Re: Which processor mode?

[Gerald Henriksen <ghenriks@xxxxxxxxxx>]

On Fri, 02 May 2003 11:45:42 -0600, you wrote:

>> 
>> So to summarize, yes Long mode *can* run 32 bit apps (using
>> compatibility mode), or native 64-bit apps. 
>
>Then I assume the os is compiled without the needed REX prefixes that
>address 64 bit registers.  This way, users can choose compatibilty mode
>in the bios, and the os will work fine with 32 bit code as well.  But in
>the case that they want *THEIR* code to access the larger register space,
>they can choose 64-bit only mode and the os apps will simply ignore the
>larger register space.

No, nothing to do with the BIOS.  Again from the AMD manual Vol 1
(pages 8 and 9, sections 1.2.2 and 1.2.3) we get the following:

The 64-bit OS (like GinGin64) decides on a program by program basis
which sub-mode is used.  Either sub-mode is enabled on an individual
code-segment basis.

So under GinGin64 you could run say an mpeg4 encoder in compiled with
the x86-64 version of gcc to take advantage of the new registers and
the OS would run the encoder in 64-bit mode while at the same time the
OS runs your IA32 version of Mozilla in compatibility mode (so mozilla
doesn't see the new registers, but can run flash).  

This is of course (along with price) why so many people are excited
about x86-64 compared to IA64 (Itanium*).  Opteron, unlike Itanium,
doesn't give you a performance penalty for running legacy applications
alongside your new 64-bit applications because Opteron can behave like
an existing IA32 processor.