Read register value to variable, with one asm command

Question

How can I read register value to variable with one inline assembler command? I am using gcc on old freeBSD system (v2.1 i386).

I have such code:

static volatile unsigned long r_eax, r_ebx;
asm ("movl %%eax, %0\n" :"=r"(r_eax));
asm ("movl %%ebx, %0\n" :"=r"(r_ebx));

As result I get this:

mov    %eax,%eax
mov    %eax,0x1944b8
mov    0x1944b8,%eax
mov    %ebx,%eax
mov    %eax,0x1944bc
mov    0x1944bc,%eax

But i need just:

mov    %eax,0x1944b8
mov    %ebx,0x1944bc

How can I achieve this result?

Answer 1

This does it for me (as long as r_eax / r_ebx are static)

asm ("movl %%eax, %0\n"
     "movl %%ebx, %1\n"
     : "=m"(r_eax), "=m"(r_ebx));

Beware that, unless you specify assembly language statements within the same asm() bracket, the compiler might decide to do all sorts of "interesting optimizations" in-between, including modifications to these regs.

Answer 2

Notice you are using constraints instructing gcc to put the result into a register. So it can not directly put it into memory. Since you only want to store values from registers already there, you don't even need any instructions, just constraints, like so:

__asm__ __volatile__ ("" : "=a" (r_eax), "=b" (r_ebx));