# dosd(4)

    NAME
         dosd - DOS file as a virtual disk

    DESCRIPTION
         The dosd* devices allow one to use a DOS file as a disk.  If for instance
         the Boot Environment contains one of these pairs of settings:

              dosd = file
              dosd0 = \minix\disk0.mnx

              dosd = fat
              dosd0 = hd1:\minix\disk0.mnx

         Then the device /dev/dosd0 addresses all  the  blocks  in  the  DOS  file
         \minix\disk0.mnx.

         The "file" and "FAT" versions of the driver differ radically on how  they
         access  the  DOS  file,  the  first will use actual DOS calls to read and
         write the file, the second interprets a  FAT  file  system  to  find  the
         blocks  of the file.  One understands that the "file" driver can only run
         if Minix has been started from DOS using the  DOS  version  of  the  Boot
         Monitor.    Note   that   the  DOS  Boot  Monitor  will  also  set  dosd0
         automatically to  the  file  it  is  to  boot,  so  no  configuration  is
         necessary.

         In either case the DOS file is seen as a disk under Minix.  The disk  can
         have  one  primary  partition table allowing one to create the partitions
         dosd1 through dosd4.

         One more virtual disk may be created for the FAT driver  by  setting  the
         dosd5  variable  to  enable  the devices dosd[5-9].  The file driver also
         supports dosd10 and dosd15 for four virtual disks total.  The FAT  driver
         allows  one to use any primary partition on a normal hard disk or even on
         a floppy drive (for testing purposes).

         The FAT driver has less then 20% performance  overhead  compared  with  a
         true  partition due to the DOS file decoding.  The file driver depends on
         how well DOS does file I/O, which isn't too  bad  for  simple  reads  and
         writes  to  one file.  Overhead can be minimized if I/O is aligned to DOS
         file clusters.  For Minix file system use you are advised  to  start  the
         virtual  partitions  on  even-numbered disk sectors, so that the 2-sector
         Minix blocks  do  not  span  clusters.   Minix-vmd  swap  partitions  are
         accessed  in  units  of  the  page size, i.e. 4096 bytes = 8 sectors.  To
         minimize overhead you could let partitions start on a  cluster  boundary.
         The  virtual  disk  will appear to have a track size equal to the cluster
         size, so a partition is cluster aligned if it starts on the first  sector
         in  a  track.  It is of course best to also defragment your DOS partition
         to eliminate any extra disk seeks due to gaps between clusters.

         Replacing the "file" driver by the "FAT" driver will help performance  if
         the  normal  disk driver used by the FAT driver is a native Minix driver.
         The FAT driver used together with the BIOS disk driver will not make much
         of a difference.

    FILES
         /dev/dosd[0-9]

    SEE ALSO
         hd(4), sd(4).

    BUGS
         The file driver needs calls to read and  write  arbitrary  DOS  files  to
         allow file to be easily moved between DOS and Minix.

         The FAT driver doesn't understand long file names.

         More testing is needed on FAT-32 file systems.

    AUTHORS

      FAT driver
         Philip Homburg (philip@cs.vu.nl)

      File driver and manual page
         Kees J. Bot (kjb@cs.vu.nl)