Re: [Xen-users] 32bit vs 64bit memory usage question

["Ulrich Windl" <ulrich.windl@xxxxxxxxxxxxxxxxxxxx>]

On 5 Feb 2007 at 10:00, Tom Miller wrote:

[...]
> 64 bit pointers and 64 bit memory address slots are twice as big as
> corresponding 32 bit ones.   So unless you have 4 GB of memory or more,
> using 64bit software is actually slower for the system, not faster.

Are you sure? You are not saying that 64-bit programs are twice as large as 
32-bit 
programs, do you? Besides of that I'd expect a 64-bit CPU to load a 64 bit word 
from memory in the same speed as a 32-bit CPU would load a 32-bit word. Also 
these 
AMD/Intel CPUs seem to have additional registers in 64bit mode.

I think the memory overhead comes from (As Andrew S Tanenbaum explained in one 
of 
his books about 10 years ago) the fact that the tables to manage the huge 64 
bit 
virtual address space are a great deal larger than those for 32-bit.

Finally (as in HP-UX 11), a 64 bit OS does not mean that every program is a 
64-bit 
executable: a 32-bit Shell, Perl and Emacs is sufficient for most users I'd 
think 
;-)

Regards,
Ulrich


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