REDUCING A VIRTUAL MACHINE'S I/O OPERATIONS The number of SIO instructions executed by a virtual machine may be
substantially reduced by: • Using 4K byte blocking factors for I/O areas • Preallocating the DASD space for as or OS/VS work data sets • Using virtual storage instead of DASD work files for smaller temporary files • Building temporary files in virtual storage and letting page
out the data (if needed) • Omitting virtual printers, punches, and readers from each partition
or region in a virtual machine because records for these devices are
unblocked • Using the virtual operating system's spooling subystem (such as POWER/VS or JES) because these spooling subsystems use large I/O areas and long chains of CCWs VIRTUAL MACHINE OPTIONS VM/370 provides several optional services to virtual machines.
these options either in the OPTION control statement of the
directory program or, for many options, in the CP SET command. Specify The BMX (virtual block multiplexer) option allows an operating system running in a virtual machine to overlap multiple SIO requests on a specified channel path. The selector channel mode is the normal (and
default) channel mode for virtual machines. When the option is
given control, it applies to all channels in the virtual machine, except
to channel 0 and channels that have a channel-to-channel adapter (CTCA).
This option can be specified regardless of whether block multipleXer channels are attached to the computer. The CP DEFINE command can
redefine the channel mode for a virtual machine.
The ECMODE option allows the virtual machine to use the complete set of
virtual System/310 control registers and the dynamic address translation
feature of the System/310. Programming simulation and hardware features
are combined to allow use of all the available features in the hardware.
This option is required both for DOS/VS, OS/VS1, and OS/VS2 virtual machines and when executing VM/310 under It is also required
when executing the generalized trace facility (G7F) under as/MVT. 6 IBM VM/370 Operating Systems in a Virtual Machine

The I5AM opticn allows the virtual machine to execute the self-modifyinq CCW command sequences generated by the 05 ISAM modules in as PCP, or MVT. This option is not required for the proper functioning of 15AM in DOS or OS/VS. However, the ISAM option is required under one of
these two conditions: (1) if ISAM is run in the virtual=real area of an OS/VS virtual machine, or (2) if VM/VS handshaking is active. This option does not permit other types of self-modifying CCW sequences to
function.
Certain ISAM channel programs that execute under as/pcP, MFT, MVT, or
in a V=R region of OS/VS use a self-modifying operation that is not
allowed under normal VM/370 processing. With the I5AM option selected, VM/370 can scan the specific ISAM channel program to handle the
self-modifying sequence properlYe Only those users with the ISAM option in their VM/370 directory entry
or who have issued the CP SET ISAM ON command have their strings
checked for self-modifying operation; thus, not all users 1ncur the
additional VM/370 overhead. This option is not needed for DOS and OS/VS IS AM when run in a V=V region.
The REALTIMER option updates a virtual machine interval timer when that
virtual machine is in a self-imposed wait state. This option is
required for virtual machines running systems that wait for a timer
interruption to continue processing.
The REALTIMER, ISAM, and ECMODE options increase the amount of VM/370 overhead incurred by the virtual machines using them. Therefore, do not
specify them for a virtual machine unless they are required. These
options can be specified either in the OPTION statement in the V8/370 directory or by using the CP SET command. If a particular situation
requires an option only occasionally, use the CP SET command and not the OPTION statement. In this way, the additional overhead is incurred only
while the option is in effect.
For more information about specifying these and the other options in
the OPTION control statement, refer to the topic "Creating VM/370 Directory Entries" in this section.
DATA TRANSFER USING VMCF Virtual machines can communicate and exchange data with other virtual machines by using the virtual machine communication facility (VMCP). VMCF is the interface among communicating virtual machines. To initiate
a VMCF functicn, the operating system in the virtual machine must issue
a DIAGNOSE instruction code 1'68' and specify a register that contains
the address of a VMCF parameter list. This list contains a subfunction
code identifying the specific request and other information about that
request. For a detailed description of both the DIAGNOSE instruction
code 1'68' and the VMCF parameter list, refer to Section 1. General Considerations 7