April 1, 1981
Appendix F.VM/370 Restrictions
A virtual machine createdby VM/370 is capable of running an IBM System/360 or system/370 operating system as long as certain VM/370 restrictions are not violated. Virtual machine restrictions and certain
execution characteristics are stated in this appendix.Dynamically Modified Channel Programs
In general, virtual machines may not execute channel programs that are
dynamically modified (that is, channel programs that are changed between
the time theSTART I/O (SIO) is issued and the time the input/output
ends, either by the channel program itself or by the processor)• Exceptions (that is, dynamically modified channel programs given
special consideration byCP) are: • Those generated by the Indexed Sequential Access Method (ISAM) running under as/pcP, OS/MFT, and as/MVT • Those generated by ISAM running in an OS/VS virtual=real partition • Those generated by the as/vs Telecommunications Access Method (TeAM) Level 5, with the VM/370 option • Those containing polling sequences
The self-modifying channel programs thatISAM generates for some of
its operations receive special handling if the virtual machine usingISAM has that option speci fied in its VM/370 directory entry. There is
no such restriction forDOS ISAM, or for ISAM if it is running in an
as/vs virtual=virtual partition. IfISAM is to run in an OS/VS virtual=real partition, you must specify the ISAM option in the VH/370 directory entry for the as/vs virtual machine.
virtual machines usingOS/VS TCAM (Level 5, generated or invoked with
theVM/370 option) issue a DIAGNOSE instruction when the channel program
is modified. This instruction causesCP to reflect the change in the
virtualCCi string to the real CCW string being executed by the channel. CP is then able to execute the dynamically modified channel program
properly.When a virtual machine starts a channel program containing a polling
sequence, theccw translation sets a PC! bit in the real CCW string.
Each time the realCCW string is executed, the resulting PC! interruption causes CP to examine the corresponding virtual CCi string
for changes. Any changes to the virtualCCW string are also made to the
realCCW string while it is executing.
The restriction against dynamically modified channel programs does
not apply if the virtual machine has the virtual=real performance option
and theNaTRANS option has been set on.
Appendix F:VM/370 Restrictions 431
Appendix F.
A virtual machine created
execution characteristics are stated in this appendix.
In general, virtual machines may not execute channel programs that are
dynamically modified (that is, channel programs that are changed between
the time the
ends, either by the channel program itself or by the processor)
special consideration by
The self-modifying channel programs that
its operations receive special handling if the virtual machine using
no such restriction for
as/vs virtual=virtual partition. If
virtual machines using
the
is modified. This instruction causes
virtual
properly.
sequence, the
Each time the real
for changes. Any changes to the virtual
real
The restriction against dynamically modified channel programs does
not apply if the virtual machine has the virtual=real performance option
and the
Appendix F:
