Job Control Words (JCWs) are a subset of variables. A JCW is an integer variable in the range 0 through 65535, and contains an internal "flag" (set by one of the JCW commands or
intrinsics) that makes it recognizable to the system as a JCW. All the variable intrinsics and commands discussed above can be used to manipulate JCWs; however, the JCW commands and intrinsics will recognize and affect only those variables flagged as JCWs.
It is possible to reclassify JCWs as variables, or vice versa, by using the appropriate intrinsics to change their values. However, not all JCWs and variables can be changed.
The JCWs CIERROR and JCW cannot be reclassified as standard variables, because system processes depend upon the fact that these variables can always be accessed and altered using JCW intrinsics. Also, predefined standard variables such as HPPATH cannot be reclassified as
JCWs.
For a list of predefined variables and JCWs, refer to the MPE/iX Commands Reference Manual, Appendix A.
Using the Predefined JCW for Interprocess Communication |
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Two processes belonging to the same job or session can communicate with each other through the predefined Job Control Word JCW. This word enables a subsystem process to return information to the job or session that initiated the process. This communication mechanism is used by the command executors for :RUN, and by various subsystem commands (for example, compilers). However, you may find this control word helpful in other applications.
Using User-Defined JCWs for Interprocess Communication |
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MPE/iX allows you to establish and manipulate user-defined Job Control Words. This
capability overcomes one of the disadvantages of using the system-defined Job Control Word: MPE/iX uses JCW for status information, and you cannot be sure that MPE/iX will not modify it, thus possibly changing the information you want to pass in JCW. (MPE/iX clears bits (0:2) of JCW, but the remaining bits are user-definable.)
A user-defined JCW is a 16-bit word that resides in the session variable
table managed by MPE/iX. This table, which also holds the
system-defined JCWs, is shared by all processes in a job or session; thus any process of a job can access any JCW in the table.
