Introduced in the 1977 ANSI M[UMPS] language standard.
The value of this variable identifies the current input/output device.
NEW %IO SET %IO=$IO
would make it possible to stack the usage of a device-specifier.
Note that common practice like:
FOR ... USE ONEDEV DO SUB1 USE OTHERDEV DO SUB2 ...
is a situation where stacking of device-specifiers makes little or no sense.
The initial value of $IO is equal to the value of $PRINCIPAL if the implementation executes implicit OPEN and USE commands while creating a job, so that the initial current device is indeed the device identified by the value of $PRINCIPAL. Otherwise, the initial value of $IO is the empty string.
This document is © Ed de Moel, 1995-2005.
It is part of a book by Ed de Moel that is published under the title "M[UMPS] by Example" (ISBN 0-918118-42-5).
Printed copies of the book are no longer available.
This document describes the various special variables that are defined in the M[UMPS] language standard (ANSI X11.1, ISO 11756).
The information in this document is NOT authoritative
and subject to be modified at any moment.
Please consult the appropriate (draft) language standard for an authoritative definition.
In this document, information is included that will
The MDC cannot guarantee that these 'next' standards will indeed appear.