“Error: bad register name `%rax'” while compiling with MinGW on 64-bit Windows 7

Question

In my previous topic: How to read registers: RAX, RBX, RCX, RDX, RSP. RBP, RSI, RDI in C or C++? I asked about reading those registers. Now I wrote a code to read (just for now on) RAX and RBX.

I'm using CodeBlocks pm 64-bit Windows 7 with MinGW as a compiler and I'm working on an x86-64 CPU. When I tried to compile the below code, I got those errors:

Error: bad register name `%rax'
Error: bad register name `%rbx'

And the code:

#include <iostream>
#include <cstdlib>
#include <stdint.h>

void read(void)
{
    uint64_t rax = 0, rbx = 0;
    __asm__ __volatile__ (
        /* read value from rbx into rbx */
        "movq %%rbx, %0;\n"
        /* read value from rax into rax*/
        "movq %%rax, %1;\n"
        /* output args */
        : "=r" (rbx), "=r" (rax)
        : /* no input */
        /* clear both rdx and rax */
        : "%rbx", "%rax"
    );

    /* print out registers content */
    std::cout << "RAX = " << rax << "\n";
    std::cout << "RBX = " << rbx << "\n";

}

int main(int argc, char **argv)
{
    read();

    return 0;
}

Mats Petersson Mats Petersson · Accepted Answer · 2013-12-14T10:24:38

I suspect your error comes from the "clobber" line:

    : "%rbx", "%rax"

which should read:

    : "rbx", "rax"

(Oh, and don't ask me why!)

Edit: You will also need to compile for 64-bit, using -m64 or similar - assuming of course the MingW compiler you have is 64-bit capable in the first place.

“Error: bad register name `%rax'” while compiling with MinGW on 64-bit Windows 7

2 Answers