the full simulation. Once the IPL sequence has completed, CCW translation can be bypassed by issuing the SET NOTRANS ON command. When the virtual machine demands a page frame through normal use of CP's page tables, the paging routine recognizes the virtual=real
capability. It then assigns the virtual page to the equivalent real page frame and does not perform a paging operation, because all these pages
are resident a nd are ne ve r swa pped out.
Note: The virtual machine running with virtual=real is still run in System/310 relocate mode.
Virtual 210x lines and sense operations from the virtual machine do
not use the virtual=real function. These invoke CCW translation for the
virtual enable/disable lines and the transfer of the sense bytes. The UNLOCK command has a VIRT=REiL operand that essentially releases
the virtual=real area for normal system paging use. Once the area has
been released, it can only be reclaimed for additional virtual=real
operations only by an IPL of the VM/310 system. The size of the
virtual=real area is an installation specification that is part of the
special nucleus generation procedure that is outlined in the V!/3IQ Rlanni.D.9 and Syste,! Generati2!! Guide. The size of the area must be
large enough to contain the entire addressing space of whatever virtual
aachine wishes to occupy that spacee A virtual machine can use a smaller
space than is provided but cannot use a larger space without
regenerating the CP nucleus. DASD STORAGE !ANAGE!ENT Any virtual machine's virtual storage pages that have been referenced
but are not resident in real storage must be kept in slots on the DASD paging device. DASD page space is assigned only when the page is
selected for a page-out. Certain DASD pages may also be marked read-only. Thus, the DASD address slot initially associated with the
page should be considered to be the source of the page only. If the
page is changed after it has been read into real storage, a new slot must be obtained when .it is paged out. Examples of read-only pages are those which contain portions of pageable saved systems and pages which
are part of a system spool file. Slots can be reassigned when DMKPTRAN finds that it must swap a page out to a movable-head DASD device. In
this case, the old slot is released and the new slot is obtained.
If a new slot is required, DMKPGT is called to supply the address of an
available slot. DMKPGT maintains a chain of cylinder allocation aaps for
each cylinder that has been assigned for either virtual storage or spool
file paging. The allocation chains for spooling are kept separately
from those used for paging so that they can be check pointed in case of a
system failure. However, in other respects they are the saae. The
allocation blocks for a given volume are chained from the RDEVBLOK for
the device on which the volume is aounted. The chains of cylinder and
slot allocation blocks are initialized by D!KCPI. Each block on an
allocation chain represents one CYlinder of space assigned to paging,
and contains a bit aap indicating which slots have been allocated and
which are available. Each block also has a pointer to the next
allocation block on the chain, a cylinder number, and a record count. D!KPGT searches this list sequentially until an available slot is found; CP Introduction 1-111
its DASD address is then determined and passed back to the calling
routine. If DKKPGT cannot find a cylinder with a de-allocated slot, it
enters the cylinder allocation phase. When an available cylinder is
found, it constructs a page allocation block for this cylinder and
allocates a page to the caller. D!KPGT controls the paging and spooling I/O load of the systea by allocating cylinders evenly across all available channels and devices.
In order for a device to be considered available for the allocation of
paging and spooling space: • Its volume serial number must appear in the system's owned list. • It must have at least one cylinder of temporary space aarked as
available in the cylinder allocation block which is located on
cylinder 0, head 0, record 3. I • It must not be an KSS 3330V volume. At system initialization time, CPINIT reads in the allocation records
for each volume and constructs the chains of device allocation blocks from which D!KPGT allocates the cylinders. In managing the cylinder
allocation, D!KPGT takes three factors into consideration: device type,
device address, and possible status as a preferred paging device. A request for a cylinder of virtual storage page space is satisfied by allocating space on a preferred paging device, provided that one
exists on the system and that it has page space available. Preferred paging devices are specified by the installation at systea generation time, and generally should be devices on which excessive seek times do
not occur. A typical preferred paging device would be the IBM 2305 Pixed Head Storage facility. If the 2305 is assigned as a preferred
device, it is possible to allocate some of its space for other
high-priority data files without excessively degrading paging. An example of such usage would be for high activity read-only saved systea pages that are not shared in real storage, and high-activity systea residence disks.
It is also possible to designate movable-head DASD devices such as
the 3330, 3340, 3350 and 2314/2319 Direct Access storage facilities as
preferred paging devices. The module(s) so designated should not be
required to seek outside of a relatively narrow cylinder band around the
center of the paging areas. It is advisable to share the access ara of
a movable-head preferred paging device with only the lowest-usage data
files.
If one or more preferred devices are defined on the systea, CP allocates all of the page space available space on these before it
allocates on any other available owned volumes. Within the class of
preferred devices, space is allocated first on the fastest devices, and
these are spread out across channels and devices. Allocation on
nonpreferred devices is spread out in the same aanner. cylinders for
spooling space are not allocated from preferred devices. Allocation on
a given device is done from the relative center of the volume outward, a
cylinder at a time in a zig-Zag fashion in an attempt to miniaize seek
tiaes.
1-112 IBM V!/370 System Logic and Problem Deteraination--Voluae 1
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