as the entire 3330/3340/3350, a
that the track 3330/3340/3350 to
write record R(O) flag is set on.
due to an invalid
device) or nondedicated. For a dedicated
write R(O) is allowed, but the user must be aware
descriptor record may not be valid from one
another. For a nondedicated 3330/3340/3350, a
is replaced by a read record R(O) and the skip
This could result in a command reject condition
command sequence. 10. When performing DASD I/O, if the record field of a search ID
argument is zero when a virtual Start I/O is issued, but the search
ID argument is dynamically read by the channel program before the
search ID CCW is executed, then the real search ID uses. the
relocated search argument instead of the argument that was read
dynamically. To avoid this problem, the record field of a search
ID argument should not be set to binary zero if the search argument 1S be dynamically read or if a search ID on record 0 is not
intended.
Timing Dependencies
Timing dependencies in input/output devices or programming do not
function consistently under VM/370: 1. The following telecommunication access methods (or the designated
option) violate the restriction on timing dependency by using
program-controlled interrupt techniques and/or the restriction on
dynamically modified channel programs: • as Basic Telecommunications Access Method (BTAM) with the
dynamic buffering option. • as Queued Telecommunications Access Method (QTAM). • DOS Queued Telecommunications Access Method (QT!M). • as Telecommunications Access Method (TCAM). • OS/VS Telecommunications Access Method (TCAM) earlier, and Level 5 if TCAM is not generated or
the VM/370 option.
Level 4 or
invoked with
These access methods may run in a virtual=real machine with CCW translation suppressed by the SET NOTRANS ON command. Even if SET NOTRANS ON is issued, CCW translation will take place if one of the
following conditions is in effect: • The channel program is directed at a nondedicated device (such
as a spooled unit record device, a virtual CTC!, a minidisk, or
a console). • The channel program starts with a SENSE operation code. • The channel program is for a dialed terminal invoked by the DIAL
command. • START I/O tracing is in effect. • The CAW is in page zero or beyond the end of the virtual=real
area.
Appendix F: VM/370 Restrictions 433

(OS BTAM can be generated without dynamic buffering, in which case
no virtual machine execution violations occur. However, the BTAM reset poll macro will not execute under VM/370 if issued from third
level storage. For example, a reset poll macro has a NOP effect if
executed from a virtual=virtual storage under VS1 which is running
under VM/370.) 2. Programming that makes use of the PCI channel interrupt for channel
program modification or processor signalling must be written so
that processing can continue normally if the PCI is not recognized
until I/O completion or if the modifications performed are not
executed by the channel.
3. Devices that expect a response to an interrupt within a fixed
period of time may not function correctly because of execution
delays caused by normal VM/370 system processing. An example of
such a device is the IBM 1419 Magnetic Character Reader. 4. The operation of a virtual block multiplexer channel is timing appears available to the virtual machine operating system, and channel available interrupts
are not observed. However, operations on virtual block-multiplexing
devices should use the available features like Rotational Position Sensing to enhance utilization of the real channels.
Processor Model-Dependent Functions On the System/370 Model 158 only, the
cannot operate concurrently with the (17117) • virtual machine assist feature 7070/7074 compatibility feature
Programs written for
execute properly in the
points should be noted:
processor model-dependent functions may not
virtual machine under VM/370. The following
1. Programs written to examine the machine logout area do not have
meaningful data since VM/370 does not reflect the machine logout
data to a virtual machine.
2. Programs written to obtain processor identification (via the Store CPUID instruction, STIDP) receive the real machine value. When the STIDP instruction is issued by a virtual machine, the version code
contains the value 255 in hexadecimal ("FF") to represent a virtual
machine.
3. No simulation of other processor models is attempted by VK/370. 4. Since an operating system's channel error recovery procedures may
be processor model-and channel model-dependent, operating systems
that will run in a virtual machine may have to be generated for the
same model of processor that VM/370 will be running on. Channel Model-Dependent Functions
Channel checks (channel data check, channel control check and interface
control check) no longer cause the virtual machine to be reset. They
are reflected to the virtual machine as other I/O errors are. This
provides the operating system or other programs in the virtual machine
with the opportunity to attempt recovery or close out its operation in
an orderly manner. To take full advantage of this the virtual machine
should comply with the following requirement: 434 IBM VM/370 Planning and System Generation Guide