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From: lsr@Apple.COM (Larry Rosenstein)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Help With VBL Tasks
Keywords: VBL task; post itself; BOMB
Message-ID: <445@internal.Apple.COM>
Date: 19 Jan 89 17:33:47 GMT
References: <8343@orstcs.CS.ORST.EDU>
Organization: Advanced Technology Group, Apple Computer
Lines: 40

In article <8343@orstcs.CS.ORST.EDU> borcelf@jacobs.cs.orst.edu (Fernando Borcel) writes:

>Here's what I wrote:
>
> procedure myVbl;
>  var
>   VBLTask : QElemPtr;
> begin
>  SetUpA5;
>
>  FlashMenuBar(0);  {my "TASK"}
>
>  new(VBLTask);
>  VBLTask^ := myVBLTask^;
>  if vInstall(VBLTask) <> NoErr then  {Enqueue for Next VBL task}
>   exitToShell;
>
>  RestoreA5;
> end;

There are a couple of problems here:

(1) A VBL task runs at interrupt level, and can't call any routins that
allocate memory.  FlashMenuBar does allocate memory (it's listed in the
chart in Inside Mac).  I suspect that New could also.

(2) The technique you use to set up the task to be executed again is not
right.  You don't need to creata new task block.  Instead you shouldsimply
reset the vbl counter in the existing task block.  A pointer to the task
block is passed in A0 when your VBL code is called.  There are various ways
to get this register in Pascal, depending on the development system.

Your initial installation code looks OK to me.


-- 
		 Larry Rosenstein,  Object Specialist
 Apple Computer, Inc.  20525 Mariani Ave, MS 46-B  Cupertino, CA 95014
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