The master scheduler subsystem establishes communication between the operating system and the primary job entry subsystem, which can be JES2 or JES3.

To do its work, the master scheduler subsystem uses an MSTJCLxx member, commonly called master JCL, which contains data definition (DD) statements for all system input and output data sets that are needed to do the communication between the operating system and JES.

An initial MSTJCL00 load module can be found in the SYS1.LINKLIB library. As shipped, MSTJCL00 contains an IEFPDSI DD statement that defines the data set that contains procedure source JCL for started tasks. Normally this data set is SYS1.PROCLIB; it may be a concatenation. For useful work to be performed, SYS1.PROCLIB must at least contain the procedure for the primary JES.

If modifications are required, the recommended procedure is to create an MSTJCLxx member in the PARMLIB data set. The suffix is specified by the MSTRJCL parameter in the IEASYSxx member of PARMLIB.

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EDIT      SYS1.PARMLIB(MSTJCL00) - 01.07                  Columns 00001 00072
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000100 //MSTRJCL  JOB MSGLEVEL=(1,1),TIME=1440
000200 //         EXEC PGM=IEEMB860,DPRTY=(15,15)
000300 //STCINRDR DD SYSOUT=(A,INTRDR)
000400 //TSOINRDR DD SYSOUT=(A,INTRDR)
000500 //IEFPDSI  DD DSN=SYS1.PROCLIB,DISP=SHR
000600 //         DD DSN=CPAC.PROCLIB,DISP=SHR
000700 //         DD DSN=SYS1.IBM.PROCLIB,DISP=SHR
000800 //IEFJOBS  DD DSN=SYS1.STCJOBS,DISP=SHR
000900 //SYSUADS  DD DSN=SYS1.UADS,DISP=SHR
********************************** Bottom of Data ****************************

When the master scheduler has to process the start of a started task, the system determines whether the START command refers to a procedure or to a job. If the IEFJOBS DD exists in the MSTJCLxx member, the system searches the IEFJOBS DD concatenation for the member requested in the START command.

If there is no member by that name in the IEFJOBS concatenation, or if the IEFJOBS concatenation does not exist, the system searches the IEFPDSI DD for the member requested in the START command. If a member is found, the system examines the first record for a valid JOB statement and, if one exists, uses the member as the JCL source for the started task. If the member does not have a valid JOB statement in its first record, the system assumes that the source JCL is a procedure and creates JCL to invoke the procedure. After the JCL source has been created (or found), the system processes the JCL.

As shipped, MSTJCL00 contains an IEFPDSI DD statement that defines the data set that contains procedure source JCL for started tasks. Normally this data set is SYS1.PROCLIB; it may be a concatenation. For useful work to be performed, SYS1.PROCLIB must at least contain the procedure for the primary JES.