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From: jlee@sobeco.com (j.lee)
Subject: Re: bi-endian environments
Organization: Groupe Sobeco, Montreal, Canada
Distribution: comp
Date: Wed, 15 May 91 00:48:48 GMT
Message-ID: <1991May15.004848.11929@sobeco.com>
References: <159@titccy.cc.titech.ac.jp> <3225@spim.mips.COM> <168@titccy.cc.titech.ac.jp> <3308@spim.mips.COM>
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In <3308@spim.mips.COM> cprice@mips.com (Charlie Price) writes:

>You *can* arrange to share ONE binary format.
>I think the only reasonable one to pick is characters (i.e. bytes).

This is what "network byte order" is all about.  If I write(2) or
mmap(2) a sequence of *bytes* on my system to disk/tape/tty/socket
and you can't read(2) or mmap(2) them and get the same sequence of
*bytes*, I would argue that someone's device driver is *bust*.  On
the other hand, if we both try to interpret those same bytes as a
sequence of two-byte or four-byte (or even eight-byte) integers,
the values we see can depend on our host byte-ordering (endian-ness).

>When a page is accessed, and mapped to the process,
>the OS must swap the bytes around within the page so that
>character-stream order is preserved.
>This in-memory page is now in a special state and it has to be
>transformed back to the native character-stream order before it
>can be written back to disk.
>If a memory-mapped file page or shared memory segment page is accessed
>by processes that have different endian-ness, the page has to be put into
>the proper character-stream order before the access can be granted.

Character (byte) stream order is *normally* preserved; in order
to do the "conversion" you must shuffle the byte ordering around
-- either when you map the page of data, or when you read a value
from that page.  However, a single, fixed, map-time transformation
for *all* pages based only on "processor endian-ness" will not work;
the transformation required for each page depends on the size and
type (in the case of FP values) of the data being loaded/stored in
that page.  There is a paper by M. Stumm and S. Zhou of the University
of Toronto on this in relation to a Distributed Shared Memory system
for Heterogeneous Processors.  They discuss a DSM system which
shares data between Vaxen and Sun3s -- machines with different
endian-ness, FP encoding, and page-size!  I'm afraid that the exact
reference escapes me at the moment.  However the point is that data
sharing between heterogeneous machines *can* be done, but requires
knowledge of the structure and encoding of the data being exchanged.
(The X11 protocol is a case in point.)

I'm not sure whether or not we've tried to say the same thing, but
if so, I hope that what I have said makes the point clearer.

Jeff Lee -- jlee@sobeco.com || jonah@cs.toronto.edu