AMD64 vs i386 for FreeBSD

[Kenneth Culver]

Quoting David O'Brien <obrien at freebsd.org>:

> On Wed, Jun 30, 2004 at 01:17:35AM -0400, Kenneth Culver wrote:
>> Quoting Doug White <dwhite at gumbysoft.com>:
>> >Me either. -current actually supports running i386 binaries in amd64 mode.
>> >Thats one of the processor's features. :-)
>>
>> >You can't run amd64 binaries when booted into an i386 OS, of course.
>>
>> Yeah you can run x86 but you cant' go into regular 32 bit mode that's all.
>
> ENOPARSE, can you please restate this?

I think what I meant is that once the kernel puts the CPU into "amd64" 
mode, it
can't go back into regular x86 mode. It can run x86 binaries but it's 
not fully
back in x86 mode, and I think some of the x86 instructions are gone in 64-bit
mode, so it has to emulate them somehow. From what I understand 32-bit 
binaries
run slightly slower when the cpu is in 64-bit mode because of this.

 From what I've read, you can't make a kernel go back into normal x86 
mode until
you reboot.

You probably know about all this better than I do, it's been a long 
time since I
read any tech specs for the cpu.

Ken