Program:	DV -- Virtual Disk Handler for IAS

Author:		Shack Toms
		Technical Advisors, Inc.
		4455 Fletcher Street
		Wayne, Michigan 48184
		(313) 722-5010

Description:	DV is a program which creates an ordinary
		non-contiguous disk file, and uses the
		logical blocks therin as the physical
		sectors of an imaginary disk drive.

		This virtual disk drive can appear to
		IAS as any of the known (to the author)
		DEC disk drives.  The size of the disk
		is up to the user (the virtual disk is
		not pre-allocated.)

		Complete file control services are available
		on the virtual disk.  In particular:
		INITIALIZE works on it (if it is emulating an allowed
			device type)
		MOUNT works on it
		DISMOUNT works on it
		DSC works on it
		FLX works on it

		In general, everything, works on it except for
		disk diagnostics.

Genning for it:	To sysgen a virtual disk into your system
		edit the [11,17]SYSGEN.CMD file to include
		the following directive (with all the other
		DEV= directives.)

		DEV=DV0,<150010,0,0,1000>,,,,VRTACP

		Then perform a system generation.

Building it:	Set your default UIC to the acccount where
		this file was found.  Then type:

		PDS> @dv

		After a while pds should buzz, and the build
		will be complete.

Using it:	When a disk is first created, it looks as though
		BADBLOCKS had been run on it and found no bad
		blocks.

		The following command sequence will enable
		a virtual disk.  The disk will reside in a file
		named SY0:[1,1]MYDISK.DSK. It will emulate
		an RK05.  You probably need various priveleges
		to do this.

		PDS> install dv

		PDS> install/task:vrtacp sy0:[11,1]bigfcp ! [ or fcp ]

		PDS> mcr loa dv
		Fake disk file? mydisk/create/emulate:rk05

		PDS> allocate dv0

		PDS> initialize/index:beginning dv0 virt

		PDS> deallocate dv0

		PDS> mount/nooperator dv0 virt

		That should do it.

		When finished with the disk, use:

		PDS> dismount dv0

		PDS> mcr unl dv

		To remount the disk, use:

		PDS> mcr loa dv
		Fake disk file? mydisk

		PDS> mount/nooperator dv0 virt

		The virtual disk handler remembers that the disk
		is an RK05.

		I have had much success with this device.  I
		have many large fixed length record files, with
		short record lengths (8 bytes.)  The most
		efficient way for me to copy these files to
		mag tape is to first copy them to a virtual
		disk, and then to DSC that disk to mag tape.
		Of course, I find that whenever I copy any
		files from mag tape to disk (virtual or otherwise),
		duing peak activity, there is a great probability
		of a system crash.  (The mag tape handler apparantly
		interferes with disk UMR usage, since the crash
		generally appears when a block from mag tape
		overwrites a recently loaded task header.)

		If you have a habit of running out of UIC's,
		you may find this program helps.

		For more complete information on the operation
		and use of this program, read the blocks of
		comments in the beginning of it.