Actual benefits of amd64 over i386

David O'Brien obrien at
Tue May 24 10:53:24 PDT 2005

On Thu, May 19, 2005 at 02:01:36PM -0300, Joo Carlos Mendes Lus wrote:
> David O'Brien wrote:
> > On Fri, May 13, 2005 at 04:17:11PM -0300, Joo Carlos Mendes Lus wrote:
> > 
> >>    What about a 64 bit kernel, and mixed mode (32bit and 64bit)
> >>userland?  Solaris does this, and it sounds efficient, from the comments
> >>I've seen in this list.
> > 
> > 
> > When Sparc went from 32-bits to 64-bits the calling ABI was not changed.
> > Nor were the number of registers increased.  So it is w/o a doubt that a
> > 32-bit Sparc binary runs faster than a 64-bit one (abit 64-bit math and
> > large memory).  This is not true of AMD64 - the number of registers was
> > doubled and the calling ABI changed and optimized.
>     Would these benefits outcome the losses caused by bigger binaries?
> Isn't it possible to use 64 bit registers in a 32 bit segment?  Just
> like i386 segments, where one could define the default register size...

> > What is the difference of "i386 emulation" and "native 32 bit executables
> > in amd64 arch"??
>     IMHO, the 32bit binaries prepared to run in amd64 32bit segments are
> not the same as 32 binaries prepared to run in i386 mode.  These "32bit
> amd64 executables" would take advantage of the extra registers and 64
> bit extensions when possible.

It is not possible to access the extra registers in 32-bit mode.
-- David  (obrien at

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