Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!apple!agate!e260-1d.berkeley.edu!c60b-1eq
From: c60b-1eq@e260-1d.berkeley.edu (Noam Mendelson)
Newsgroups: comp.sys.ibm.pc.misc
Subject: Re: QUESTION ABOUT INTERRUPTS AND TIMERS
Message-ID: <1991Apr6.055003.19402@agate.berkeley.edu>
Date: 6 Apr 91 05:50:03 GMT
References: <TRUX.91Apr5123621@atop.atop.cs.caltech.edu>
Sender: usenet@agate.berkeley.edu (USENET Administrator)
Distribution: comp.sys.ibm
Organization: University of California, Berkeley
Lines: 36

In article <TRUX.91Apr5123621@atop.atop.cs.caltech.edu> trux@atop.atop.cs.caltech.edu (Flem Snopes) writes:
>Can anyone out there in netland help me?
>I am using a 12Mhz IBM AT clone.
>Here is my obstacle:
>    I wish to program an interrupt associated with a timer that calls the
>    interrupt 8000 times per second.
>Here is what I have done already:
>    I have used interrupt 1CH and changed the 8253 timer from the standard
>    18.2 hz to 8Khz but this has an adverse side affect, it calls interrupt
>    08H 8000 times per second which make my system crawl.  I dont know what
>    interrupt 08H does, but if you disable it, the system locks.  Also, if
>    I hook 08H instead of 1CH, I could theoretically call the original 08H
>    at 18.2 hz, but the system locks all the same.  So it appears that 08H
>    MUST be called every timer pulse.  
>Here are my questions:
>    why must 08H be called every timer pulse?  what does 08H do that takes
>    so long.  Can I use another timer?  

INT 08H does the following things:
	1) Increment a running count of clock ticks at 0040:006C
	2) Decrements the byte at 0040:0040, which controls the timing of
	   the diskette motor
	3) Calls interrupt 1CH

INT 08H is not very CPU-intensive but at 8,000 calls/sec, it's likely
to slow your system down considerably.
Another point: if you change the rate of the timer you _must_ create a new
INT 08H which calls the original INT 08H at the standard interval.  Your
system should not lock when you do this.  I am surprised that your
system did not lock when you changed the rate without modifying INT 08H.
Perhaps you are not doing this correctly.

+==========================================================================+
| Noam Mendelson   ..!agate!ucbvax!web!c60b-1eq | "I haven't lost my mind, |
| c60b-1eq@web.Berkeley.EDU                     |  it's backed up on tape  |
| University of California at Berkeley          |  somewhere."             |