The primary function of the prescan phase is to extract each job's resource requirements from the SWA control blocks created in the interpreter phase.

Installation exits IATUX04, IATUX05, and IATUX06 allow you to examine or change job, step, and DD information, respectively. You can use these exits to examine or change the information before processing begins.

During the prescan phase, the JCL for the job is examined for PGM=JCLTEST or PGM=JSTTEST. If PGM=JCLTEST is found on an EXEC statement, the JCL is interpreted and the job is then express canceled on completion of the CI DSP. If PGM=JSTTEST is found on an EXEC statement, the job is processed through the prescan and postscan phases, a printed format of the job summary table (JST) is printed on the JESYSMSG data set, and the job is then canceled-with-print on completion of the CI DSP. For more information on JCLTEST and JSTTEST, see z/OS JES3 Diagnosis.

If the job is being processed in a C/I FSS, JES3 determines whether the job is eligible to have catalog searches performed on the main where the C/I FSS is executing. If catalog searches can be performed on that main, locate processing occurs in the C/I FSS and control then returns to the JES3 global address space for the remainder of postscan processing.