Re: Which processor mode?

[Gerald Henriksen <ghenriks@xxxxxxxxxx>]

On Thu, 01 May 2003 15:41:38 -0600, you wrote:

>I hope you mean "long" mode.  Compatibility mode is technically a 64 bit
>mode as well.  Long mode *Cannot* run 32 bit apps, so this is an important
>distinction.  

To clarify the terms, the following is from the AMD x86-64 Programmers
Manual Volume 1 (page 3):

There are 2 main operating modes, Long and Legacy.

Legacy mode is used when running a 32-bit OS, and offers the usual
sub-modes from the IA32 architecture.

Long mode is the native 64-bit mode for x86-64.  There are 2 sub-modes
offered in Long, 64-Bit mode and Compatibility mode.  Long mode
requires a 64-bit OS (like GinGin64).

Of the 2 sub-modes in Long:
 64-Bit mode - requires applications compiled specifically for it, and
offers 64-bit address size as well as the register extensions (16
general purpose, 16 XMM, 8 MMX).

 Compatibility mode - what is used to run existing 32-bit IA32
application binaries, limited to 32-bit address size and the normal
IA32 registers.

So to summarize, yes Long mode *can* run 32 bit apps (using
compatibility mode), or native 64-bit apps.