delphiassemblydividebasm

ASM/Delphi - Divide

I am trying to divide two numbers 50 and 5. This is my code:

function Divide(Num1, Num2: Integer): Integer;
asm
  MOV   EAX, Num1
  CDQ
  MOV   ECX, Num2
  IDIV  ECX
  MOV   @RESULT, ECX
end;

It gives me a DivisionByZeroException exception in Delphi. Can someone tell me what am I doing wrong ?

Solution

  • It's the CDQ instruction. From an online reference:

    Converts signed DWORD in EAX to a signed quad word in EDX:EAX by extending the high order bit of EAX throughout EDX

    The problem is, Num2 being the second parameter, is stored in EDX, and since you're running CDQ before loading EDX to ECX, what ends up in ECX is 0. Rewrite it like this, and your routine works as expected:

    function Divide(Num1, Num2: integer): integer;
asm
  MOV EAX, Num1
  MOV ECX, Num2
  CDQ
  IDIV ECX
  MOV @Result, EAX
end;