Last stretch of rounding function in ASM

Question

What I essentially have to do is make what is in Main work.

I'm on my last stretch of this assignment (which will likely take just as long as it did for me to get here) I'm having trouble figuring out how to pass the roundingMode that is passed to roundD and using it in ASM.

Also, there is a block of just comments, as far as I can tell, that's all I have left to do. does that sound right?

#include <stdio.h>
#include <stdlib.h>

#define PRECISION           3
#define RND_CTL_BIT_SHIFT   10

// floating point rounding modes: IA-32 Manual, Vol. 1, p. 4-20
typedef enum {
    ROUND_NEAREST_EVEN =    0 << RND_CTL_BIT_SHIFT,
    ROUND_MINUS_INF =       1 << RND_CTL_BIT_SHIFT,
    ROUND_PLUS_INF =        2 << RND_CTL_BIT_SHIFT,
    ROUND_TOWARD_ZERO =     3 << RND_CTL_BIT_SHIFT
} RoundingMode;

double roundD(double n, RoundingMode roundingMode)
{
// do not change anything above this comment

    int             oldCW = 0x0000;
    int             newCW = 0xF3FF;
    int             mask  = 0x0300;
    int             tempVar = 0x0000;


    asm("   push    %eax                            \n"
        "   push    %ebx                            \n"
        "   fstcw   %[oldCWOut]                     \n"     //store FPU CW into OldCW
        "   mov     %%eax, %[oldCWOut]              \n"     //store old FPU CW into tempVar
        "   mov     %[tempVarIn], %%eax             \n"
        "   add     %%eax, %[maskIn]                \n"     //isolate rounding bits
        "   add     %%eax, %[roundModeOut]          \n"     //adding rounding modifier
                                                            //shift in old bits to tempFPU
                                                            //do rounding calculation
                                                            //store result into n
        "   fldcw   %[oldCWIn]                      \n"     //restoring the FPU CW to normal
        "   pop     %ebx                            \n"
        "   pop     %eax                            \n"

        :   [oldCWOut]      "=m"    (oldCW),
            [newCWOut]      "=m"    (newCW),
            [maskOut]       "=m"    (mask),
            [tempVarOut]    "=m"    (tempVar),
            [roundModeOut]  "=m"    (roundMode)

        :   [oldCWIn]       "m"     (oldCW),
            [newCWIn]       "m"     (newCW),
            [maskIn]        "m"     (mask),
            [tempVarIn]     "m"     (tempVar),
            [roundModeIn]   "m"     (roundMode)
        :"eax", "ebx"
    );

return n;

// do not change anything below this comment, except for printing out your name
}

int main(int argc, char **argv)
{
    double  n = 0.0;

    if (argc > 1)
        n = atof(argv[1]);

    printf("roundD even %.*f = %.*f\n",
           PRECISION, n, PRECISION, roundD(n, ROUND_NEAREST_EVEN));
    printf("roundD down %.*f = %.*f\n",
           PRECISION, n, PRECISION, roundD(n, ROUND_MINUS_INF));
    printf("roundD up   %.*f = %.*f\n",
           PRECISION, n, PRECISION, roundD(n, ROUND_PLUS_INF));
    printf("roundD zero %.*f = %.*f\n",
           PRECISION, n, PRECISION, roundD(n, ROUND_TOWARD_ZERO));

    return 0;
}

Answer 1

While C might like to pretend that enum is not just an integer, it is just an integer. If you can't use roundingMode directly in the assembly, create an integer local variable and set it equal to the roundingMode parameter.

I'm just offering this as a suggestion to you. I've never used inline assembly before and I've never used x86 assembly before, but if all you need to do is reference the parameter, what I said above should work.