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From: pnl@hpfinote.HP.COM (Peter Lim)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Why use MS-window ?
Message-ID: <19450063@hpfinote.HP.COM>
Date: 2 Jun 90 17:40:43 GMT
References: <10699@medusa.cs.purdue.edu>
Organization: Hewlett Packard CICD
Lines: 45

Hi Peter,

>   Don't say they just make the time slices Real Thin or something;
>   that doesn't work.  In Windows 3.0 the foreground task is guaranteed 
>   50% of the CPU.   In Desqview the minimum time-slice is an 18th
>   of a second and they recommend using bigger chunks than that.
>   But even an 18th of a second at 1200 baud is enough time to 
>   drop 7 characters.    So how do they do it?   I've always wondered
>   about this.      (Assume a 386SX with 4 meg of RAM running Windows
>   3.0 in enhanced mode if that helps.) 
>   
I think this has something to do with INTERRUPT. When data is knocking
at the door of the incoming serial port, the serial port generates an
interrupt which is captured by the communication software sitting in
the background. That is why you must not allow the communication
software code to be swapped out when it goes into the background.
This has nothing to do with time-slicing which goes around distributing
CPU power to process not propelled by interrupts.

For a little bit more details, on UART system which has FIFO, you can 
set the interrupt to occur when the FIFO is filled to certain level 
instead of interrupting whenever a character arrive at the serial port.
That is why, smarter comm cards with chips like 16550 imposed much less 
CPU overhead than dumb card like those with 8250 (did I get the number 
correct ?).  Of course your comm software has to be aware of the FIFO 
feature in the smart comm card, or else, it will go to waste.

It is the combination of interrupts and time-slicing that make these
multi-tasking systems tick.

Another 2 cents to the net.


>   Thanks in advance.   
>
You are welcome  :-).


Regards,                       ## Life is fast enough as it is ........
Peter Lim.                     ## .... DON'T PUSH IT !!          >>>-------,
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