Q..Eliol! Tbe ISAM cfticn allows VM/370 to handle self-modifying channel programs
that IS!M generates for some of its operations. If this option is not
sFecified in the directory entry, users can obtain the facility by
issuing the CP SET command with the ISAM operand:
#cp set isam on
Tbe ISAM oFtion must be specified if a user is: • Using ISAM in an OS/PCP, OS/MFT, or OS/MVT virtual machine • Using ISAM in a V=R parition or region of an OS/VS virtual machine • Using VS1 bandshaking with VS1 nonpaging
Do not specify the ISAM option if a user is: • using I5AM in a DOS or DOS/VS virtual machine • Using ISAM in a V=V region of an OS/VS virtual machine Enter the REALTI8ER option if the virtual timer is to be updated during
virtual wait time as well as during virtual processor time. This option
is required for operating systems running applications where certain
interruptions are timer driven. If this option is not specified in the
directory entry, a user can obtain this timing facility by issuing the CP SET command with the TIMER operand: icp set timer real
To turn cff the option, issue: #cp set timer off Q..E.!:i.Q!! Tbe SVCOFF oFtion specifies that V8/370, rather than
assist or ECPS: VM/370, handle all SiC interruptions.
option, issue the CP SET command:
#cp set assist svc
virtual machine
To override this !Qte: If the operating system uses SVC 76 for error recording, VM/310 bandIes the SVC 76 interruptions whether SVCOFF is in effect or not.
Tbe virtual=real option may be desirable, undesirable, or mandatory. It
is desirable when running a virtual machine operating system (like DOS/VS or OS/VS) that performs its own paging because it eliminates the
possibility of double paging. Thevirtual=real option is not desirable
wben running an operating system in nonpaging mode with V8/VS bandshaking in a virtual machine. It is mandatory to use the
virtual=real option to allow programs that execute self-modifying
cbannel programs or have a certain degree of hardware timing
dependencies to run under V8/370. Section 1. General Considerations 41

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