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From: ech@spuxll.UUCP (Ned Horvath)
Newsgroups: net.micro
Subject: Re: ANOTHER 32-BIT MACHINE???
Message-ID: <655@spuxll.UUCP>
Date: Sat, 4-May-85 01:29:02 EDT
Article-I.D.: spuxll.655
Posted: Sat May  4 01:29:02 1985
Date-Received: Mon, 6-May-85 11:43:59 EDT
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clif@intelca.UUCP (Clif Purkiser) writes:

> 	As a Mac owner, and largely an Apple fan, I found it interesting that
> the Mac is really a segmented machine, except that they restrict segments
> to 32K!!.  So, I really don't think segmentation is bad, just the fact that
> aren't as large as you want.

Whoops, I can't let that go by: there is nothing in the architecture of the
68k, or its deployment in the Mac, that limits one to 32k text segments.
That is strictly a limitation of the Pascal compiler, which prefers to
generate only PC-relative text references (which use signed 16-bit offsets).
In turn, that allows segments up to 32k to move around in memory without
being relocated (as long as you don't try to cache absolute code pointers!).
In fact, one does need to exercise some minimal care -- code only floats if
you cut it loose.  The SUMacC system creates arbitrary-sized segments
(although all hell breaks loose if the thing is allowed to move -- the compiler
there DOES compile absolute addresses which get relocated at launch time).

Indeed, intersegment calls in the Mac, even using Pascal, can be to
absolutely anywhere in a 2^24 byte namespace; the particular trick they use is
a jump table pointed to by a dedicated address register, thus
	jsr a5(offset)
actually calls the instruction
	jmp [absolute address of entry]
the last being a full 24 bit address.

My sole affiliation with Motorola is as a satisfied end-user and programmer
of 68k-based products (Mac & CT Miniframe).

=Ned=