From:	Hans-Bernhard Broeker <broeker AT physik DOT rwth-aachen DOT de>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: problem in inline assembler
Date:	28 Aug 2001 10:33:08 GMT
Organization:	Aachen University of Technology (RWTH)
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"±θΗΠΑΦ" <kimguy00 AT orgio DOT net> wrote:

> and I thought %0, %1, %2.. are just a kind of a mapping sequencely.

They are. The sequence is just counting a bit differently from what
you seemt expect: it counts constraints, not C variables mentioned in
them.  The idea is that GCC will evaluate those expressions
(optimizing the evaluation as it sees fit, possibly mixing the inline
assembly statements with the generated statements), and make them
available to the assembly by substituting some register name or memory
address on the stack for the %0...%1 operands.

You may want to have a look at the generated assembly (gcc -O2 -S) to
see how this works out.

> So I thought %0 is gate_addr, %1 is dpl, %2 is type

No. 

%0 is (*((long *)(gate_addr)))
%1 is (*(1+(long*)(gate_addr)))
%2 is ((short) (0x8000+(dpl<<13)+(type<<8)))
%3 is ((char *) (addr)),"a" (KERNEL_CS << 16)

> #define _set_gate(gate_addr,type,dpl,addr) \
> __asm__ __volatile__ ("movw %%dx,%%ax\n\t" \
>  "movw %2,%%dx\n\t" \
>  "movl %%eax,%0\n\t" \
>  "movl %%edx,%1" \
>  :"=m" (*((long *) (gate_addr))), \
>   "=m" (*(1+(long *) (gate_addr))) \
>  :"i" ((short) (0x8000+(dpl<<13)+(type<<8))), \
>   "d" ((char *) (addr)),"a" (KERNEL_CS << 16) \
>  :"ax","dx")


-- 
Hans-Bernhard Broeker (broeker AT physik DOT rwth-aachen DOT de)
Even if all the snow were burnt, ashes would remain.

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