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Re: Merge back of the MIPS sources

To: Ralf Baechle <ralf@mailhost.uni-koblenz.de>
Subject: Re: Merge back of the MIPS sources
From: "William J. Earl" <wje@fir.engr.sgi.com>
Date: Tue, 10 Jun 1997 10:48:39 -0700
Cc: mende@piecomputer.engr.sgi.com, ralf@Julia.DE, ariel@sgi.com, linux@yon.engr.sgi.com
In-reply-to: <199706101059.MAA18571@informatik.uni-koblenz.de>
References: <199706091836.LAA02114@fir.engr.sgi.com> <199706101059.MAA18571@informatik.uni-koblenz.de>
Sender: owner-linux@cthulhu.engr.sgi.com
Ralf Baechle writes:
 > >       If the MIPS system is running big-endian (as under RISCos), it can
 > > be binary-compatible with Indy linux, just as RISCos 5.01 -systype svr4
 > > binaries are binary-compatible with Indy IRIX 5.1 and later systems.
 > > If the MIPS system is running little-endian (as under NT), it cannot
 > > be binary-compatible with Indy linux, without adding bi-endian support
 > > (for running opposite-endian binaries, which is feasible, but messy).
 > > We prototyped bi-endian support in RISCos when before the Magnum
 > > was built, and it worked, but the code was very ugly in the streams
 > > area.  linux would likely be easier, but still a hassle.
 > 
 > Now that the overhead of accessing the userspace from the kernel under
 > Linux is very close to zero I fear that the additional overhead of the
 > byteorder conversion will show up very clearly in benchmarks unless
 > someone comes up with very clever ideas how do this.  Do you have
 > numbers how much the impact on RISC/os performance was?

     I don't have the numbers ready to hand, but it was not huge for most
things, given the hack of running the disk wrong-endian.  CPU overhead
for non-disk DMA I/O was higher, of course, but there is no extra
overhead for operations where all the arguments are in registers.
(An integer in a register is the same, no matter which endian you are.)
getpid(), for example, is unaffected.  Similarly, read() and write()
to disk are unaffected.  On the other hand, stat() is a little slower.

     Note that endian data conversion is not blind byte swapping for
structures such a stat.  Instead, it is a field-by-field operation,
since the MIPS reverse-endian mode affects only addressing, not the
layout of the bytes of, say, an integer in memory.  A 32-bit integer
will be at a different offset from the base of the structure (4, say,
instead of 0), but the byte order in memory will be the same.  The
byte numbering of memory is different, which is how it appears that
the bytes have been swapped.  The structure conversion routines can
thus be compiled fairly efficiently, avoiding runtime structure offset
calculations for the most part (except for arrays embedded in
structures).

     If this starts to look worth doing, I can supply more imformation,
such as examples of how to efficiently convert structures.  

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