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From: randy@cbmvax.cbm.UUCP (Randy Weiner)
Newsgroups: net.micro.amiga
Subject: Re: Clarifications for the Ultra Fast Timer
Message-ID: <299@cbmvax.cbmvax.cbm.UUCP>
Date: Wed, 28-May-86 09:21:17 EDT
Article-I.D.: cbmvax.299
Posted: Wed May 28 09:21:17 1986
Date-Received: Fri, 30-May-86 06:51:33 EDT
References: <724@watcgl.UUCP>
Reply-To: randy@cbmvax.UUCP (Randy Weiner)
Distribution: net
Organization: Commodore Technology, West Chester, PA
Lines: 66

In article <724@watcgl.UUCP> fdfishman@watcgl.UUCP (Flynn D. Fishman) writes:
        I have removed most of the article... now to get on to
        the heart of the matter.
>
>I have some questions now:  I am in the infansy of my C learning, could
>someone please explain to me about the busy wait.
>
        Busy-wait is the technique where your program loops forever
until some condition is met. Typically, you would test the state
of some variable to determine when to exit your loop. In a real-world
situation, you would also want some kind of time-out that would force
you to exit the loop after some period of time, otherwise you are
hung.

        In terms of the Amiga Exec, a busy wait might consist of
sending a command to an IO device, then checking its status with a
CheckIO(), looping until the IO has completed.

        Busy-wait is NOT a recommended programming technique on the
Amiga. Mostly because it is a waste of CPU time, and secondly, if
you have spawned other tasks (or processes), they will not get
a chance to run since your task is hogging all the time for itself.
Instead, you should take advantage of the message facilities
of the exec and Wait(), or WaitIO() for a desired set of signals,
or messages from cooperating tasks, and/or devices.

        I refer you to the Rom Kernal Manual for more information.
Look at the sections on Messages and Ports, chapter 1.
many examples which use this technique.

>  Does anybody know where the Amiga micro-second timer is?  I have the
manuals
        The micro-second timer is just the normal timer device
opened  in microhertz mode. Refer to RKM, starting on page 3-29. The
last paragraph refers to the MICROHZ timer. But then, this does
not really apply to your application. You need to determine the
interval between two events.

        The Amiga timer device is basically a delay timer. There
is no easy way to accurately time an interval to a +/- 1 microsecond
accuracy. I refer you to Volume 2 of the RKM, page B-82:
        the microhertz timer "has a precision down to about 2
        microseconds, but will drift at system load increases.
        The timer is typically accurate to within 5%"

Have you considered reading the system time and using time
arithmetic to get the interval. The only problem is that
this will not provide a 1 or 2 microsecond resolution, and
is probably not repeatable, either.

>>The interrupt interfacing will have to be done in assembly.
>
>I was afraid someone would say that. I just (am still) learning C, I hope
>someone can come up with some ideas (in C preferably)
>
        While it is possible to do an interrupt interface in C,
it will depend on your specific requirements. Using C will incur quite
a bit more overhead than if using assembler.

--
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Randy Weiner -- Commodore Business Machines <<Amiga Technical Support>>
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