Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ucbvax!UNCAEDU.BITNET!svermeulen%Janus.MRC.AdhocNet.CA
From: svermeulen%Janus.MRC.AdhocNet.CA@UNCAEDU.BITNET (Steve Vermeulen)
Newsgroups: comp.sys.amiga
Subject: VBlank interrupt servers
Message-ID: <880911141731.00s@Janus.MRC.AdhocNet.CA>
Date: 11 Sep 88 20:17:31 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 47

Sorry to post this in c.s.a but I have lost c.s.a.tech read capability
for the moment...

I am busy writing parts of the next AMUC Demo, last year I put togeather
the AMUC Scrolling Demo (Fish 117) and this year the project is
a Video Wall - or the first public showing of the Amiga 9000 :-)

As part of this 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.

First question:  how much drift between VBLANK counts between machines
am I likely to see over 15 minutes, 1 hour, 8 hours?

I looked into the timer device first and found that the unique time stamp
feature of this device might prevent this exact time keeping.  So
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.

This was almost exactly what I needed.

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"???




                      Stephen Vermeulen
                      Author: Express Paint
                      Chairman: AMUC.