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From: ephram@violet.berkeley.edu
Newsgroups: comp.sys.ibm.pc
Subject: Re: Reading scan codes
Message-ID: <3559@jade.BERKELEY.EDU>
Date: Thu, 14-May-87 23:37:36 EDT
Article-I.D.: jade.3559
Posted: Thu May 14 23:37:36 1987
Date-Received: Tue, 19-May-87 01:29:09 EDT
References: <476@catnip.UUCP> <311@nonvon.UUCP>
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Reply-To: ephram@violet.berkeley.edu ()
Organization: University of California, Berkeley
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in article <476@catnip.UUCP>, ben@catnip.UUCP (Bennett Broder) says:
> 
> I am writing a program using Quick Basic, and I would like to be able to
> determine if the shift keys are pressed.  I figure that there must be
> a way to read the port that the keyboard is connected to in order to
> determine this, but I am unsure of exactly what must be done.  Could
> someone shed a little light on the subject?
> 
> Thanks.
> 
> -- 
> 
> Ben Broder
> {ihnp4,decvax} !hjuxa!catnip!ben
> {houxm,clyde}/

The status of things like shift keys, alt key ctrl key and some others are
all bitmapped into a byte called the keyboard status flag.  The byte is at
segment 40H offset 17H.  the bit are as follows

80H   insert state is active
40H   Caps lock state has been toggled
20H   num lock   "     "   "     "
10H   scroll lock state "  "     "
08H   alternate key depressed
04H   ctrl key depressed
02H   left shift key depressed
01H   right shift key depressed

 above source PC tech ref july 82

in order to test to see if both shift keys are depressed (no pun)
 do something like the pascal construct

if (40:17 and 3) eq 3 then
     {code to do if both keys are depressed simultaneously (sp?) }
  else
     {some other action}
  endif;

Ephram Cohen

ephram@violet.berkeley.edu