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COBOL Tutorial

COBOL UNIT - Quick Reference

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Overview

UNIT is a system/JCL concept indicating a device class or number. COBOL files are typically assigned via DDNAMEs, with UNIT specified in JCL for physical allocation.

Common Patterns

  • Device class - UNIT=SYSDA, UNIT=TAPE
  • COBOL mapping - SELECT ASSIGN TO ddname; JCL DD binds device
  • Portability - Keep device specifics outside COBOL source

Syntax and Usage

COBOL File Assignment

cobol
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* COBOL file assigned to a DDNAME
       ENVIRONMENT DIVISION.
       INPUT-OUTPUT SECTION.
       FILE-CONTROL.
           SELECT INFILE ASSIGN TO "INFILE".
       DATA DIVISION.
       FILE SECTION.
       FD  INFILE.
       01  IN-REC PIC X(80).
       PROCEDURE DIVISION.
           OPEN INPUT INFILE
           READ INFILE AT END DISPLAY "EOF" END-READ
           CLOSE INFILE
           STOP RUN.

The JCL DD named INFILE will control device allocation (UNIT, SPACE, etc.).

JCL UNIT Parameter

jcl
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//* JCL example
//STEP1    EXEC PGM=COBPROG
//INFILE   DD  DSN=MY.DATASET,DISP=SHR,UNIT=SYSDA
//OUTFILE  DD  DSN=*.TEMP,DISP=(,CATLG),SPACE=(CYL,(5,5)),UNIT=SYSDA

UNIT selects the device class; SMS may override explicit UNIT based on policies.

Best Practices

  • Abstraction - Keep device specifics in JCL or deployment configs
  • Standards - Follow site allocation rules and SMS classes
  • Testing - Validate on the target system with real allocations

UNIT Quick Reference

AspectDescriptionExample
Device classUNIT= parameterUNIT=SYSDA
DD mappingCOBOL ASSIGN TO DDASSIGN TO "INFILE"
PortabilityKeep in JCLDD with UNIT set

Test Your Knowledge

1. What does UNIT typically refer to in mainframe contexts?

  • A COBOL arithmetic verb
  • A device class or device number used by the operating system/JCL
  • A COBOL intrinsic function
  • A screen control command

2. How does COBOL usually connect to a device/unit?

  • Directly specifying UNIT in the ENVIRONMENT DIVISION
  • Through file assignment (SELECT ASSIGN TO DDNAME) with JCL controlling UNIT
  • Using PERFORM UNIT
  • By setting UNIT in the IDENTIFICATION DIVISION

3. Which JCL parameter commonly controls device allocation?

  • UNIT=
  • CLASS=
  • REGION=
  • PARM=

4. What is a best practice for portability regarding UNIT?

  • Hard-code physical devices in the COBOL source
  • Use DDNAMEs in COBOL and configure UNIT in JCL or environment
  • Always use tape devices
  • Avoid JCL entirely

5. What happens if UNIT is omitted in JCL?

  • The COBOL runtime fails immediately
  • The system selects defaults based on installation standards
  • The program switches to screen mode
  • The dataset is always temporary

Frequently Asked Questions

Related Concepts

File Assignment

SELECT/FD mapping and DD names.

JCL DD Statement

Dataset allocation and parameters.

SMS Classes

Storage/management classes overview.

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