UNIQUE DIRECTORY ENTRY CONSIDERATIONS This topic lists directory control statements in their logical order of
appearance in a directory entry. It describes only those statements with unique ccnsideraticns for running an operating system in a virtual
machine.
The USER centrol statement has no unique considerations for running an
operating system in a virtual machine.
The ACCOUNT centrel statement has no unique considerations for running
an operating system in a virtual machine.
The OPTION central statement allows virtual machine users to specify
certain optiens and features for their virtual machines.
The ACCT option allows one user to charge another user for virtual
machine reseurces. For example: If the machine is performing a batch
type operation, a virtual machine user can generate job accounting cards
fer each jcb processed. To generate accounting cards for a virtual
user, refer to the for a discussion of
DIAGNOSE code X'4C'. Specify the ECMODE option if the virtual machine uses an operating
system that: • Runs in extended control mode • Uses dyna mic add re ss tra nsla tion (DA T) • Uses extended control registers other than zero • Addresses I/O channels 6 through 15
If this optien is not specified in the directory, a user can enter EC mede by issuing the CP SET command with the ECMODE operand: #cp set ecmode on When the ECMODE option is specified for a virtual machine, the saved segments cf the virtual operating system are shareable. 40 IBM VM/370 Operatin1 Systems in a Virtual Machine

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