17. ADC Examples

1. The following fragment adds two 8-bit integers (FFh + FFh), producing a 16-bit sum in DL:AL, which is 01h:FEh.

       mov dl, 0
       mov al, 0FFh
       add al, 0FFh  ; AL = FEh, CF = 1
       adc dl, 0     ; DL += CF, add "leftover" carry
   

    

2. Similarly, the following instructions add two 32-bit integers (FFFFFFFFh + FFFFFFFFh).

   The result is a 64-bit sum in EDX:EAX, 0000000lh:FFFFFFFEh,

       mov edx, 0
       mov eax, 0FFFFFFFFh
       add eax, 0FFFFFFFFh
       adc edx, 0    ; EDX += CF, add "leftover" carry
   

    

3. The following instructions add two 64-bit numbers received in EBX:EAX and EDX:ECX:

   • The result is returned in EBX:EAX.

   • Overflow/underflow conditions are indicated by the Carry flag.

       add eax, ecx ; add low parts EAX += ECX, set CF
       adc ebx, edx ; add high parts EBX += EDX, EBX += CF
       ; The result is in EBX:EAX
       ; NOTE: check CF or OF for overflow (*)
   

    

4. The 64-bit subtraction is also simple and similar to the 64-bit addition:

       sub eax, ecx ; subtract low parts EAX -= ECX, set CF (borrow)
       sbb ebx, edx ; subtract high parts EBX -= EDX, EBX -= CF
       ; The result is in EBX:EAX
       ; NOTE: check CF or OF for overflow (*)
   

• _____________

• (*) The Carry flag CF is normally used for unsigned arithmetic.

•     The Overflow flag OF is normally used for signed arithmetic.