PRIORITY The VM/370 operator can assign specific priority values to different
virtual machines. In so dOing, the virtual machine with a higher priority is considered for dispatching before a virtual machine with a
lower priority. User priorities are set by the following class A
command:
SET PRIORITY userid nn
where userid is the user's identification and nn is an integer value
from 1 to 99. The value of nn affects the user's dispatching priority
in relation to other users in the system. The priority value (nn) is
one of the factors considered in VM/370's dispatching algorithm.
Generally, the lower the value of nn, the more favorable the user's
position in relation to other users in VM/370's dispatch queues. RESERVED PAGE FRAMES VM/370 uses chained lists of available and pageable Fages. Pages for
users are assigned from the available list, which is replenished frem
the pageable list. Pages that are temporarily locked in real storage are not available
or pageable. The reserved page function gives a particular virtual
machine an essentially "private" set of pages. The pages are not
locked; they can be swaFped, but only for the specified virtual machine. Paging proceeds using demand paging with a "reference bit" algorithm to
select the best page for swapping. The number of reserved page frames
for the virtual machine is specified as a maximum. The page selection
algorithm selects an available page frame for a reserved user and marks
that page frame "reserved" if the maximum specified for the user has not
been reached. If an available reserved page frame is encountered for the
reserved user selection, it is used whether or not the maximum has been
reached.
The maximum number of reserved page frames is specified by a class A
command of the following format:
SET RESERVE userid xxx
where xxx is the maximum number required. If the page selection
algorithm cannot locate an available page for other users because they
are all reserved, the algorithm forces the use of reserved pages. This
function can be specified in only one virtual machine at anyone time. Note: xxx should never approach the total available pages, since CP
overhead is substantially increased in this situation, and excessive
paging activity is likely to occur in other virtual machines. VIRTUAL=REAL For this option, the VM/370 nucleus must be reorganized to provide an
area in real storage large enough to contain the entire virtual=real machine. In the virtual machine, each page from page 1 to the end is in
its true real storage location; only its page zero is relocated. The Part 2. Control Program (CP) 95

virtual machine is still run in dynamic address translation mode, but
since the virtual page address is the same as the real page address, no ccw translation is required. Since CCW translation is not performed, no
check is made to ensure that I/O data transfer does not occur into page
zero or any page beyond the end of the virtual=real machine's storage. Systems that are generated
system loader (DMKLDOOE). virtual=real system, see the with the virtual=real option use the
For information about generating a Figure 13 is an
virtual=real option.
example of a real storage layout with
The V=R area is 128K and real storage is 512K. Virtual Storage Addresses I CP PAGE 0 (MODULE DMKPSA) 4KI I Virtual Page 1 I I VIRTUAL=REAL AREA
/
/ SIZE = 128K BYTES I (Minimum size is 32K bytes.) 128KI OKI Virtual Page 4K, 132K, / REMAINDER OF CP NUCLEUS / , I I / DYNAMIC PAGING AREA
/ and I FREE STORAGE I I I I I I /
/ , I 0 I I I /
/ I I I /
/ I Real Storage Addresses OK 4K 128K
132K (DI1KSLC) End of CP Nucleus (DMKCPE) 512K (End of real
storage)
Figure 13. Storage Layout in a Virtual=Real Machine the
There are several considerations for the virtual=real option that
affect overall system operation:
1. The area of contiguous storage built for the virtual=real machine
must be large enough to contain the entire addressing space of the
largest virtual=real machine. The virtual=real storage size that a VM/370 system allows is defined during system generation when the
option is selected.
2. The storage reserved for the virtual=real machine can only be used
by a virtual machine with that option specified in the VM/370 directory_ It is not available to other users for paging space, nor
for VM/370 usage until released from virtual=real status by a
system operator via the CP UNLOCK command. Once released, VM/370 must be loaded again before the virtual=real option can become
active again. 96 Systeill