Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!batcomputer!itsgw!steinmetz!uunet!auspex!guy
From: guy@auspex.UUCP (Guy Harris)
Newsgroups: comp.lang.c
Subject: Re: Address error quietly fixed on 680x0 systems
Keywords: 68k, sparc, portability
Message-ID: <903@auspex.UUCP>
Date: 28 Jan 89 05:16:12 GMT
References: <302@puivax.UUCP>
Reply-To: guy@auspex.UUCP (Guy Harris)
Distribution: usa
Organization: Auspex Systems, Santa Clara
Lines: 18

>Looking at the code generated by cc I see a long move: no smarts
>in the compiler therefore. So the only mechanism that I can be fixing
>this is an address error trap that quietly moves unaligned objects
>byte-by-byte as requested.

Wrongo.  The 68020 (as appears in the Sun-3), and a variety of other
chips/machines, don't *take* address error traps when you give them an
unaligned address; they just fetch the data.  RTFM (TFM being, in this
case, Motorola's 68020 book).  Furthermore, the 68000 (as appears in
most Macs) and 68010 only require 2-byte alignment, even for 4-byte
quantities (TFM being, in this case, Motorola's 68000/68008/68010 book). 

>Simple timing experiments on the Sun3 don't appear to bear this out,
>however; I saw a 50% increase in runtime with writes to an odd address
>but would have expected more (the trap mechanisms on the 68k aren't
>super fast after all). Enlightenment?

Off-boundary references are, however, slower than on-boundary references.