HP 3000 Manuals

Getting Started as an MPE/iX Programmer Programmer's Guide

Control Codes (JCWs) 

A Job Control Word (JCW) is one word (16 bits) of memory used to pass
information between job steps and to determine the status of the previous
job step.  Figure 3-5  shows the structure of a JCW. It is composed of
a 2-bit type and a 14-bit modifier.

The space allowed for JCWs is determined by the space allowed for your
session or job after file equations, temporary files, and data space is
taken.  There is always one system JCW and one user-defined JCW named
CIERROR that is actually defined by Command Interpreter (CI). You can
define as many user-defined JCWs as remaining space allows.

You can use the MPE/iX command :SHOWJCW to see the value of JCWs.  If you
have not defined any JCWs of your own, this shows only the value of the
system JCW and CIERROR.


          Figure 3-5.  Job Control Word (JCW) Structure 

System JCW 

The system JCW is set by MPE/iX and some subsystems.  You can set the
system JCW programmatically by using intrinsics or interactively at the
Command Interpreter (CI) level by using the :SETJCW command.

MPE/iX checks the system JCW before each step (process) in a session or
job is executed.  Based on the system JCW value, MPE/iX may abort the
session or job.  You can interactively check the system JCW at the CI
level by using the :SHOWJCW command.  You can programmatically check by
intrinsics (calling :FINDJCW) or by using an :IF/THEN, :ELSE, :ENDIF
construct.

The system JCW type provides information on the severity of an error.
The modifier can be set to identify the cause or to pass information to
subsequent job steps.  However, information placed there by the user may
not be preserved, because MPE/iX can change it.  Thus it is frequently
expedient to place information that you want preserved in a user-defined
JCW instead of in the system JCW. When set by the user, the modifier may
be modified by MPE/iX. The types are the bit settings given below,
accompanied by their keywords, and descriptions.

00:OK      No error occurred in the previous step.

01:WARN    An unusual event occurred, but not necessarily fatal.

10:FATAL   Program aborts, under its own control.

11:SYSTEM  System aborts user process due to a problem outside
               the process's direct control.

The modifier can be set to any number from 0 to 65535, inclusive.  These
numbers are divided up for use by each of the four types given above:

          OK      0-16383
          WARN    16384-32767
          FATAL   32768-49151
          SYSTEM  49152-65535

JCW Notation 

A JCW value can be described with three kinds of notation:

   *   A keyword, followed by a constant.  The constant is the decimal
       equivalent of the octal number in the modifier.

   *   Modifier appended to the type to form one constant.

   *   The numerical value of the modifier.

Figure 3-6  shows examples of JCW notations.


          Figure 3-6.  JCW Notation Examples 

Using a System JCW 

The following job shows an example of using a system JCW:

        :JOB STUDENT.INTRO/PASSWORD
        :CONTINUE
        :FTNXL ABC
        :IF   JCW<FATAL THEN
        :     LINK $OLDPASS,ABCPROG
        :     SAVE ABCPROG
        :     RUN ABCPROG
        :ELSE
        :     SAVE $OLDPASS,ABCOBJ
        :ENDIF
        :SHOWJCW
        :EOJ

This job compiles an HP FORTRAN 77/iX program.  If it works, you want to
link the object file to your program file and run the program.  If a
problem occurs during the compile, you want to save the file and end the
job.

If an error occurs, HP FORTRAN 77/iX sets JCW = FATAL, so the job tests
the value of the JCW against the value FATAL (FATAL is the same as FATAL0
or 32768).  The :CONTINUE command causes the job to proceed even if the

next step produces an error.  The :IF/THEN, :ELSE, :ENDIF structure
allows you to make a decision based on the JCW value following the :FTNXL
job step.  The system-defined JCW that CI creates is called CIERROR. CI
sets the JCW modifier to the last CI error number.  Sometimes, CI aborts
a job based on the value of the CIERROR modifier.

User-defined JCWs 

User-defined JCWs allow you to supply input to programs by using Command
Interpreter (CI) and get status from programs that can be examined in CI.
Characteristics of user-defined JCWs include:

   *   Available to each process in the same session or job.

   *   Have names different from JCW or CIERROR.

   *   Are created and set interactively with :SETJCW or programmatically
       with the PUTJCW intrinsic.

   *   Can be examined interactively with :SHOWJCW or programmatically
       with the FINDJCW intrinsic.

MPE/iX does not examine user-defined JCWs.  The value of a user-defined
JCW has meaning only to the user.  Keywords and definitions assigned to
the type and modifier can be identical to those assigned to system JCWs
or different.  You determine what action to take based on your own
definitions.

Using a User-defined JCW 

The following job shows an example of using a user-defined JCW.

          :JOB STUDENT.INTRO/PASSWORD
          :SETJCW UPDATE=OK
          :RUN UPDATEDB
          :IF UPDATE=OK THEN
          :    RUN REPORT
          :ELSE
          :    SHOWJCW
          :    TELLOP REPORT NOT RUN
          :    TELLOP TOO MANY INPUT ERRORS
          :ENDIF
          :EOJ

In this example, the job stream begins by defining and setting a
user-defined JCW called UPDATE to the value OK ( OK is the same as OK0 or
0).  Running the program named UPDATEDB edits a database.  If too many
errors occur, UPDATEDB changes the value of UPDATE by using the PUTJCW
intrinsic.  The :IF/THEN, :ELSE structure tests the value of UPDATE. If
the program is successful (in other words, if there are not too many
errors), it runs another program called REPORT. If too many errors did
occur in the UPDATEDB program, the job shows the JCW values, sends two
messages to the console operator, and ends.

MPE/iX 5.0 Documentation