CODEWITHSUNDEEP
assemblyprimesdivisionmasm
caboose103
caboose103

Reputation: 33

Prime numbers program error (MASM)

Having a slight error in this program. It's a simple program that allows the user to input a number higher than 2 and determine if it's prime or not. It works fine expect for the numbers of 3 and 4. 3 is said to be not a prime and 4 is a prime. I'm simply showing the code for the calculations.

;ecx = divisor, esi = max divisor, edx = remainder, eax = quotient

mov ebx, eax
shr eax, 1
mov esi, eax   ;Max divisor value
mov ecx, 1

 isPrime:
      inc ecx
      cmp ecx, esi
      je Prime

      mov edx, 0
      mov eax, ebx
      div ecx
      cmp edx, 0
      jz NotPrime

      jmp isPrime

I'm not sure if the shr might have something to do with the error. Any explanations? I'm trying to avoid making jumps just for those values.

Thank you for your time.

Upvotes: 0

Views: 513

Answers (1)

rkhb
rkhb

Reputation: 14409

je Prime is wrong. The value of ESI is the half of EAX (rounded down). If EAX is 3, then ESI will become 1. ECX starts effectively with 2 (mov ecx, 1 inc ecx). So it will loop over the whole 32 bit range before it will be equal to ESI. When ECX becomes 3 then the loop will break to NotPrime.

If EAX==4, the loop will break to Prime at once: the first ECX is 2 and ESI is 2.

Remedy: Change je Prime to ja Prime.

Upvotes: 2

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