classiccmp-digest V1 #163

From: Eric Smith <eric_at_brouhaha.com>
Date: Tue May 9 13:39:18 2000

David Greelish wrote:
> The Mac OS has never been fully contained in ROM. Starting with the Lisa in
> 1983 and the original Mac in 1984, Apple used a 64k ROM that contained GUI
> program routines (the Macintosh ToolKit). These machines still had to boot a
> floppy which made calls to the ROM.

The Lisa only had 16K of ROM, which did NOT contain anything but a
bootstrap loader and diagnostics. The Lisa operating system and
applications did not make any calls into the ROM, with the possible
exception of one function to set MMU registers. Macintosh software
certainly did not make any calls to the Lisa ROM, even on Lisas that
were sold under the name "Macintosh XL".

In the original Macintosh (64K ROM, 128K RAM), the OS was *almost*
entirely contained in ROM. The System file primarily contained fonts,
desk accessories, defprocs, and bug fixes. It was in fact possible
to write Macintosh applications that could be booted directly with no
System file, yet take full advantage of the Macintosh "OS".

The Macintosh OS has gone through multiple cycles of adding large
chunks of new functionality to the System file, then moving those
pieces into ROM. Unfortunately when the code gets moved into ROM,
it doesn't usually get removed from the System file, because it still
is needed for older Macs since Apple typically does not offer ROM
upgrades.

With System 7.5 (or was it 7.1), the size of the System file was
getting sufficiently out of hand that Apple moved a lot of platform-
specific code into "System Enablers".

Nowdays Macs don't have *any* of the OS in ROM; they use the OpenBoot
firmware to load the entire OS from disk.
Received on Tue May 09 2000 - 13:39:18 BST