April 1, 1981
Appendix F. VM/370 Restrictions
A virtual machine created by 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 the START 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 by CP) 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 that ISAM generates for some of
its operations receive special handling if the virtual machine using ISAM has that option speci fied in its VM/370 directory entry. There is
no such restriction for DOS ISAM, or for ISAM if it is running in an
as/vs virtual=virtual partition. If ISAM 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 using OS/VS TCAM (Level 5, generated or invoked with
the VM/370 option) issue a DIAGNOSE instruction when the channel program
is modified. This instruction causes CP to reflect the change in the
virtual CCi 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, the ccw translation sets a PC! bit in the real CCW string.
Each time the real CCW string is executed, the resulting PC! interruption causes CP to examine the corresponding virtual CCi string
for changes. Any changes to the virtual CCW string are also made to the
real CCW 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 the NaTRANS option has been set on.
Appendix F: VM/370 Restrictions 431

l"dge or: AS upaated April 1, 1981 by TNL GN25-0837 Minidisk Restrictions
The following restrictions exist for minidisks:
1. In the case of read home address with the skip bit off, VM/370 modifies the home address data in user storage at the completion of
the channel program because the addresses must be converted for
minidisks; therefore, the data buffer area may not be dynamically
modified during the input/output operation.
2. On a minidisk, if a ccw string uses multitrack search on
input/output operations, subsequent operations to that disk must
have preceding seeks or continue to use multitrack operations.
There is no restriction for dedicated disks.
3. as/pcP, MFT, and MVT ISAM or OS/VS ISAM running virtual=real may be
used with a minidisk only if the minidisk is located at the
beginning of the physical disk (that is, at cylinder 0). There is
no such restriction for DOS ISAM or OS/VS 151M running v iJ: t udl..;..y 1.1. t ual. 4. Because the VS1 system does no paging, any ISAM programs run
under VS1 are treated by VM/370 as though they are running in an ADDRSPC=REAL partition. VM/370 does not return an end-of-cylinder condition to
machine that has a virtual 2311 mapped to the top half
tracks 0 throuqh 9) of 2314 or 2319 cylinders. a virtual
(that is,
5. If the user's channel program for a minidisk does not perform a
seek operation, then to prevent accidental accessing, VM/370 inserts a positioning seek operation into the user's channel
program. Thus, certain channel programs may generate a condition
code (CC) of 0 on a SIO instead of an expected CC of 1, which is
reflected to the virtual machine. The final status is reflected to
the virtual machine as an interrupt.
6. A DASD channel program directed to a 3330, 3340, or 3350 device may
give results on dedicated drives which differ from results on
minidisks having non-zero relocation factors if the channel program
includes multiple-track operations and depends on a search 10 high
or a search 10 equal or high to terminate the program. This is
because the record 0 count fields on the 3330, 3340, and 3350 must
contain the real cylinder number of the track on which they reside,.
Therefore, a search ID high, for example, based on a low virtual
cylinder number may terminate prematurely if a real record 0 is
encountered.
1. Minidisks with non-zero relocation factors on 3330, 3340, and 3350 devices are not usable under OS and OS/VS systems when
the minidisk contains a VTOC of more than one track. The
locate catalog management function employs a search 10 equal
or high CCW to find the end of the VTOC. Since VM/370 does
not permit the guest to write RO, the VTOC search ends
prematurely.
7. On a 3330, 3340, or 3350, an OS/VS, or OS minidisk must start at
real cylinder 0 unless the VTOC is limited to one track. 432 IBM VM/370 Planning and System Generation Guide