7. IMUL Examples

1. The following fragment computes 8-bit signed multiplication (48 × 4):

       mov  al, 48
       mov  bl, 4
       imul bl      ; AX = 00C0h (decimal +192), OF = 1
   

   Because AH is not a sign extension of AL, the Overflow flag is set to 1.

    

2. The following instructions perform 8-bit signed multiplication of (-4 × 4), producing -16 in AX:

       mov  al, -4
       mov  bl, 4
       imul bl      ; AX = FFF0h, OF = 0
   

   AH is a sign extension of AL, meaning that the signed result fits within AL, therefore the Overflow flag is cleared.

    

3. The following instructions perform 16-bit signed multiplication (48 × 4), producing +192 in DX:AX:

       mov  ax, 48
       mov  bx, 4
       imul bx      ; DX:AX = 0000h:00C0h , OF = 0
   

   Here, DX is a sign extension of AX, so the Overflow flag is cleared.

    

4. 32-bit signed multiplication example:

       .data
       signed_val SDWORD ?  ; Data type is signed doubleword
       .code
       mov  eax, +4823424
       mov  ebx, -423
       imul ebx             ; EDX:EAX = FFFFFFFFh:86635D80h, OF = 0
       move signed_val, eax ; store the result
   

   EDX is a sign extension of EAX, so the Overflow flag is cleared.