Re: [PATCH] 2.1.131: i386 tweak to switch_to()

Horst von Brand (vonbrand@inf.utfsm.cl)
Tue, 15 Dec 1998 13:47:48 -0300

Andrea Arcangeli <andrea@e-mind.com> said:
> On Sat, 12 Dec 1998, Jamie Lokier wrote:
> >+ :"ebx", "ecx", "esi", "edi", "cc", "memory"); \
> ^^^^^^^^^^^^^^
>
> What is "cc" and why we need to declare "memory" clobbered after using
> __switch_to()?

"cc" is condition flags; "memory" is clobbered each time you write to
something that is not in the arguments. Clobbering "memory" is used to
singal gcc that it can't assume that memory is unchanged (i.e., reuse a
value in memory that just happens to be in a register), and also has the
side effect that it acts as a barrier: No reordering of instructions will
move stuff over the asm() because of the above.

-- 
Dr. Horst H. von Brand                       mailto:vonbrand@inf.utfsm.cl
Departamento de Informatica                     Fono: +56 32 654431
Universidad Tecnica Federico Santa Maria              +56 32 654239
Casilla 110-V, Valparaiso, Chile                Fax:  +56 32 797513


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