NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part2/16109

Internet Message Format | 1996-08-05 | 1.7 KB

Path: keats.ugrad.cs.ubc.ca!not-for-mail
From: [e-mail] (Kazimir Kylheku)
Newsgroups: comp.lang.c++,comp.lang.c
Subject: Re: Allowing interrupts in interrupt code
Date: 9 Apr 1996 09:03:01 -0700
Organization: Computer Science, University of B.C., Vancouver, B.C., Canada
Message-ID: <[e-mail]>
References: <[e-mail]>
NNTP-Posting-Host: keats.ugrad.cs.ubc.ca

In article <[e-mail]>,
Steve Lowe  <[e-mail]> wrote:
>I have a routine attached to the timer interrupt.

Timer interrupts are not defined by the C language. You cannot write interrupt
service routines in ANSI C. Your question is likely platform-specific.

>However, sometimes it takes more than one clock tick to execute - which
>causes endless headaches.
>
>Is there anything I can put in this routine so that an new interrupts
>can carry on?

Yes, you can put in a semaphore flag to indicate that you are currently doing
busywork inside the interrupt handler. Then you reenable interrupts. When an
interrupt arrives, it sees that the flag is set and will not enter the same
region of code, but will instead do only basic housekeeping things like
updating counters, calling other handlers in the interrupt chain and whatnot.
It will not fiddle with the data that the first invocation is working on.

The real solution is to do time-slicing so that your lengthy task can be done
as a thread dispatched by a scheduler. In a typical (single-CPU) OS
implementation, tasks that can be done quickly are done with interrupts
disabled. Tasks that take longer are done inside a critical region protected by
semaphores rather than by interrupt masking, so that other processes can
execute, and interrupts can be handled.
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