these situations but is unable to provide predictable protection to
other virtual machines. In addition, violation of this restriction
may compromise the integrity of the system. The results are
unpredictable.
2. A two-channel switch can be used between the IBM System/370 running
a virtual machine under VM/370 and another processor.
3. The DIAGNOSE instruction cannot be issued by the virtual machine
for its normal function. VM/370 uses this instruction to allow the
virtual machine to communicate system services requests. The
Diagnose interface requires the operand storage addresses passed to
it to be real to the virtual machine issuing the DIAGNOSE instruction. For more information about the DIAGNOSE instruction in
a virtual machine, see the !M/370 4. A control unit normally never appears busy to a virtual machine.
An exception exists when a forward space file or backward space
file command is executed for a tape drive. Subsequent I/O operations to the same 7irtual cor.trol unit result in a control unit busy condition until the forward space file or backward space
file command completes. If the real tape control unit is shared by
more than one virtual machine, a control unit busy condition is
reflected only to the virtual machine executing the forward space
file or backward space file command. When a virtual machine
attempts an I/O operation to a device for which its real control
unit is busy, the virtual machine is placed in I/O wait
(nondispatchable) until the real control unit is available. If the
virtual machine executed a SIOF instruction (rather than SIO) and
was enabled for block-multiplexing, it is not placed in I/O wait
for the above condition.
5. The CP IPL command cannot simulate self-modifying IPL sequences off
dedicated unit record devices or certain self-modifying IPL sequences off tape devices.
6. The VM/370 spooling facilities do not support punch-feed-read,
stacker selection, or column binary operations. Detection of
carriage control channels is supported for a virtual 3211 only.
7. VM/370 does not support count
operator's console.
control on the virtual 1052 8. Programs that use the integrated emulators function only if the
real computing system has the appropriate compatibility feature. VM/370 does not attempt simulation. The DOS emulator running under OS or OS/VS is not supported under VM/370. 9. The READ DIRECT and WRITE DIRECT instructions are not supported for
a virtual machine. 10. The System/370 SET CLOCK instruction cannot be simulated and,
hence, is ignored if issued by a virtual machine. The System/370 STORE CLOCK instruction is a nonprivileged instruction and cannot
be trapped by VM/370; it provides the true TOD clock value from the
real processor.
11. The 1050/1052 Model 2 Data Communication System is supported only
as a keyboard operator's console. Card reading, paper tape I/O, and other modes of operation are not recognized as unique, and
hence may not work properly. This restriction applies only when
the 1050 system is used as a virtual machine operator's console.
It does not apply when the 1050 system is attached to a virtual
machine via a virtual 2701, 2702, or 2703 line.
436 IBM VM/370 Planning and System Generation Guide

12. The pseudo-timer (usually device address OFF, device type TIMER)
does not return an interrupt from a start I/O; therefore, do not
use EXCP to read this device.
13. A virtual machine device 1PL with the NOCLEAR option overlays one
page of virtual machine storage. The 1PL simulator uses one page
of the virtual machine to initiate the 1PL function. The starting
address of the overlaid page is either the result of the following
formula:
virtual machine size
-------------------- = starting address of the overlayed page
2
or the hexadecimal value 20000, whichever is smaller. 14. To maintain system integrity, data transfer sequences to and from a
virtual system console are limited to a maximum of 2032 bytes.
Channel programs containing data transfer sequences that violate
this restriction are terminated with an interrupt whose CSW status
indicates incorrect length and a channel program check.
1. A data transfer sequence is defined as one or more read or
write CCws connected via chain data. The introduction of
command chaining defines the start of a new data transfer
sequence. Data chain seek CCWs with counts of less than four
are inconsistent with data security of VM/370 and therefore
will give an inconsitent error.
2. Data chained seek CCWs with counts of less than four are
inconsistent with the data security of VM/370 and therefore
will give an inconsistent error when attempting to use.
15. When an I/O error occurs on a device, the System/370 hardware
maintains a contingent connection for that device until a SENSE channel command is executed and sense data is recorded. That is, no
other I/O activity can occur on the device during this time. Under VM/370, the contingent connection is maintained until the SENSE command is executed, but 110 activity frOm other virtual machines can begin on the device while the sense data is being reflected to
the virtual machine. Therefore, the user should be aware that on a
shared disk, the access mechanism may have moved during this time.
16. The mode setting for 7-track tape devices is maintained by the
control unit. Therefore, when a virtual machine issues the SET MODE channel command to a 7-track tape device, it changes the mode
setting of all 7-track tape devices attached to that control unit.
This has no effect on virtual machines (such as OS or DOS) that
issue SET MODE each time a CCW string is to be executed. However,
it can cause a problem if a virtual machine fails to issue a SET MODE with each CCW string executed. Another virtual machine may
change the mode setting for another device on the same control
unit, thereby changing the mode setting of all 7-track tape devices attached to that control unit. 17. OS/VS2 is supported in uniprocessor mode only.
1 A • A shared system or one that uses discontiquous saved segments
cannot be loaded (via IPL) into a virtual machine running in the
virtual=real area.
Appendix F: VM/370 Restrictions 437