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Date: Fri, 30 Nov 2001 17:23:03 -0500 (EST)
From: Alfred M Kriman <akriman@darwin.helios.nd.edu>
Message-Id: <200111302223.fAUMN3b14938@darwin.helios.nd.edu>
To: classics@u.washington.edu
Subject: Re: Mac OS 10 improvements

When comparing Macintosh operating systems, it is not fair (so far
as I know still *) to compare the speed of the original OS with that
of a later user-installed OS.  The reason is that Macs, unlike PC's
(+), have toolbox functions and other high-use pieces of the OS in
ROM.  When you install a new OS from disk, any modification of
functions originally in ROM introduces two kinds of delay:

 (a) An initial start-up delay, as OS code is loaded from (usually)
 disk into RAM (which is, or at least is treated as volatile).  To
 play it safe and for practical reasons, modified systems may bypass
 large chunks of ROM OS, so even minor system upgrades may put
 substantial code in RAM and noticeably degrade performance for this
 reason and those described under (b)...

 (b) Generally slower operation of every program (including most
 applications; nonmasochist programmers call system functions rather
 than reinvent the wheel and the dialog box).  There are multiple
 reasons for the slower performance: less RAM is available for
 application code and data, sometimes forcing the machine to page-swap.
 RAM is usually slower than ROM (_always_ slower, at a given level of
 fabrication technology).  There are additional communication traffic
 delays that depend in detail on the architecture, which tends to be
 optimized for original-OS-in-ROM.

(*) I haven't been a regular Mac user in five years, but I think the
preceding is still accurate.

(+) By PC's, I mean generic machines that use Microsoft or similar OS's.
These have traditionally used much less ROM, mostly for BIOS, afaik.
There is no technical reason why the machine designer couldn't use the
Mac ROM-intensive strategy (Atari also did it), but there are costs.
Lisa had a GUI but used multiple OS's, and so had less OS in ROM.

AMK
.