Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Micro Timing Critical sections HELP!
Message-ID: <14567@cbmvax.commodore.com>
Date: 20 Sep 90 18:56:51 GMT
References: <4934@uqcspe.cs.uq.oz.au>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 55

In article <4934@uqcspe.cs.uq.oz.au> thyssen@batserver.cs.uq.oz.au writes:

>   I calculated that the `E' line should be runing at .715Mhz (1/10th
>of cpu clock rate) and physical measurments (a stop watch over ~20mins)
>give a figure of .709 +/- .001 Mhz (3 measurements were made).

You're right, actually, on both accounts. If you have a PAL machine, the 
CPU clock is 7.09MHz, so you get a 0.709MHz E clock.  If you have an NTSC
machine, you have a 7.159MHz CPU clock, which results in a 0.716MHz E clock.

>   Timing a fixed loop of code (see below) however produced varing
>results which I beleave is due to the interupt handling of the CPU.

There are a variety of reasons why any arbitrary chunk of code will take
varying amounts of time to execute.  The most obvious reason is that Amiga
CPUs vary from 7.09MHz 68000s to 50MHz 68030s.  Thus, the policy that no 
CPU-speed dependent code is permitted in Amiga applications starts to make
some real-world sense.  If you need timing, use a timer, which will be at
least reasonably consistent (like, within 1% or so) between all Amigas.

>QUESTIONS :-
> 1)  What is the differance bettween Disable-Enable
>   and Forbid-Permit exec library calls?

Forbid()/Permit() disables multitasking -- it lets you hog the CPU, for 
whatever reasons.  In general, the only valid reason for this is that you have
a chunk of code that's examining some shared data structures that aren't
locked by a semaphore.  Disable()/Enable() turn off all interrupt processing,
which as a result also happens to disable multitasking.  Some I/O devices will
use this, but there's just about no situation in which an application program
will use these functions.

> 3)  Is there any other interupts which can be disabled? How?

Disable()/Enable(), by definition, will disable all valid interrupts.  Or, to
invert this argument, interrupts that don't get disabled by Disable() aren't
supported by the AmigaOS.  The only such interrupts you could get that are
members of this class are expansion-board-generated INT1*, INT4*, INT5*, or
INT7* on an A2000.

> 4)  Under the above can memory still be allocated/deallocated - Important?

Memory can be allocated and freed during a Forbid(), and I believe during a
Disable() as well, though I have never used that one myself.

>  Anthony Thyssen - (Dragon Computing!)         thyssen@batserver.cs.uq.oz.au

So you know your dragons.  Think of the OS as you walking down the road,
unarmed.  Forbid() would be like your average angry Wyvern meeting you on
this road.  Disable() is more like Smaug or Tiamat coming along to greet
you.
-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
      Get that coffee outta my face, put a Margarita in its place!