April 1, 1981 CMSSEG USAGE OPTIONS At system
segment.
contained
Gu generation time, you may choose not to generate the CMSSEG If you choose to use it, information on how to generate it is
in the publication VM/370 Plan!!!M: gnd Once generated, users also have the option of choosing whether or not
to use CMSSEG. The IPL command provides the facility to either use or
not use CMSSEG. When you IPL your CMS virtual machine, you can request
that CMSSEG be used by specifying IPL CMS PARM SEG=CMSSEG SEG=CMMSEG is the default option.
To request that CMSSEG not be used for your virtual machine, specify
an invalid segment name in the 1PL command, for example, IPL CMS PARM SEG=DUMMY When the CMSSEG segment is not loaded, the routines that perform the
Editor, EXEC, and OS simulation functions execute in the CMS user area.
The SET command also can be used to control the use of the CMSSEG segment. If the CMS system is loaded (via IPL) with an invalid CMSSEG segment name specified, DMSSVT is loaded in the CMS user area to provide
support for OS simulation routines. In this case, the Editor and EXEC modules must be available on the S-disk or another accessed disk, in
this case.
In addition, for the CMSSEG segment only, you can indicate an
alternate segment to be loaded on the IPL command. The format of the IPL command to support this is: 1PL cuu PARM SEG=segmentname
systemname
SEG=seqmentname indicates the name of the saved segment to be loaded
whenever the CMS Editor, EXEC processor, or OS simulation routines
are needed. Eight characters must be entered for segmentname;
either assign an 8-character segment name when you code the NAMESYS macro for your installation, or be sure that the operator enters
trailing blanks if segmentname is less than 8 characters long.
The CMS Batch Facility loads whatever segment is specified
first 1PL command issued for the batch virtual machine. Thus,
first IPL command for a CMS Batch Facility machine is: IPL CMS PARM SEG=CMSSEG02 the same segment name (CMSSEG02) is loaded.
on the
if the When the command "SET SYSNAME CKSSEG segmentname" is specified (where
seqmentname is the name of a defined CMS discontiguous saved segment) ,
free storage containing OS simulation routines is released and OS routines contained in the CKSSEG segment are used by the
virtual machine to provide OS simUlation fUnctions.
314 IBM VM/3 7 0 System Programmer's Guide

Saved System Restrictions for eMS There are several coding restrictions that must be imposed on CMS if it
is to run as a saved system. CMS may never modify, with a single machine instruction (except MYCL), a section of storage that crosses the boundary between two pages
with different storage keys. This restriction applies not only to SS instructions, such as MVC and ZAP, but also to as instructions, such as STM, and to RX instructions, such as ST and STD, which may have
nonaligned addresses on the System/370. It also applies to I/O instructions. If the key specified in the CCW is zero, then the data area for input may not cross the boundary between
two pages with different storage keys.
If you intend to modify a shared CMS system, be sure that all code
that 1S to be shared in the shared segment, CMS Nucleus (X'10000'-X'20000'). To make room for additional code in the eMS Nucleus, you may have to move some of the existing code. You can use the USERSECT area of DMSNUC to contain nonshared instructions. CP does not permit a user of a shared system to set storage keys via
the Set Storage Key (SSK) instruction. Thus, one user cannot prevent
other users from accessing shared storage. Part 3. Conversational Monitor System (eMS) 315