C Code Examples: In-line Assembly

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  • DevynCJohnson
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    In-lining Assembly into C code allows developers to incorporate faster and/or low-level code into their C programs.

    The following in-line x86 Assembly code is supported by GNU-GCC and Clang. Other compilers may not support the given code.

    Get CPU Vendor ID

    const char *get_vendor_id(void) {
    // Return the vendor ID of the x86 CPU (Assumes little-endian)
    int ebx, edx, ecx;
    __asm__ (
        "movl $0,%%eax;"
        "cpuid;"
        "movl %%ecx,%0;"
        "movl %%edx,%1;"
        "movl %%ebx,%2;"
        : "=r" ( ebx ), "=r" ( edx ), "=r" ( ecx )  // Output C variable
        :  // No input variables
        : "ebx", "edx", "ecx"  // Clobbered Register
    );
    static char vendor[16];
    vendor[13] = '\0';
    vendor[12] = '\0';
    vendor[11] = (char)((ebx >> 24) & 0xFF);
    vendor[10] = (char)((ebx >> 16) & 0xFF);
    vendor[9] = (char)((ebx >> 8) & 0xFF);
    vendor[8] = (char)(ebx & 0xFF);
    vendor[7] = (char)((edx >> 24) & 0xFF);
    vendor[6] = (char)((edx >> 16) & 0xFF);
    vendor[5] = (char)((edx >> 8) & 0xFF);
    vendor[4] = (char)(edx & 0xFF);
    vendor[3] = (char)((ecx >> 24) & 0xFF);
    vendor[2] = (char)((ecx >> 16) & 0xFF);
    vendor[1] = (char)((ecx >> 8) & 0xFF);
    vendor[0] = (char)(ecx & 0xFF);
    return vendor;
}

    Get CPU Stepping

    signed int get_cpu_stepping(void) {
    // Get the stepping value of the CPU
    register signed int stepping;
    __asm__ (
        "movl $0x01,%%eax;"
        "cpuid;"
        "andl $0b00000000000000000000000000001111,%%eax;"
        "movl %%eax,%0;"
        : "=r" ( stepping )  // Output C variable
    );
    return stepping;
}

    Test if the x86 CPU has a FPU

    int is_fpu_aval(void) {
    // Test if the CPU supports `fpu`
    register int truth;
    __asm__ (
        "movl $0x01,%%eax;"
        "cpuid;"
        "andl $0b00000000000000000000000000000001,%%edx;"
        "movl %%edx,%0;"
        : "=r" ( truth )  // Output C variable
    );
    return truth;
}

    Test if the CPU supports PSE

    int is_pse_aval(void) {
    // Test if the CPU supports `pse` (Page Size Extension)
    register int truth;
    __asm__ (
        "movl $0x01,%%eax;"
        "cpuid;"
        "andl $0b00000000000000000000000000001000,%%edx;"
        "sarl $3,%%edx;"
        "movl %%edx,%0;"
        : "=r" ( truth )  // Output C variable
    );
    return truth;
}

    Test if the CPU Supports CMOV

    int is_cmov_aval(void) {
    // Test if the CPU supports `cmov` (Conditional move and FCMOV instructions)
    register int truth;
    __asm__ (
        "movl $0x01,%%eax;"
        "cpuid;"
        "andl $0b00000000000000001000000000000000,%%edx;"
        "sarl $15,%%edx;"
        "movl %%edx,%0;"
        : "=r" ( truth )  // Output C variable
    );
    return truth;
}

    Test if the CPU Supports SSE3

    int is_sse3_aval(void) {
    // Test if the CPU supports `sse3` (Prescott New Instructions-SSE3 (PNI))
    register int truth;
    __asm__ (
        "movl $0x01,%%eax;"
        "cpuid;"
        "andl $0b00000000000000000000000000000001,%%edx;"
        "movl %%edx,%0;"
        : "=r" ( truth )  // Output C variable
    );
    return truth;
}

    Get a Random Number from RDRAND

    #ifdef __BMI2__
ulint rdrnd(void) {
    // Get a random number from the rdrand assembly instruction
    if ((is_rdrnd_aval()) == 0) return (ulint)0;
    register ulint rand_num;
    __asm__ volatile ( "rdrand %0;" : "=r" ( rand_num ) );
    return rand_num;
}
#endif

    Further Reading

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