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

["Li, Xin B" <xin.b.li@xxxxxxxxx>]

>> 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).

Still a little bit confusing if eax is 64bit here :-(, people need keep
this in mind when programming on x86_64 xen.

>
>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... 
>

Agree! And a little bit hate of the zero-fill. 
Thanks
-Xin

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