11. Overflow Flag Examples

• Examples of 8-bit signed and unsigned addition and subtraction:

              .DATA
      mem8    BYTE    39       ;                                           0010 0111   27
                               ;
              .CODE
      ; Addition + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
                               ;                    signed    unsigned     binary      hex
              mov   al, 26     ; Start with register   26       26         0001 1010   1A
              inc   al         ; Increment             +1       +1
                               ;                     ----     ----
                               ;                       27       27         0001 1011   1B
              add   al, 76     ; Add immediate        +76      +76         0100 1100   4C
                               ;                     ----     ----
                               ;                      103      103         0110 0111   67
              add   al, [mem8] ; Add memory           +39      +39         0010 0111   27
                               ;                     ----     ----
              mov   ah, al     ; Copy to AH          -114      142         1000 1110   8E (OF) (SF)
              add   al, ah     ; Add register      + -114     +142         1000 1110   8E
                               ;                     ----     ----
                               ;                       28       28         0001 1100   1C (OF) (CF)
      ; Subtraction- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
                               ;                    signed    unsigned     binary      hex
              mov   al, 95     ; Load register         95       95         0101 1111   5F
              dec   al         ; Decrement           -  1     -  1
                               ;                     ----     ----
                               ;                       94       94         0101 1110   5E
              sub   al, 23     ; Subtract immediate  - 23     - 23         1110 1001   E9
                               ;                     ----     ----
                               ;                       71       71         0100 0111   47
              mov   [mem8],122 ; Load memory
              sub   al, [mem8] ; Subtract memory    - 122    - 122         0111 1010   7A
                               ;                     ----     ----
                               ;                      -51      205         1100 1101   CD (SF) (CF):borrow
              mov   ah, 119    ; Load register        
              sub   al, ah     ; and subtract       - 119    - 119         0111 0111   77
                               ;                     ----     ----
                               ;                       86       86         0101 0110   56 (OF)
  

• (OF) overflow flag indicates that result is too large to fit in the 8-bit destination operand:

  • the sum of two positive signed operands exceeds 127.

  • the difference of two negative operands is less than -128.

• (CF) carry flag indicates that the sum of two unsigned operands exceeded 255.

• (SF) sign flag indicates that result goes below 0.

• To verify the above conditions, try ADD_SUB_EFLAGS.exe demo program ADD_SUB_EFLAGS.ASM ( download source here.)