The ORIGIN option of the INCLUDE command is used to cause the
included file to overlay a previously loaded file. The address at which
a phase begins must be a double.ord boundary. For example, if the rcct
phase were X'2BD' bytes long, starting at virtual storage location X'20000', then location xxxxxx would be the next doubleword boundary, or X'202CO'. The STR option, which is specified in the GENMOD of the root phase,
specifies that whenever that module is brought into storage with the LOADMOD command, the Storage Initialization routine should be invoked.
This routine initializes user free storage pointers.
At execution time of the prestructured overlay program, each phase is
brought into storage with the LOADMOD command. The phases can call LOADMOD. The OS macros LINK, LOAD, and ICTL normally invoke the INCLUDE command, which loads TEXT files. These macros will invoke LOADMOD if a
switch, called COMPSWT, in the CMS Nucleus Constant area, NUCON, is
turned on. With COMPSWT set, overlay phases that use LINK, LOAD, and XCTL must be pre structured MODULE files. DYNAMIC LOAD OVERLAY The dynamic load method of using an overlay structure is to have all the
phases in the form of relocatable object code in TEXT files or of a TEXT library, filetype TXTLIB. The OS macros, LINK, LeAD, and XCTL may then be used to pass control from one phase to another. The XCTL macro causes the calling program to be overlayed by the called program
except when it is issued from the root phase. When issued from the root
phase, CMS treats XCTL as it would a LINK macro, adding the new code at
the end of the root phase.
The COMPSWT flag in OSSFLAGS must be off when the dynamic load method
is used.
Part 3. Conversational Monitor System (CMS) 325

326 IBM VM/370 System programmer's Guide