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

BTAM AUTOPOLL CHANNEL PROGRAMS If an operating system is executing BTAM channel programs, VM/370 checks
each BTAK autcpoll CCW string to see if it has been dynamically changed.
It does this each ti.e the string is executed. To bypass this checking,
issue the CP command:
set autopoll on Whenever the BTAM autopoll CCWs are .odified, OS/VS1 Release 6 and DOS/VS Release 34 with the Advanced Functions-DOS/VS program Product
(Program NO. 5746-XE2) use the DIAGNOSE instruction code X'28' to notify VK/370. The combination of the SET AUTOPOLL ON command and the use of the
diagnose interface reduces VM/370 overhead and improves the overall
performance for that particular user. However, both of these facilities aust be active.
If a user has specified SET AUTOPOLL ON and the operating system does
not use the diagnose interface, a channel program modification goes
undetected. The results are unpredictable.
If a user has specified SET AUTOPOLL OFF and the operating system
uses the diagnose interface, the unnecessary checking results in
performance degradation. !gte: Set DOS/VS AUTOPOLL on when using VM/VS handshaking.
8 VM/370 operating Systems in a Virtual Machine