Page of GC20-'807-7 As Updated April 1, 198' by TNL GN25-0829
LOGICAL UNIT ASSIGNMENT A logical unit is a symbolic name by which a program may refer to a real I/O device without knowing the device address. Two examples of logical
units are SYSRDR and SYSPCH. The DOS/VS supervisor uses two control blocks, the logical unit block (LUB) and the physical unit block (PUB), to map the symbolic name to the
real device address. An entry in the LUB for a particular logical unit,
such as SYSRDR, contains a pointer to a PUB entry. The PUB entry
contains the address of the reader, X'OOC'. Thus, all programs that
read from the logical unit SYSRDR actuallY read from the device at
address X'OOC'. On a real DOS/VS machine, logical unit assignments are made
dynamically via the ASSGN job statement or the ASSGN operator command. When usinq CMS/DOS, the CMS ASSGN command performs a similar
function. The ASSGN command in CMS/DOS assigns (or unassigns) a system
or programmer logical unit to (or from) a virtual I/O device. If a disk
is being assigned to a logical unit, the disk must have been previously
accessed via the ACCESS command. As in DOS/VS, you are not allowed to
assign the system residence volume via the ASSGN command. SYSLOG is the default value assigned to the terminal when SET DOS ON is issued.
The valid system logical units that can be assigned are: SYSRDR SYSIPT SYSPCH SYSLST SYSLOG SYSIN SYSOUT SYSSLB SYSRLB SYSCAT SYSCLB The following DOS/VS system logical units cannot be assigned: SYSRES SYSUSE SYSLNK SYSREC SYSVIS An error message is issued and the command terminated if any of these
last five system logical units are specified in the ASSGN command. If SYSIN is specified, both the SYSIPT and SYSRDR LUB and PUB entries are
filled in. If SYSOUT is specified, both the SYSLST and SYSPCH LUB and PUB entries are filled in.
If you wish to use DOS/VS private relocatable, core image or source
statement libraries, you must assign SYSRLB, SYSCLB or SYSSLB, respectively. You can assign programmer units SYSOOO through SYS241 with the ASSGN command. This deviates from DOS/VS, where the number of programmer
logical units varies according to the number of partitions. ASSGN creates a DOS Logical Unit Block (LUB) and Physical Unit Block (PUB) entry if the device is unassigned or alters the existing LUB/PUB relationship if the device is already assigned. ASSGN fills in a
one-byte index in the LUB, which points to the proper PUB entry. This PUB entey contains the channel, unit, and device type information. When a system or programmer logical unit is assigned to READER, PUNCH, oe PRINTER, the reference is to a spooled unit record device.
Card reader and terminal I/O data must not be blocked.
288 IBM VM/370 System Proqrammer's Guide

Page of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829
The ASSGN command is also used to ignore (IGN) or unassign (UA) a
logical unit. An I/O operation for a logical unit that IGN status is effectively a NOP. When a logical unit is unassigned, its pointer to
the PUB table is removed.
Part 3. Conversational Monitor System (eMS) 288.1