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

1Q. The DUMMY feature for VSAM data sets is not supported and should
not be used at program execution time. Specifying this option on
the DLBL command will cause an execution-time OPEN error 20. The 3066 is supported as a 3215. It is not supported as a graphics
editor; therefore, it is recommended that the NODISP option of the
EDIT command be used when editing in a 3066. 21. The Program Controlled Interruption (PCI) FETCH option for load
module retrieval is not supported for OS/MFT or VS1. MSS Restrictions
1. There are two os/vs system data sets associated with Mass Storage System: The mass storage volume inventory and the mass storage
volume control journal. There is one copy of each data set per Mass storage System, not necessarily one per opera ting system. If more than one oS/vs on either native or in a virtual machine) is connected to a common Mass Storage System, then
the OS/VS systems must share a common inventory and journal.
2. When a real 3330V device is dedicated to a virtual machine as a
virtual 3330V, the prbgramming support in the virtual machine must
recognize and access the virtual device as a 3330V. 3. The following must be compatible; the definition of 3330V addresses
in the MCS tables; the DMKRIO module; and the IOGEN for any as/vs
system running in a virtual machine with a dedicated MSC port. The
reason for this, and the way to ensure it, is explained in the 4. Fach active volume in the MSS must have a unique volume number. If
you wish to have two or more user volumes having the same volume
serial (such as different versions of an OS/VS2 system residence
volume both having a volume serial of VS2037), then create two MSS volumes having different volume serials and allocate the user
volumes as minidisks.
5. Mass Storage System volumes may not be used for VM/370 residence,
paging, spooling, or temporary disk space.
6. You must not change the volume of a real 3330V volume (the volume
serial as known by the MSC) except by using the OS/VS access method
services utilities. If, for example, cylinder 0 of a 3330V is
dedicated to a virtual machine and that virtual machine alters the
volume serial using DDR, then the volume cannot be mounted. eMS Restrictions
The following restrictions apply to CMS, the conversational subsystem of VMI 3
7 0: 1. eMS executes only on a virtual IBM System/370 provided by VM/370. 438 IBM VM/370 Planning and System Generation Guide