RE: [Xen-devel] Definition of eax and rax

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Li, Xin B
> Sent: 13 December 2006 12:13
> To: Keir Fraser
> Cc: Xen Development Mailing List
> Subject: [Xen-devel] Definition of eax and rax
> 
> Keir, just noticed the macro:
> #define __DECL_REG(name) union { uint64_t r ## name, e ## name; }
> So, rax and eax are both 64bit, but I think we'd better define eax to
> 32bit, how do you think?

I believe the purpose of the double-declaration is to have the ability
to do generic code that uses eax for both 32- and 64-bit code, avoiding
having to write two different pieces of code for 32- and 64-bit code. 

If you want to use the lower 32-bits of rAX in 64-bit mode, I'd suggest
a more explicit way (like casting it to 32-bit size, for exmple).

Note also that the x86_64 specification says that a 32-bit write to a
64-bit register should zero-fill the upper 32 bits, which won't happen
if you have :

        union { uint64_t rax; uint32_t eax } rAX;
        rAX.eax = 7;

You'd end up with whatever was there from before in the upper bits of
rAX.rax... 
        
--
Mats
> -Xin
> 
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