The virtual=real area is set up at V8/370 IPL. The primary V8/370 system operator can release the area for use as part of the dyna.ic paging area. Once released, it cannot be reclaimed except by reloading V8/370. The virtual=real area must be released in total; that is,
unused pages of the area cannot be selected for release.
There are several ways to use the virtual=real option effectively on
a data communication system with no CCW translation (SET 10THAIS 01) that has multiple ports. Dedicate either the transmission control unit
or communications lines to' that system via the ATTACH co •• and or by V8/370 directory assignment. Conversely, on a multiple port data cOllmunication virtual=real opera tion, virtual 270x lines (that is, lines
assigned and used by the CP DEFIlE and DIAL co.mands) operate with CCW translation. When V8/370 detects the use of nondedicated co •• unication
lines, it ignores the SET 10TRABS ON co.mand.
For general information on specifying a virtual=real .achine and on
defining the VIRT=REAL option in a virtual machine's directory entry,
refer to §uide. !Qte: Portions of the DOS/VS supervisor and OS/VS nucleus must be
relocated above page zero to keep input operations from compro.ising V8/370's page zero. For a more detailed description of the problem and
the suggested solutions for DOS/VS and OS/VS, refer to the "System Generation Recom.endations" topic in the sections for DOS/VS and OS/VS in a virtual machine in this publication.
By specifying the YIRT=REAL option, the virtual .achine is eligible to occupy V8/370's low storage. Thus, with the exception of page 0, all
other virtual storage addresses correspond to the real storage
addresses. V8/370's page 0 occupies the first 4096 bytes of real
storage, and Y8/370 relocates virtual page 0 to a position i •• ediately
fcllowing the area set aside for V=R operation (see Figure 14).
Real Storage
r I I I 1 1-, , Virtual machine's page 0 -->1 I I 1-------------- I Virtual Storage 1 t 1-------------- I 1 I 1 I I I ---------------------1---> I 1->1 V8/370's page 0--->1 I • Figure 14. Virtual=Real 8achine V8/370's V=R area • This option can be specified for many virtual machines; however, only one virtual machine can occupy the V=R area at one time. If this option
is sFecified and the V=R area is occupied when logging on, V8/370 creates the virtual machine in virtual=virtual mode and sends a message
to inform the user of this development.
42 IBM VM/370 Operating Systems in a Virtual Machine

If a virtual .achine runs one operating system most of the time, a user
can have that system automatically loaded each time he logs on. Use the IPL statement to identify the operating system to VM/370, such as by the following:
ipl 350 The virtual address 350 contains the system to
operating system has been
specify:
ipl DOSVS represents the address of the device that
be loaded. or, if the virtual machine's
"saved" using the CP SAVESYS command), Where DOSVS is the name under which the system was saved. !Qte: For the V"/370 system operator to automatically log onto a virtual
machine (by using the class A or B AUTOLOG command), the virtual machine directory entry must contain an IPL control statement.
The CONSOLE central statement has no unique considerations for running
an system in a virtual machine.
The MDISK centrol statement has no unique considerations for running an
oFerating system in a virtual machine.
The SPOOL centrol statement has no unique considerations for running an system in a virtual machine. Use the DEDICATE control statement to provide a virtual machine with a
corresponding real device. The virtual machine has sole use of the
dedicated device. A device such as a magnetic tape driYe can be used by
only one virtual machine at a time; therefore, specify it in a directory
entry with a DEDICATE statement.
Example: The directory specifies:
DEDICATE 181 281
This statement allows the operating system to access the device at real
address 281 via a virtual address of 181.
Section 1. General Considerations 43