Each virtual channel should map to real channels of a single type.
In other words, the virtual devices on a virtual channel should all
map to real devices on real channels of a single type and model.
These real channels should all be the same as each other, but not
necessarily the same as the virtual channel.
If the I/O configuration of a virtual machine does not meet the above
requirement, no warning message is issued and the virtual machine will run successfully until a channel check occurs. In this case, when a
channel check occurs, there is a possibility that the channel extended
logout data may be inconsistent with the data provided by the store
channel id (STIDC) instruction. Virtual machines running CMS do not need to comply with these
requirements. Here, only unit record spooling and diagnose 110 are performed; For unit record spooling there are no channel checks and for
diagnose I/O, CP attempts to perform the error recovery itself. When the store channel id instruction (STIDC) is executed in a
virtual machine, it returns information from an arbitrary channel, one
of several the specified virtual channel may map to. The type, model,
and logout length data returned by the STIDC are the same as the real
channel except that when a real channel is a block multiplexer and the
virtual channel is a selector, the type field returned by STIDC
indicates a selector channel.
Since the STIDC returns identifying data from
channel model-dependent error recovery procedures
identify the channel.
the real channel,
can use STIDC to
Channel extended logouts are reflected to the virtual machine in a
manner that is processor model-and channel model-dependent and
consistent with the data returned by STIDC (provided that the
virtual-to-real channel mapping complies with the requirement stated
previously) .
A deviation in the handling of channel extended logouts occurs if the
virtual machine uses the bit in control register 14 to mask out channel extended lcgcuts. In a virtual machine, any channel extended logouts
that are masked out by control register 14 are lost rather than kept
pending, and the logout pending bit (bit 5) in the CSi is never set.
However, channel extended logouts will not be lost when they are kept
pending along with their associated 1/0 interrupts by the channel masks
in control register 2 and the PSi. Regardless of whether or not the
setting of the virtual machine's control register 14 causes it to lose
the channel extended logout, CP will still successfully record the
logout in its own error recording cylinders. Virtual Machine Characteristics
other characteristics that exist for a virtual machine under are
as follows:
1. If the virtual=real option is selected for a virtual machine,
input/output operations specifying data transfer into or out of the
virtual machine's page zero, or into or out of storage locations
whose addresses are greater than the storage allocated by the
virtual=real option, must not occur. The storage-protect-key
mechanism of the IBM System/370 and channels operates in
Appendix F: VM/370 Restrictions 435

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