What does ALLOCATE do?

A TSO command that creates a new dataset, or connects an existing one to your session.

ALLOCATE

Creates a new dataset and puts its name in the catalog.

Also written ALLOC

Why it matters. Everything you build starts as an empty dataset. ALLOCATE is how you make one.

See it

Read it left to right

ALLOC: Make a dataset
DSN('…'): Its name, in brackets and quotes
NEW: A new one
DSORG(PO): A library. DSORG(PS) for a single file
DIR(5): Room for the library's list of members
SPACE(1,1) TRACKS: One track to start, one more each time it fills

Syntax

{ALLOCATE | ALLOC} {DATASET | DSNAME}(dsname) [OLD | SHR | MOD | NEW]
ALLOCATE ... [DSORG(PS | PO)] [RECFM(FB)] [LRECL(n)] [BLKSIZE(n)] [SPACE(primary,secondary) TRACKS | CYLINDERS] [DIR(n)] [FILE(ddname)] [LIKE(model-dsname)]

DATASET(dsname): The dataset name. DA and DSN are the short forms. Quote the name. A library member can go in brackets.
NEW: Make a new dataset. Without it, ALLOCATE looks for one that exists (OLD, SHR or MOD).
DSORG(PS | PO): PS is a plain sequential file. PO is a library with members.
SPACE(primary,secondary): How much disk to ask for. Add TRACKS or CYLINDERS to say the unit.
DIR(n): Directory blocks for a library (PO). It is needed unless a data class supplies a default.
RECFM, LRECL, BLKSIZE: Record format, record length and block size. FB with LRECL(80) is usual for JCL and source.
LIKE(model): Copy the attributes of an existing cataloged dataset.
FILE(ddname): The file name (ddname) the dataset is connected to for this session.

Try it ALLOCATE

1
Create a library called ZB0001.DEV.JCL that can hold members.
ALLOC DSN('ZB0001.DEV.JCL') NEW DSORG(PO) DIR(5) SPACE(1,1) TRACKS
2
Now create a plain file called ZB0001.DATA.
ALLOC DSN('ZB0001.DATA') NEW DSORG(PS) SPACE(1,1) TRACKS
3
Run the same command again, to see what happens when the name is taken.
ALLOC DSN('ZB0001.DATA') NEW DSORG(PS) SPACE(1,1) TRACKS

ZedBank practice system

READY

Nothing here is saved

Remember

NEW makes a new dataset. Without it, ALLOCATE looks for one that exists.
DSORG(PS) is one file. DSORG(PO) is a library of members.
Names are unique. A second ALLOCATE of the same name fails.

The usual mistake

ALLOC DSN('ZB0001.DATA') DSORG(PS)

Without NEW, ALLOCATE looks for a dataset that is already there. It does not create one.

ALLOC DSN('ZB0001.DATA') NEW DSORG(PS) SPACE(1,1) TRACKS

On real z/OS

MF/OS shows a 'Mainframe101 sim:' note when a new dataset is made; real TSO prints nothing. The storage-class operands (DATACLAS, MGMTCLAS, STORCLAS) and tape operands are not modelled, so every dataset lands on the simulator's own disk, RES001. IGD17101I is an SMS message, so it appears on systems where SMS manages the dataset.

Where can I practise ALLOCATE?

Right here, at ZedBank. You take a junior job at a simulated bank and use ALLOCATE in real work, with Zed beside you. There's nothing to install.

Start at ZedBank