When loading a Vs1 virtual machine that has the handshaking feature,
the VS1 initialization routines determine whether the handshaking
feature is available. VS1 checks for V8/370 by issuing an STIDP (store processor ID) instruction. When STIDP returns the version code X'FF', VS1 is running under V8/370. On receiving version code X'FF', VS1 issues a DIAGNOSE code X'OO' instruction tc store the V8/370 extended-identification code. If V8/370 returns a code to VS1, VM/370 supports handshaking; otherwise, VM/370 does not support handshaking.
The psuedo page fault portion of handshaking may be turned on and off
with the CP SET PAGEX command. When using the CP IPL command again, PAGEX is turned off.
A page fault is a program interruption that occurs when a page marked
"not in storage" is referred to by an instruction within an active page. V8/370 places the virtual machine operating system referring to the page
in a wait state while the page is brought into real storage. Without handshaking, V8/370 places the entire VS1 virtual machine in page wait
until the needed page is available.
However, with handshaking, a multiprogramming (or multitasking) VS1 virtual machine can dispatch one task while waiting for a page request
tc be answered for another task. V8/370 passes a pseudo page fault
(program interruption X'14') to VS1. When VS1 recognizes the pseudo
page fault, it places only the task waiting for the page-in page wait
and can dispatch any other VS1 task. Thus, when 1S1 uses pseudo page its execution under the control of V8/37C more closely resembles
its execution on a real machine. When a Fage fault occurs for an vs1 virtual machine, VM/370 ensures
(1) that the Fseudo page fault portion of handshaking is active and (2)
that the 151 virtual machine is in 2C mode and enabled for I/O interruptions. Then, V8/370 reflects the page faults to VS1 by: Storing the virtual machine address that caused the page fault at
location X'90' (the translation exception address) Reflecting a Frogram interruption (interruption code X'14') to VS1 Removing the VS1 virtual machine from page and execution wait When VS1 recognizes program interruption code X'14', it places the
associated task in a wait state. VS1 can then dispatch other tasks. When the requested page is available in real storage, VM/370 reflects
the same program interruption to VS1, except that a bit in the
translation exception address field is set to one to indicate ccmFletion. VS1 removes the task from page wait, and the task is then
eligible to be dispatched.
Note: VS1 with handshaking is not designed to run in a machine that is itself running under VM/370. Otherwise,
page fault occurs, the second-level V8/370 system (18/370 dces not recognize the page fault and terminates with a
code. VM/370 virtual
when a pseudo
under VM/370) PRG020 abend Section 4. OS/VS in a Virtual Machine 111
When both VM/310 and VS1 support handshaking, full handshaking results
when VS1 runs in nonpaging mode. However, handshaking does not require
nonpaging mode. When VS1 is run under the control of it
executes in nonpaging mode if: Its virtual storage space is equal to the storage space of the VM/310 virtual machine. Its virtual machine storage space is at least 1 megabyte. VM/VS handshaking is available.
The VS1 virtual machine operator initiates nonpaging mode by issuing the CP SET PAGEX ON command. Provided that the above conditions are satisfied when VS1 is loaded
under control of VM/370, VS1 issues a special message after the IEA160A SPECIFY VIRTUAL STORAGE SIZE message:
IEA788I NON-PAGING MODE OF OS/VS1 UNDER VM/370 When VS1 executes in nonpaging mode, it uses fewer privileged
instructions and avoids duplicate paging. The VS1 nucleus
initialization program (NIP) fixes all VS1 pages to avoid duplicate
paging. !2ig: The VM/310 working set size (the estimated number of real storage pages that a virtual machine needs to execute) may be larger for a VS1 virtual machine in nonpaging mode than for one not in nonpaging mode.
Considerations unique to nonpaging mode are:
An EOS or U response to the VS1 SPECIFY VIRTUAL STORAGE SIZE message
automatically sets the VM/370 virtual storage space equal to the VS1 virtual machine storage space, provided the latter is 1 megabyte or
larger. Storage limits for ncnpaging mode are the same as for VS1 itself: VS1 Release 6 has a 16 megabyte storage limit in nonpaging mode
and an 8 megabyte limit in paging mode (the limit is equal to the
real machine size or configuration as supported by VS1). During IPL, VS1 issues a message that specifies these limits. VS1 Release 5 has a 4 megabyte storage limit and issues a
message to that effect. VS1 Release 4 has a 4 megabyte storage limit, but issues no
message to that effect.
A minimum size VS1 nonpaging mode. It
partitions.
nucleus tends to be
provides more space
more suitable
for problem
112 IBM '5/370 Operating Systems in a Virtual Machine for the program
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