dosd - MS-DOS file as a virtual disk
The dosd* devices allow one to use an MS-DOS file as a disk. If for
instance the Boot Environment contains the setting:
dosd0 = hd1:/minix/disk0
Then the device /dev/dosd0 addresses all the blocks in the DOS file
\minix\disk0 on the first partition of the Winchester hard disk. This
device may have one primary partition table to create the partitions
dosd1 through dosd4.
One more virtual disk may be created by setting the dosd5 variable to
enable the devices dosd[5-9]. The MS-DOS file may be on any DOS
partition on either a winchester or a SCSI disk.
These devices allow one to run Minix within a DOS file, so there is no
need to repartition the disk to make Minix partitions. The Boot Monitor
has an MS-DOS version that can boot Minix from a virtual disk.
This driver has less then 20% performance overhead compared with a true
partition due to the DOS file decoding. 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 swap
partitions are accessed in units of the page size, i.e. 4096 bytes = 8
sectors. To minimize any overhead ever 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.
Philip Homburg (email@example.com)
Kees J. Bot (firstname.lastname@example.org)