Hi, I've been stuck on this for a while and need some help. My program is supposed to do what this blurb of c++ does:
int add(short * a, const short *b, const short *c, int n)
{
for (int i=0; i<n; i++)
a[i] += b[i] + c[i] + max(b[i],c[i]);
return count-of-overflows;
}It is supposed to return a 1 if the short a overflows, otherwise it returns 0. The value of a only matters if there is no overflow. Needless to say, it doesn't work how I want it to. Here's what I have so far:
of.s
.text
.global _add
_add:
// prolog
pushl %ebp
movl %esp, %ebp
pushl %ebx
pushl %esi
pushl %edi
// 8(%ebp) - a (arg1) pointer to value of a[0]
// 12(%ebp) - b (arg2) pointer to value of b[0]
// 16(%ebp) - c (arg3) pointer to value of c[0]
// 20(%ebp) - n (arg4) # of iterations
// ------------------------------------------
mov 8(%ebp), %ebx // ebx = *a
mov 12(%ebp), %ecx // ecx = *b
mov 16(%ebp), %edx // edx = *c
mov $0, %esi // i = 0
mov $0, %eax // overflow count = 0
begin:
cmp %esi, 20(%ebp) // if(%esi [i] < 20(%ebp) [n])
jle quit // continue, else jump out of loop
movb (%ebx, %esi, 2), %al // %al = a[i]
addb (%ecx, %esi, 2), %al // %al += b[i]
jo overflow // if the OF flag is set, jump to overflow:
addb (%edx, %esi, 2), %al // %al += c[i]
jo overflow // if the OF flag is set, jump to overflow:
mov (%edx, %esi, 2), %edi // mov the value c[i] into %edi
cmp (%ecx, %esi, 2), %edi // if b[i]>c[i]
jl else // {
addb (%ecx, %esi, 2), %al // %al += b[i]
jo overflow // if the OF flag is set, jump to overflow:
jmp skip // }
else: // else c[i]>=b[i]
addb (%edx, %esi, 2), %al // { %al += c[i]}
skip:
mov %al, (%ebx, %esi, 2) // a[i] = %al
jmp next
overflow:
inc %eax //increment overflow counter
next:
inc %esi // i++
jmp begin //jump back to begin:
quit:
// ------------------------------------------
// return %eax, which should = 1 if overflow occured, otherwise it should = 0
// epilog
popl %edi
popl %esi
popl %ebx
popl %ebp
retI use the following .cpp file in conjunction with the command: g++ -Wall testof.cpp of.s -o testof.exe
It fails the assert in line 29
testof.cpp
#include <stdio.h>
#include <memory.h>
#include <assert.h>
extern "C" int add(short *a,const short *b,const short *c,int n);
short a[10000];
short b[10000];
short c[10000];
int main()
{
int result;
memset(a,0,sizeof(a));
memset(b,0,sizeof(b));
memset(c,0,sizeof(b));
// simplest test
result = add(a,b,c,0);
assert(result==0);
// there are others
// just one
a[7] = 5;
b[7] = 9;
c[7] = 3;
result = add(a+7,b+7,c+7,1);
assert(result==0); //<-------------*this is where it fails*
assert(a[7] == 26);
assert(b[7] == 9);
assert(c[7] == 3);
printf("you did it!\n");
}Any help at all would be very appreciated. If you can't help, then thanks for at least reading this far!