GENCSEG is a
utility program for VM and CMS users that installs LOADLIB members in a Discontiguous
Saved Segment (DCSS), hereafter referred to as a segment. Once
installed, these members may be dynamically loaded by using the
addsrch
and
loadm
functions.
There are several reasons for using the GENCSEG utility.
For example, programs that reside in a segment can be attached outside of
the virtual machine's address range and, therefore, do not occupy memory within
the virtual machine. Also, several users can share a copy of the segment.
Further, since dynamic loading from a segment uses CP to fetch load modules,
this type of installation is useful for programs that rely heavily on dynamic
loading.
The GENCSEG utility operates as follows. First, GENCSEG
accumulates a list of LOADLIB members. The next step is to determine the
address range of the segment. Next, GENCSEG allocates a directory space and
then begins to load the members in sequential order. As the members are loaded,
their names are added to the directory. By default, the members are added
sequentially, but GENCSEG provides parameters to specify load order, alignment,
and spacing. The load address for the first member always begins immediately
following the directory. The load address is then updated according to requested
alignment and other options.
The
loadm
function is used
to dynamically load external load modules. Under
CMS,
loadm
searches for the load module in several locations. The search
is controlled by the
addsrch
function, which defines a set of locations
where dynamically loaded modules can be found. The CMS_DCSS argument to
addsrch
specifies that modules can be found in a segment that has been created
by GENCSEG.
A user
with CP class E privileges can create and maintain new
segments. Once a segment is defined or available, you can use GENCSEG to install
members of one or more LOADLIBs into a single segment. The installation can
be controlled by GENCSEG parameters that specify which members are to be installed,
the names of the LOADLIBs to be used, and the order in which the members are
to be installed.
GENCSEG always loads the members in your virtual machine,
creating an image of the segment to be saved. This means that GENCSEG must
be invoked in a virtual machine that has enough virtual storage defined to
contain the entire segment, plus the memory required by GENCSEG to operate
and the memory required by CMS. Before installing any members, GENCSEG initializes
the segment image by writing binary 0s into the entire image area.
GENCSEG always assumes that the segment is designated
as shared and protected under VM/SP. In a VM extended architecture system,
this implies that the SR parameter is used in the DEFSEG command. After the
image area has been set to 0, GENCSEG performs the equivalent of the SETKEY
command, setting all storage keys in the segment image to key 0. This ensures
that the code and data in the segment do not become corrupted during use.
Since the code and data will reside in protected memory, the code must be
reentrant. Non-reentrant code in a shared segment causes protection exceptions
when executed. Use the SAS/C Compiler
option
rent
to allow reentrant modification of static and external variables.
See Compiler Options.
The
loadm
function searches for modules in a segment
by reading a directory in the segment itself. This directory is created by
GENCSEG during the installation process and begins at the first address in
the segment. There is one entry in the directory for each member or alias
installed in the segment.
GENCSEG saves the segment name and the current date
and time in the first 16 bytes of the directory. The directory is terminated
by an entry containing all binary 1s. Each directory entry is four words
long. The first two words contain the name of the member or alias. The third
word contains the address of the module entry point. The fourth word is all
binary 0s and is reserved for use by the library.
The GENCSEG load parameters LOADLIB, ALIGN, PAGE, and
SPACE control the installation process. These parameters are processed from
left to right in the order they are entered on the command line. Upon invocation,
GENCSEG initializes the segment and then begins processing the load parameters.
As members are loaded, GENCSEG maintains the value of
the next available address in the segment. This value, called the load address or loadaddr, has an initial value equal to
the first address following the segment directory. After each member is loaded, loadaddr is incremented by the size of the member. The ALIGN,
PAGE, and SPACE parameters can be used to modify the value of loadaddr. However, loadaddr cannot
be decremented, and at no time can the value of loadaddr
become larger than the last address defined in the segment. If this occurs,
GENCSEG terminates with a diagnostic message, and the segment is not saved.
GENCSEG installs a LOADLIB member by relocating the
code and data based on loadaddr and creating a segment directory
entry for the member. This process is repeated until all the members specified
are installed. GENCSEG then issues a SAVESYS command for the segment.
As the installation proceeds, GENCSEG types a variety
of diagnostic messages to the terminal. These messages report on the parameters
being used, the state of the virtual machine, where the members are being
loaded, and how many pages are used. GENCSEG also issues diagnostics when
something unexpected occurs or when it is unable to continue the installation.
When all of the load parameters are processed (and no
errors have occurred), GENCSEG invokes the CP SAVESYS (under VM/SP) or SAVESEG
(under VM/XA or VM/ESA) command to save the segment image into DASD storage.
The SAVESYS command must be issued from a userid with a CP privilege class
of E. If the userid does not have CP class E privileges, the command fails
and the segment is not saved.
Copyright © 2001
by SAS Institute Inc., Cary, NC, USA. All rights reserved.
