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

7. Segment zero must always have the exclusive access attribute “Ex”. If a
shared (“Sx”) page descriptor code is used for any pages in segment zero, the
command is rejected.
8. A CP DCSS (SC mode) contains CP-writable pages, to which virtual machine
users have just read-only access. CP writes data into these pages, which may
then be referred to by virtual machines. Serialization of data in the CP-writable
pages, if desired, must be arranged between the CP function providing the
data and the virtual machines referring to it.
9. A CP DCSS may not be loaded in nonshared (exclusive) mode by DIAGNOSE
X'64'.
10. Saved segments are loaded through DIAGNOSE X'64'. Named saved systems
(NSS) are loaded through the IPL command.
11. Only one skeleton (class S) DCSS can exist with a specific name. If one
already exists and a second DEFSEG command is issued specifying the same
name, the command is rejected.
12. Each name may be used for only one of the following: a segment space, a
member saved segment, or a DCSS.
13. Defining members that use undefined areas within other members of the same
segment space is not recommended. However, in support of multiple products
requiring both shared and exclusive segment types and to minimize storage
use, definition of members with interleaved ranges is supported.
14. You can use the SAMERANGE operand only with the SPACE operand.
Note: You cannot use the SAMERANGE operand with a dcssname containing
SW, SN, EW, or EN page types.
15. A DEFSEG with a SAMERANGE operand can be the first definition of a
segment space only if the specified member is a class S file.
16. A DEFSEG with a SAMERANGE operand does not need a corresponding
SAVESEG if the member saved segment is already saved. If the member
saved segment is not saved, you need to use the SAVESEG command.
17. A member of a segment space cannot overlap any of the ranges specified for
an existing member within the same segment space. Also, the members
cannot have conflicting descriptor codes for ranges defined within the same
segment of storage.
18. You cannot specify a DEFSEG with the SAMERANGE operand for a segment
space that already has a member saved segment with that name.
19. Putting a member name in a directory statement has no effect; use the
segment space name.
Security Notes:
DEFSEG
1. When RSTD is specified (indicating a restricted saved segment), only those
users whose directory entries contain a NAMESAVE statement specifying the
name of the restricted system data file are allowed to load this saved segment.
If RSTD was specified without the SPACE operand, the directory entries must
contain a NAMESAVE statement specifying the same DCSS name as
dcssname. If RSTD was specified with the SPACE operand, the directory
entries must contain a NAMESAVE statement specifying the segment space
name as spacename. This allows an installation to ensure data integrity by
restricting access to saved segments that contain shared writable storage
(defined with the SW and SN page descriptor codes) or to CP DCSSs (defined
with the SC page descriptor code), which may contain sensitive data.
Chapter 2. CP Commands 209