Characteristics of the IBM-Supplied External Writer

The external writer has the following features:

It runs as a started task, in its own address space, in 24-bit addressing mode.
It processes only those data sets that meet its selection criteria. You set some of these in a cataloged procedure (see The External Writer Cataloged Procedure) but you can override them with the START and/or the MODIFY operator commands (see z/OS MVS System Commands for the general description of these commands).
If you do not specify data set selection criteria, the external writer will be used to process all the output in the system.
It removes data sets from the JES spool. That is, it dynamically allocates the data sets, reads them, writes them to output devices, and then dynamically deallocates them. A large format spool data set can be shared.
It supports basic format, extended format and large format sequential data sets on DASD.
It supports checkpoint and spool on 3390 Model 9 and Enterprise Storage Server (ESS).
It accesses data sets according to one or more of the following criteria:
- Output class
- Job ID
- Forms specification
- Destination (LOCAL, or remote workstation name)
- The name of your output writing routine
The usual technique however, for setting data set selection criteria, is to build a list of eligible SYSOUT classes for the devices that will use the external writer.
If no class is specified, then we will not start selecting any output and will wait for a MODIFY command to be issued. For example, if you have the following in process:
```
//IEFPROC  EXEC  PGM=IASXWR00,PARM='P',REGION=20K
```
No class is specified. An S XWTR will cause the external writer to be started, but idle. Then, a F XWTR,F=ABC may be specified to have the external writer select by form type. If another F XWTR,C= is issued, then everything on the output queue will be selected because a null class is specified.