In my university project i have to use binary representation of float number in x86 assembly for arithmetic operations. Using FPU is forbidden so i try to read float number and return it as DWORD but whatever i try to do i get "-nan". Any advices?
Edit: I use gcc and it's 32 bit code
Declaration in C (i can't change that)
extern "C" float func(float num);
*.asm file
section .text
global func
func:
;prolog
push ebp
mov ebp, esp
; zapamiętanie rejestrów zachowywanych
push ebx
push esi
push edi
mov eax, DWORD [ebp+8]
;mov eax, 0xffffffff i checked that but i still get the same result
; odtworzenie rejestrów, które były zapamiętane
pop edi
pop esi
pop ebx
;epilog
pop ebp
ret
Example result (for 256)
01000011100000000000000000000000
11111111110000000000000000000000
num1: 256.000000
num2: -nan
Edit:
C code without checking bits part
#include <stdio.h>
extern "C" float func(float num);
int main()
{
float num1;
float num2;
scanf("%f", &num1);
num2=func(num1);
printf("num1: %f\nnum2: %f\n", num1, num2);
return 0;
}
If you declare the return type func as float the result will be returned in the FPU (ST0). For returning a value in EAX you have to declare it as an integer type. For printf you have to fake a float. Example:
caller.c:
#include <stdio.h>
#include <stdint.h>
extern float asmfunc1(float);
extern uint32_t asmfunc2(float);
int main (void)
{
printf ("asmfunc1: %f\n", asmfunc1(456.78));
uint32_t ifl = asmfunc2(123.45);
float* pfl = (float*) &ifl; // Faking a float
printf ("asmfunc2: %f\n", *pfl);
return 0;
}
callee.asm:
section .text
global asmfunc1, asmfunc2
asmfunc1:
fld dword [esp+4]
ret
asmfunc2:
push ebp
mov ebp, esp
mov eax, [ebp+8]
leave
ret
Build & run:
nasm -felf callee.asm
gcc -m32 callee.o caller.c
./a.out
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