12. The IDIV Instruction

• The IDIV (signed divide) instruction performs signed integer division, using the same operands as the DIV instruction.

• For both DIV and IDIV, all of the arithmetic status flags are undefined after the operation.

• When doing 8-bit division, you must sign-extend the dividend into AH before using IDIV.

• For example, (-48/5):

      .DATA
      byte_val SBYTE -48
      .CODE
      mov al, byte_val ; dividend
      cbw              ; sign-extend AL into AH
      mov bl, 5        ; divisor
      idiv bl          ; quotient AL = - 9 , remainder AH = -3
  

   

• Similarly, l6-bit division requires that AX be sign-extended into DX.

• For example, (-5000/256):

      .DATA
      word_val SWORD -5000
      .CODE
      mov ax, word_val ; dividend, low part
      cwd              ; sign-extend AX into DX
      mov bx, 256      ; divisor
      idiv bx          ; quotient AX = -19, remainder DX = -136
  

   

• Similarly, 32-bit division requires that EAX be sign-extended into EDX. The next example

• For example, (-50000/256):

      .DATA
      dword_val SDWORD -50000
      .CODE
      mov eax, dword_val ; dividend, low
      cdq                ; sign-extend EAX into EDX
      mov ebx, 256       ; divisor
      idiv ebx           ; quotient EAX = -195, remainder EDX = -80