IBM OS/390 manual Job Control Language JCL Differences and Considerations

Because there is no concept of permanent ASSGN or specification of standard label facilities, all resource requirements for each job step must be included explicitly in each job step. In OS/390, these definitions can be included as procedures which can reduce the repetitive coding of JCL statements (specifically DD statements).

To understand the structure and philosophy of MVS job control language, you must first understand the workflow process for batch jobs. With OS/390, there are separate phases for each of the following:

1Input Service

The job′s JCL and any instream data sets are read by JES and stored on spool as related spool files. No procedures or included JCL statements are referenced or verified at this time.

JES control statements (JECL) are read, validated and attributes are recorded for later processing. (These are converted to JCL comment cards so the subsequent stages do not process them.)

2JCL Conversion

The job control statements are ªconvertedº into structured text units, and most (but not all) syntax checking is performed. This step usually takes place immediately after input service, and any JCL errors result in the job being queued for output processing, bypassing execution, with a JCL error message being sent to the submitter.

3Job Scheduling

After conversion, the job is queued for initiation, and selected by either a JES2 or WLM managed initiator on a priority, class, and first-come,

first-served basis.

4Job and Step Initiation

When the job is selected for initiation, the converter-interpreter text units are read and any data set references are resolved. Any errors such as ªdata set not foundº are determined at this point and the job is flushed - queued for output processing - and the programmer is notified.

Once the interpreted control blocks are read into memory, each job step is given control one at a time based on the conditional JCL processing options. At step initiation time, all data sets referenced by JCL statements are allocated. This is unlike VSE when data sets are allocated when they are opened.

5Output and Hardcopy Processing

There are actually two steps here with JES2. When the job finishes execution, the output created on JES spool is grouped into output elements according to destination and similar attributes, and queued for hardcopy processing.

These output elements are then individually scheduled for printing, punching, online viewing, or transmission to another node.

6Purge Processing

After all the output for a job is disposed of, the job is deleted from the JES queues and the spool space is freed up.

Chapter 4. Job Control Language (JCL) Differences and Considerations 71