Download IBM RT PC Virtual Resource Manager Programming Reference

When page level protection is selected and a protection violation occurs, a program check
interrupt indicating a protection exception will be given to the virtual machine.
Mapped Page Ranges
A mapped page range gives you the capability to define the relationship between the pages
of a segment and logical disk blocks in the minidisk. Once defined, the virtual address
space of a mapped page range is referenced in a normal fashion by the processor. Accessing
the logical disk blocks is accomplished by load, store, and processor instruction fetches. The
paging supervisor manages the transfer of the logical disk blocks to and from main storage.
The virtual machine must attach a minidisk in the normal fashion before creating a
mapped segment on a minidisk. The virtual machine is responsible for the integrity of the
minidisk in terms of conflicting accesses to blocks on the minidisk, whether those accesses
are implicit as a result of mapped segments or explicit as a result of Start I/O SVCs.
The virtual machine must also unmap or destroy the minidisk’s segment before closing the
minidisk.
Three types of mapped page ranges can be defined. They are Read/Write (RW), Write-New
(W), and Copy-on-Write (CW).
RW
References to virtual addresses cause data to be paged-in without explicit disk
reads and buffering. Changes to the data are implicitly written back to the disk
when the underlying pages are paged out. You can explicitly tell VRM to update
the disk by way of a virtual memory SVC.
W
Data that did not previously exist on the disk can be written out by the paging
system without the original contents of the disk blocks being read in first. This
mode can be used to move data from the paging space minidisk (previously
mapped CW) to the virtual machine’s file system minidisk.
CW
This type is similar to RW, except that when data is paged out it is not written
back to the same disk block(s) it originally came from. Instead, it is written to
the VRM page space. You must explicitly preserve this data if you require a
permanent copy.
A read-only page range can be set up as a special case of a read-write page range. To set up
a read-only page, map a page range as RW, then prevent the pages from being written to
with page protection functions. A page range of this type can be used, for example, to map
read-only data and program code.
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VRM Programming