From:	CRDGW2::CRDGW2::MRGATE::"SMTP::CRVAX.SRI.COM::RELAY-INFO-VAX" 28-JUL-1990 02:58:35.78
To:	MRGATE::"ARISIA::EVERHART"
CC:	
Subj:	Re: BACKUP and control-T: how does it do that?

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Date: Fri, 27 Jul 90 20:06 MST
From: KAPLAN@ccit.arizona.edu
Subject: Re: BACKUP and control-T: how does it do that?
To: INFO-VAX@KL.SRI.COM
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>I've been wondering lately just how BACKUP manages to spit out several
>lines of info when you enter control-T.  

Yes, and once you find out how it works, you can just "imagine the 
possibilities"! Qute stuff.

>I could imagine, for example,
>that it simply issues an AST-driven single-character read to the keyboard,
>in "transparent" mode so VMS won't interpret it.  Is this what it
>actually does, or is there some obscure system service with which one can
>snatch control-T, like what can be done with control-C?

No, and yes!  In the I/O User's Manual - PART I (that's PROGRAMMING manual 
set, volume number 7A of the "full VAX/VMS manual set), you'll find some very 
interesting reading starting on page 8-47 under the heading of "Enable 
Out-of-Band AST Function Modifier".  There are 32 "special" characters that
the terminal driver can treat specially for you (a la the Church Lady on
Saturday Night Live?.  I guess that they are called "out of band" since the
normal "bandwidth" of keystrokes does not usually contain them (i.e.: control 
characters and the like).  

At any rate, you can dispatch control to any code that you would like by
asking the terminal triver to queue an AST for your process when the
"out-of-band" character of your choice is typed.  This is done with
IO$_SETMODE and IO$M_OUTBAND function modifiers for your friendly neighborhood
QIO.  The manual actually has a nice programming example to follow. 

Many, many possibilities here!  I've seen people build everything from
performance monitors (i.e.: "show me what locks I hold" to security-breachers 
(i.e. "I'm ready to have inner-mode access now, please") out of this nice
facility. Please tell us what you build with it. 

Happy hacking!

	Ray 8-|)}

Ray Kaplan - I know what I don't know
W) Computer Center - University of Arizona - Tucson, AZ, 85751 - (602) 621-2857
H) P.O. Box 32647 - Tucson, Arizona  85751 - (602) 323-4606
BITNET:    KAPLAN@ARIZRVAX 
INTERNET:  KAPLAN@RVAX.CCIT.ARIZONA.EDU
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