Path: utzoo!utgpu!watmath!att!dptg!rutgers!gatech!prism!vsserv!loligo!pepke
From: pepke@loligo (Eric Pepke)
Newsgroups: comp.sys.mac
Subject: Re: The REAL reason the mouse is jerkey!
Message-ID: <140@vsserv.scri.fsu.edu>
Date: 30 Aug 89 14:47:03 GMT
References: <4254@ogccse.ogc.edu> <34413@apple.Apple.COM>
Sender: news@vsserv.scri.fsu.edu
Reply-To: pepke@loligo.UUCP (Eric Pepke)
Organization: Supercomputer Computations Research Institute
Lines: 35

In article <34413@apple.Apple.COM> tecot@Apple.COM (Ed Tecot) writes:
>down.  No one seemed to mind.  These days, when interrupts are diasabled
>on the CPU, all the other input devices keep chugging along; they have
>their own processors.  When the CPU "wakes up", the mouse reports the new
>movement, causing the cursor to appear "jerky".  The latter is more accurate,
>but less desirable.

No, this would be acceptable, if that's all that were happening.  

What I think is really happening is this:
1) CPU goes to sleep for a while.
2) The ADB devices keep humming blithely along.
3) CPU wakes up.
4) CPU gets the new coordinates from the mouse.
5) CPU wrongly assumes that it always polls the mouse at regular intervals
   and runs these new x and y deltas through its cursor speed scaling routines.
6) The cursor jumps WAY out of proportion to the actual mouse movement.

Now, I don't really know any of this, because I don't work for Apple, and I
haven't cared enough to disassemble the code.  However, it is consistent with
the behavior I have observered.  The tablet cursor, which does not do mouse 
scaling, works just fine when interrupts are disabled. 

It is step 5 that causes the jerkiness which I find unpleasant.  The fix is
pretty simple.  Just change the CPU routines so that they have some basic
awareness of how long it has been since the last time they did anything and
scale what they do to this time period.

Eric Pepke                                     INTERNET: pepke@gw.scri.fsu.edu
Supercomputer Computations Research Institute  MFENET:   pepke@fsu
Florida State University                       SPAN:     scri::pepke
Tallahassee, FL 32306-4052                     BITNET:   pepke@fsu

Disclaimer: My employers seldom even LISTEN to my opinions.
Meta-disclaimer: Any society that needs disclaimers has too many lawyers.