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From: ade@pucc-i (D. Kakarigi)
Newsgroups: net.lang.pascal
Subject: Turbo Pascal feature (?)
Message-ID: <1094@pucc-i>
Date: Thu, 1-Aug-85 15:10:57 EDT
Article-I.D.: pucc-i.1094
Posted: Thu Aug  1 15:10:57 1985
Date-Received: Sat, 3-Aug-85 02:15:02 EDT
Reply-To: ade@pucc-i.UUCP (D. Kakarigi)
Organization: Purdue University Computing Center
Lines: 30

How would you expect the following loop to work?
                             
                             { Applicable to V 2 and 3/MS-DOS.        }
	repeat               { This may seem like useless code but    }
	  ...		     { it is conceiveably needed and it causes}
	  writeln(....);     { a very uncomfortable feature (?) of    }
	  ...		     { Turbo Pascal to surface to my surprise.}
	until keypressed     { BTW, this problem was pointed to me by }
                             { my colleague at work.  Thanks Bill.     }

	It should do the ... writeln ... until you press any key (with the 
exception of ^S if the C compiler flag is on, and ^C), right?. Well, it 
doesn't.  The code for writing to stdout itself checks for keypressed using
interrupt 16 and 1 in AL, appropriatelly.  Then, if in fact a key was pressed,
it goes and grabs the character with, again, interrupt 16 and 0 in AL,
inappropriately. 
	What happens is that the input buffer pointers get updated, (if AL=0)
and the fact that a key was pressed during the write operation, is not the fact
any more when your until keypressed line wants to examine it.  That is OK if
the key(s) pressed was ^S, but if it is something else, the loop is almost an
infinite loop unless you happen to press a key while your program is executing
somewhere between the writeln and the until keypressed.
	Instead, when the write code detects the keypressed (with AL=1),
it should take the character which is automatically returned anyway (with the
same INT 16 and AL=1), examine it, and only if the character was ^S should the
code go and actually get it with INT 16 and AL=0 (in order to update
pointers), otherwise do nothing.

P.S.  Another thing is that the write code checks for ^S after outputing the
character instead of before.  Matter of taste?