Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!apple!phil
From: phil@Apple.COM (Phil Ronzone)
Newsgroups: comp.sys.mac.programmer
Subject: Re: PMMU and low memory sharing
Message-ID: <31133@apple.Apple.COM>
Date: 19 May 89 19:13:46 GMT
References: <811@key.COM> <30353@apple.Apple.COM> <4666@okstate.UUCP> <1787@internal.Apple.COM> <7266@hoptoad.uucp> <13472@dartvax.Dartmouth.EDU> <7321@hoptoad.uucp> <30935@apple.Apple.COM> <18-May-89.104411@192.41.214.2> <2065@oakhill.UUCP>
Organization: Apple Computer A/UX Group
Lines: 30

In article <2065@oakhill.UUCP> davet@oakhill.UUCP (David Trissel) writes:
>Let's take the slow "Classic" Mac which runs at an effective MC68000 rate of 
>about 5.2 Mhz once the standard vertical interrupt handler and screen refresh 
>are taken into account. Let's use worst case assumptions here and say your 
>Mac is running 50(!) applications. The 60'th second tick update should 
>certainly take less than 100 cycles to find the next application's low memory 
>global base and write the new value in.  A simple calculation shows that 
>5,000 cycles per second amounts to only .1% lost processing power.

The case in point was dealing with clock ticks. The point was a process, say
a game, wanted to wait say 32 milliseconds for display/animation purposes. If
it watched the low memory global, waiting for it to changed, it essentially
had to run "un-context-switchable", otheriwise animation would be "jerky".

If it told the operating system it wanted control back in 32 milliseconds,
things would happen (hopefully) a lot smoother.

An Excel app recalculating in the background isn't helped by an idiot app
that is spinning on a low memory global. Tens apps spinning can make like
miserable for background processes.

Philosophical point more than a Mac OS specific point.

+-----------------------------------------------------------------------------+
|Philip K. Ronzone, Apple Computer, 10440 Bubb Rd, MS 58A, Cupertino, CA 95014|
|{amdahl,decwrl,sun,voder,nsc,mtxinu,dual,unisoft,...}!apple!phil             |
+-----------------------------------------+-----------------------------------+
| All "IMHOs" disclaimed and copyrighted. | Self defense is a human right ... |
+-----------------------------------------+-----------------------------------+