CODEWITHSUNDEEP
cgccx86-64inline-assemblycpu-registers
user9370454
user9370454

Reputation:

Accessing the low 32-bits of r8 through r15

How can I write a 32-bit value to the low doubleword of the registers r8-r15 with asm()? The following code won't compile:

#include <stdlib.h>
#include <stdio.h>
int main()
    {
    float f0,f1,f2=-2.4f;
    asm volatile
    (
        "movl %2, %%r8\n"
        "movl %%r8, %1\n"
        "movl %1, %%r15\n"
        "movl %%r15, %0"
        :"=r"(f0,f1)
        :"r"(f1,f2)
        :"%r8,%r15"
    );
    printf("%f\n",f0);
    system("pause");
    return 0;
    }

The error I get:

unknown register name '%r8,%r15' in 'asm'

Note that It is a x64 program so r8-r15 registers should be available.

Upvotes: 1

Views: 2235

Answers (1)

prl
prl

Reputation: 12435

Use r8d to access the low 32 bits of r8. Note that when you write to r8d (or any 32-bit register in 64-bit mode), it clears the upper 32 bits. There is no way to write to the low 32 bits and preserve the upper 32 bits.

Upvotes: 7

Related Questions