.TH DOSD 4
.SH NAME
dosd \- MS-DOS file as a virtual disk
.SH DESCRIPTION
The
.B dosd*
devices allow one to use an MS-DOS file as a disk.  If for instance the
Boot Environment contains one of these pairs of settings:
.PP
.RS
.nf
.BR dosd " = " file
.BR dosd0 " = " \eminix\edisk0
.sp 0.4v
.BR dosd " = " fat
.BR dosd0 " = " hd1:\eminix\edisk0
.fi
.RE
.PP
Then the device
.B /dev/dosd0
addresses all the blocks in the DOS file
.BR \eminix\edisk0 .
.PP
The "file" and "FAT" versions of the driver differ radically on how they
access the DOS file, the first will use actual MS-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 MS-DOS using the DOS version of the Boot Monitor.
Note that the DOS Boot Monitor will also set
.B dosd0
automatically to the file it is to boot, so no configuration is necessary.
.PP
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
.B dosd1
through
.BR dosd4 .
.PP
One more virtual disk may be created for the FAT driver by setting the
.B dosd5
variable to enable the devices
.BR dosd[5-9] .
The file driver also supports
.B dosd10
and
.B 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).
.PP
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.
.PP
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.
.SH FILES
/dev/dosd[0\-9]
.SH "SEE ALSO"
.BR hd (4),
.BR sd (4).
.SH AUTHORS
.SS "FAT driver"
Philip Homburg (philip@cs.vu.nl)
.SS "File driver and manual page"
Kees J. Bot (kjb@cs.vu.nl)