z/VM: CP Commands and Utilities Reference - z/VM - IBM

DEFSEG
Usage Notes
208 z/VM: CP Commands and Utilities Reference
The LOADNSHR operand cannot be used with the RSTD operand. It also
cannot be used with the SC page range descriptor code.
SECURE
indicates that the userid that created the DCSS will be the only userid allowed
to SPXTAPE DUMP or SPXTAPE LOAD that DCSS file.
SPACE
specifies the definition of a segment space. Use this operand to make the
dcssname that you are defining a member of this segment space.
spacename
is the name of the segment space (a one- to eight-character alphanumeric
string). You must specify this if you use the SPACE operand. The spacename
must be different from any dcssname known to CP.
1. CP can process only one DEFSEG or SAVESEG command at any given time.
If CP is already processing a DEFSEG or SAVESEG command, for any name,
processing of a subsequent DEFSEG or SAVESEG command is delayed.
2. A saved segment defined at a high storage address that is shared by many
virtual machines may affect real storage availability. For every virtual machine
larger than 32 MB, CP creates a segment table in real storage to represent the
virtual machine storage. Because CP dynamically expands the size of a virtual
machine to incorporate a saved segment loaded at an address outside the
virtual machine, the segment table for the virtual machine also expands. If the
dynamically-expanded virtual machine is larger than 32 MB but less than or
equal to 1024 MB, one real storage frame is allocated for the segment table. If
the dynamically-expanded virtual machine is larger than 1024 MB, two
consecutive real storage frames are allocated for the segment table. CP
creates the segment table at the start of the real storage frame, and any
storage remaining beyond the end of the segment table may be used for CP
free storage. But in the case of the dynamically-expanded virtual machine, the
portion of the segment table between the defined end of the virtual machine
and the start of the saved segment represents real storage that is unavailable
to the system for other uses. However, the virtual machine may load other
saved segments there.
3. Pages either defined as, or defaulted to, the no data saved attribute are
initialized to a page of zeros when first referred to. If the pages are in an
exclusive segment, changes are seen only by the one user. If the pages are in
a shared segment, any changes previously made by any sharing user (if
allowed) are visible to all other users sharing that same page.
4. Storage keys are not saved for pages defined as, or defaulted to, the “no data
saved” attribute. When first referred to, they have keys of zero.
5. A segment space has a range that may span one or more physical megabyte
boundaries. The beginning address of a segment space is rounded down to
the nearest MB boundary of the member having the lowest page value. The
ending address of a segment space is rounded up to the last address of the
last page within the MB occupied by the member having the highest page
value.
6. Each member saved segment has a range that may span one or more pages.
The beginning address of a member saved segment is determined by its
lowest page value. The ending address is determined by the last address of
the highest page value.