Path: utzoo!attcan!uunet!cbmvax!steveb
From: steveb@cbmvax.UUCP (Steve Beats)
Newsgroups: comp.sys.amiga
Subject: Re: VBlank interrupt servers
Message-ID: <4703@cbmvax.UUCP>
Date: 12 Sep 88 13:59:40 GMT
References: <880911141731.00s@Janus.MRC.AdhocNet.CA>
Reply-To: steveb@cbmvax.UUCP (Steve Beats)
Organization: Commodore Technology, West Chester, PA
Lines: 42

In article <880911141731.00s@Janus.MRC.AdhocNet.CA> svermeulen%Janus.MRC.AdhocNet.CA@UNCAEDU.BITNET (Steve Vermeulen) writes:
>
>.............. I needed an exact, stable, time base that would track
>across 9 Amigas for an extended period of time without drifting
>between machines.  To do this I am proposing to use the VBLANK signal
>and essentially keep a count of all the VBLANKS that occur.
>
>remembering the DevCON notes I looked at Steve Beats article on using
>68K assembler on the Amiga.  In which Steve creates a VBLANK server
>that counts vblank events and Signals another task every 60 vblanks.
>
>Comparing his article to the Exec RKM on page 60 where it states:
>
>   "Servers have different register values passed than handlers do.
>    A server cannot count on the D0, D1, or A6 registers containing
>    any useful information."
>
>This has caused me some head scratching, as Steve's interrupt server
>USES A6 under the assumption it has a valid value of ExecBase in it
>when it is called.  You see, I started with the example on pages 61-62
>of RKM Exec and then added in the "jsr _LVOSignal(a6)" call and things
>went nuts.  I finally traced it down to this: when my server (at priority
>of -60) is called A6 is trashed (as the RKM states) but Steve's
>server is added at priority +20 and this must put it high enough in the
>list (probably at the head) that A6 still contains a valid pointer
>to ExecBase.
>
>Second Question: What I want to know is "AM I BRAIN DAMAGED, or is this the
>correct explaination"???
>

Er, cough, ahem ... You spotted the intentional mistake then ?

Sorry, that was a real, honest-to-goodness cock up on my part.  It did work
though, which is probably why I missed it.  I've been looking through these
manuals so long now that I often make assumptions on how something will work
(based on the way other routines in the same sub-system are called).  In most
cases this is OK, but obviously not in all of 'em.

So don't worry, your brain appears undamaged and functional.

	Steve