate 1
 
• If you are trying to read a whole disk image using fdrawcmd, you can read both sides of a single track in one command. It will not, however, step to the next track. It's a bad idea to do the stepping inside the fdrawcmd read command, as this will force a seek to track zero. Here is an example of a reasonably efficient way to read an image of a 5.25 inch double density 8 sector/track double sided 80 track disk mounted in drive /dev/fd1:
          #!/bin/bash
          cat /dev/null > $1
          floppycontrol --resetnow 2
          fdrawcmd drive=/dev/fd1 rate=1 readid 1 need_seek track=0
          for ((i=0;i<80;i++)) do
              fdrawcmd drive=/dev/fd1 seek 1 $i
              fdrawcmd drive=/dev/fd1 rate=1 read 1 $i 0 1 2 8 0x1b 0xff length=8192 >> $1
              done
          gzip $1
  To change the number of sectors/track, or the number of sides, you just replace the 8 with the new number of sectors and replace the 8192 with 512 * <number of sectors> * <number of sides>. To change the number of tracks, just change the 80. The above version does no useful error checking. A more robust approach checks that the reads read the correct number of bytes.
          #!/bin/bash
          cat /dev/null > $1
          errfile=/tmp/$$_errs
          track=-1
          floppycontrol --resetnow 2
          fdrawcmd drive=/dev/fd1 rate=1 readid 1 need_seek track=0 2> /dev/null
          for ((i=0;i<80;i++)) do fdrawcmd drive=/dev/fd1 seek 1 $i 2> /dev/null
            fdrawcmd drive=/dev/fd1 rate=1 read 1 $i 0 1 2 8 0x1b 0xff length=8192 >> $1 2> $errfile
            awk '/remaining=/ {if ( $2 != 0 ) { print "**ERROR**" ; exit (2) } }' $errfile || { track=$i ; break ; }
          done
          rm $errfile
          if (( track == -1 )) ; then gzip $1
            exit 0
          else rm $1
            echo "** READ FAILED AT TRACK $track **"
            exit 2
          fi
  Finally, if you want to read 40 track disks on an 80 track drive, you need to do a bit of arithmetic for the seek.
               #!/bin/bash
              cat /dev/null > $1
              errfile=/tmp/$$_errs
              track=-1
              floppycontrol --resetnow 2
              fdrawcmd drive=/dev/fd1 rate=1 readid 1 need_seek track=0 2> /dev/null
              for ((i=0;i<40;i++)) do let "p = 2 * i"
                fdrawcmd drive=/dev/fd1 seek 1 $p 2> /dev/null
                fdrawcmd drive=/dev/fd1 rate=1 read 1 $i 0 1 2 9 0x1b 0xff length=9216 >> $1 2> $errfile
                awk '/remaining=/ {if ( $2 != 0 ) { print "**ERROR**" ; exit (2) } }' $errfile || { track=$i ; break ; }
                done
              rm $errfile
              if (( track == -1 )) ; then gzip $1
                exit 0
              else rm $1
                echo "** 360k READ FAILED AT TRACK $track **"
                exit 2
              fi
   
• Once you have your disk image, you need to interpret it. The mtools will do a good job on most MSDOS-like formats (but see below). These other tools might be useful for some legacy formats:
  • CP/M: http://www.moria.de/~michael/cpmtools/
  • UCSD P-system: http://www.threedee.com/jcm/diskutil/index.html
  • Sinclair QL: QLtools at http://www.daria.co.uk/
  • Tripos: my own crude effort, triposfs.c
  • INMOS TDS for Sage and Stride: my simple tdsfs.c and the accompanying tdsfile.c to parse the folded files.
  • Legacy UNIX archive formats, including bar can be manipulated using cpio from the Heirloom Toolchest.
     
• MSDOS-style disks, also used by the Atari ST
  The following table may be useful. All modern MSDOS-style floppies have a BIOS parameter block in the first 512 byte sector. This gives complete information about the layout of the disk. Very early versions of MSDOS produced floppies without this block and relied on the media descriptor, the first byte of the second sector, to give the disk layout. The most common MSDOS formats are shown in green.
   

  FORMAT	MEDIA DESCRIPTOR	USUAL MEDIA	TRACKS	SECTORS	SIDES	CLUSTER SIZE	FAT TYPE	FAT	DIRECTORY
  160K	FE	5.25 inch	40	8	1	1 = 512 byte	12 bit	1 sector	4 sectors = 64 entries
  180K	FC	5.25 inch	40	9	1	1 = 512 byte	12 bit	2 sector	4 sectors = 64 entries
  320K	FF	5.25 inch	40	8	2	2 = 1024 byte	12 bit	1 sector	7 sectors = 112 entries
  360K	FD	5.25 inch	40	9	2	2 = 1024 byte	12 bit	2 sector	7 sectors = 112 entries
  720K	F9	3.5 inch	80	9	2	2 = 1024 byte	12 bit	3 sector	7 sectors = 112 entries
  1.2Meg	F9	5.25 inch HD	80	15	2	1 = 512 byte	12 bit	7 sector	14 sectors = 224 entries
  1.44Meg	F0	3.5 inch	80	18	2	1 = 512 byte	12 bit	9 sector	14 sectors = 224 entries
  2.88Meg	F0	3.5 inch HD	80	36	2	2 = 1024 byte	12 bit	9 sector	15 sectors = 240 entries
  	F8	fixed disk					12/16/32 bit

• If you have a 8-inch drive, it should work too, although I have not tried out mine yet. The most common formats were 77 tracks with 26 sectors either single or double sided, recorded at 500kHz (rate=0) either FM with a sector size of 128 or MFM with a sector size of 256.