Xref: utzoo comp.arch:7785 comp.misc:4626 comp.lang.misc:2468 comp.protocols.misc:440
Path: utzoo!attcan!uunet!lll-winken!ames!elroy!cit-vax!wen-king
From: wen-king@cit-vax.Caltech.Edu (King Su)
Newsgroups: comp.arch,comp.misc,comp.lang.misc,comp.protocols.misc
Subject: Re: "big endian" and "little endian" - first usage for computer
Message-ID: <9036@cit-vax.Caltech.Edu>
Date: 6 Jan 89 15:40:12 GMT
References: <2766@cbnews.ATT.COM> <10147@well.UUCP> <13045@cup.portal.com> <20264@ames.arc.nasa.gov> <2695@rti.UUCP>
Reply-To: wen-king@cit-vax.UUCP (Wen-King Su)
Organization: California Institute of Technology
Lines: 44

In article <2695@rti.UUCP> bcw@rti.UUCP (Bruce Wright) writes:
>My immediate thought on seeing the VAX instruction set when it first came
<out was that by making the byte order "little endian" it allowed something
>like a Fortran compiler to take a statement like:
<
>	call sub (1)
<
>and pass a number to it (a longword - 4 bytes) which would be interpreted
<correctly whether the receiving formal parameter was a byte, a word (2 bytes),
>or a longword (4 bytes).  This is not possible in a "big endian" machine -
<you have to know how many bytes of high order 0's to write before you get to
>the low order byte.

This cannot have anything to do with byte-ordering because the two
byte-ordering conventions are totally symmetrical and isomorphic.  Any
difference between two machines must have been a result of some asymmetries
that was imposed on on the machine when the machine was designed.  In
the example above, the asymmetry was imposed when the following question
is answered:

   If a data unit is consisted of a sequence of bytes, what should the
   address of the data unit be: the address of the MSByte or the address
   of the LSByte.

In VAX, and in most little-endian machines, the address of the LSByte was
used to represent the address of the data unit.  In 68K and most big-endian
machines, the address of the MSByte was used.  The choice is quite arbitrary,
but the important thing is that it imposes an asymmetry.  The supposed
"advantage" of the little-endian byte-ordering is really the advantage of
choosing the address of the LSByte to be the address of a multi-byte unit.

We can build a big-endian machine with exactly the same advantage if we make
the same choice for it as we have made for VAX.  In this case, a 'long' that
occupies byte address 0x20 0x21 0x22 0x23, will have 0x23 as its address.

In general, given any little-endian machine, we can build a big-endian
machine that is exactly as good as the little-endian machine (in fact,
they will be duals), and vice versa.  Byte-ordering should cease to be the
focal point of any arguments; talks about the decisions that lead to the
asymmetries should replace it.
-- 
/*------------------------------------------------------------------------*\
| Wen-King Su  wen-king@vlsi.caltech.edu  Caltech Corp of Cosmic Engineers |
\*------------------------------------------------------------------------*/