I'm trying to convert the following C++ code into Assembly (MASM, Irvine32):
const int SIZE = 10;
int numbers[SIZE] = {10,60,20,33,72,89,45,65,72,18};
int limit = 50;
int index = 0;
int sum = 0;
while( index < SIZE )
{
if( numbers[index] <= limit )
{
sum = sum + numbers[index]; // sum += array[index];
}
index++;
}
If anyone could clear up where I'm going wrong -- I'm getting errors at L1: it's just spouting out "+10". I believe it's because I cannot translate sum=sum+numbers[index] into Assembly. If anyone could help me do that it would be fantastic. My attempt at translating it (lines starting from "total: mov esi, offset numbers" to "inc index") is obviously incorrect.
.data
SYZ = 10
numbers DWORD 10, 60, 20, 33, 72, 89, 45, 65, 72, 18
limit DWORD 50
index DWORD 0
sum DWORD 0
.code
main PROC
mov eax, index
mov ebx, SYZ
top: cmp eax, ebx
jae next
jb total
total: mov esi, OFFSET numbers
mov ecx, limit
cmp [esi], ecx
jbe L1
L1: add eax, ebx
inc index
jmp top
next: mov edx, sum
call WriteInt
exit
main ENDP
END main
Your conditional branch that implements the if is wrong. It should look like:
top:
...
cmp [esi], ecx
ja L1 ; conditional jump *over* an ADD instruction
add eax, [esi] ; [esi] is array[index] if you increment ESI properly...
L1: inc index
jmp top
In your C++, you can see that if numbers[index] <= limit then you want to update the sum, otherwise just increment the index and go back to the "top"; aka recheck the stopping condition.
Your original asm code was doing a condition check and then continuing regardless of the result.
cmp [esi], ecx
jbe L1 ; jump or fall-through to L1, condition irrelevant
L1: add eax, ebx
The C++ equivalent of your original asm is:
if( numbers[index] <= limit )
{
}
sum += ebx;
index++;
I'm not sure if this will solve all of your problems, but it will definitely solve one of them.