Editing Job Control Language (section)

==DOS JCL==

===Positional parameters===
<syntaxhighlight lang="jcl">
//TLBL TAPEFIL,'COPYTAPE.JOB',,,,2
//ASSGN SYS005,200
//DLBL DISKFIL,'COPYTAPE.JOB',0,SD
//EXTENT SYS005,VOL01,1,0,800,1600
</syntaxhighlight>

DOS JCL parameters are positional, which makes them harder to read and write, but easier for the system to parse.
* The programmer must remember which item goes in which position in every type of statement.
* If some optional parameters are omitted but later ones are included, the omitted parameters must be represented by commas with no spaces, as in the TLBL statement above.

DOS JCL to some extent mitigates the difficulties of positional parameters by using more statements with fewer parameters than OS JCL. In the example the ASSGN, DLBL and EXTENT statements do the same work (specifying where a new disk file should be stored) as a single <code>DD</code> statement in OS JCL.

===Device dependence===
In the original DOS/360 and in most versions of DOS/VS one had to specify the model number of the device which was to be used for each disk or tape file&mdash;even for existing files and for temporary files which would be deleted at the end of the job. This meant that, if a customer upgraded to more modern equipment, many JCL files had to be changed.

Later members of the DOS/360 family reduced the number of situations in which device model numbers were required.

===Manual file allocation===
DOS/360 originally required the programmer to specify the location and size of all files on [[Direct-access storage device|DASD]].  The <code>EXTENT</code> card specifies the volume on which the extent resides, the starting absolute track, and the number of tracks. For z/VSE a file can have up to 256 extents on different volumes.