Date: Sat, 29 Dec 2007 12:39:10 -0500 From: Mike Meyer <mwm-keyword-freebsdhackers2.e313df@mired.org> To: freebsd-hackers@freebsd.org Subject: Re: Architectures with strict alignment? Message-ID: <20071229123910.3ed3c68b@bhuda.mired.org> In-Reply-To: <20071229124120.GA23415@owl.midgard.homeip.net> References: <fl4c8o$vpu$1@ger.gmane.org> <47760132.5040306@pacific.net.sg> <b1fa29170712290014w780448bh8da93006a629b7b4@mail.gmail.com> <47761B63.4010407@pacific.net.sg> <20071229124120.GA23415@owl.midgard.homeip.net>
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On Sat, 29 Dec 2007 13:41:21 +0100 Erik Trulsson <ertr1013@student.uu.se> wrote:
> On Sat, Dec 29, 2007 at 06:03:15PM +0800, Erich Dollansky wrote:
> > All RISC based designs need the alignment so that the CPU can fetch a CPU
> > word in one go. CISC based designs do not have this limitiation.
> >
> > I also do not know of any other CISC based design which made it to
> > mainstream.
I think this would be more correctly expressed as "still survive in
the mainstream."
> Not quite true. Take for example the venerable Motorola 68000 CPU (used in
> many different computers in the early and mid 80's).
> It required all 16-bit (and 32-bit) accesses to be aligned on a 16-bit
> boundary. This was later relaxed in the M68020 and later CPUs.
The same is true of the IBM 360 architecture - had it initially, and
dropped it in later revisions - which was at one time the most popular
architecture in the world, complete with competitors cloning it to
take advantage of the software base.
FWIW, it was also the first architecture where it made sense to ask
this question. Prior to the 360, computers were either business
machines, and only did things with/to bytes, or they were scientific
computers, and only did things with/to words. It's ability to both is
one of the reasons it became the most popular architecture in the
world, and that basic design now dominates the industry.
<mike
--
Mike Meyer <mwm@mired.org> http://www.mired.org/consulting.html
Independent Network/Unix/Perforce consultant, email for more information.
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