linux-mips-fnet
[Top] [All Lists]

Re: m700-10 success

To: linux-mips@fnet.fr
Subject: Re: m700-10 success
From: tor@spacetec.no (Tor Arntsen)
Date: Thu, 9 May 1996 13:12:09 +0200
In-reply-to: Dominic Sweetman <dom@algor.co.uk> "Re: m700-10 success" (May 9, 10:04)
On May 9, 10:04, Dominic Sweetman wrote:
>Scientific manner, chaps!  LE and BE are two different *arbitrary*
>choices about how you represent bigger-than-byte integers in a
>byte-organised memory.  Neither one is right or wrong; I find LE
>easier because I began my computing career on DEC and got into
>low-level code on Intel CPUs.  Tor finds BE natural, I suspect,
>because he started off on Motorola?

Well, microprosessors weren't much used when I started, and I don't know 
even today what an Alpha LSI used for byte endianity..  however, when I 
began to do things where I had to know about such things I was working with 
VAX and another mini with Motorola endianity, at roughly the same time.

>Arguably DEC and IBM (whose bit numbers increase the same way as their
>byte addresses) are righter than Motorola (whose bit numbers go down
>as byte addresses go up).  It would be nice if everyone did the same,
>of course - one less thing to go wrong.

IMHO it doesn't matter much which way the bits are counted, however I
prefer that the sequence is linear also when crossing byte boundaries
(e.g. bit 7 sits beside bit 8 is more important than which comes 'first').
One reason is that I worked with satellite data where we had to write code
to extract say 47 bits from a particular window inside a set of 111 bits.
For me it was always easier to code that on big endian computers.  There are
others though who argue that LE is easier, although I never really got it :-)
This is no big deal though, and I've been using my Linux/Intel PC since 1992
without suffering too much :-)

>> I would really like to switch my Olivetti M700-10 over to big endian
>> at some stage, if that's at all technically possible.
>
>Don't even think about it!

The real reason I'm interested in BE for Linux/MIPS is as I said that Sun
and SGI use it.  So are you saying that it can't be done for Jazz boxes? 
That's bad news :-(

>PS: the "change endianness in user mode" feature was a late addition
>    to the R3000 chip, by disgruntled MIPS engineers who wanted to be
>    able to run DECstation binaries on MIPS Unix boxes.  The system
>    software to make the hardware feature usable was never included in
>    any MIPS OS, and I think has never been used by anyone.

Interesting! I didn't know it could be done in user mode for MIPS, I thought 
it had to be done no later than during boot (I seem to remember a pin on the 
chip for this?).  I know PPC can be switched at any time, one task can run BE 
and the other LE -- at the same time even.  At least that's what I've been 
told.

Cheers,
-Tor

<Prev in Thread] Current Thread [Next in Thread>