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From: jcmorris@mbunix.mitre.org (Joseph C. Morris)
Newsgroups: comp.lang.asm370
Subject: Re: Question on IBM terminology
Message-ID: <64363@linus.UUCP>
Date: 16 Aug 89 20:55:38 GMT
References: <12734@brunix.UUCP>
Sender: news@linus.UUCP
Reply-To: jcmorris@mbunix (Morris)
Organization: The MITRE Corporation, McLean, VA.
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In article <12734@brunix.UUCP> rac@cs.brown.edu (Robert A. Chekaluk) writes:
>
>Does anyone know the origin of the IBM term "vary," meaning to attach
>or detach a device ("Vary the drive on," "Vary the device off") ??

Plain English: you're telling the system to VARY the status of the device.
The MVS command doesn't refer to attaching or detaching the device.
The original (OS/360 PCP) command had only two operands: ONLINE and OFFLINE.
To bring a device online, you entered:

     VARY cuu,ONLINE

and so forth.  Over the years the VARY command has accumulated a number of
additional uses, including the control of TP lines; it also generates an
artificial device end interrupt for tape and DASD units when ONLINE is 
specified.  

The command was ported to VM (some would say that it contaminated VM)
when IBM began supporting VTAM in the VM world.  Since the concept of
attaching and detaching devices exists under VM, there is a relationship
between attaching a device to a virtual machine and, if that machine is
an MVS intruder or the VTAM robot, the VARY command could be used to tell
the vm that it had a new device to play with.