Arithmetic and the flags

Arithmetic instructions put their result in the first operand: ADD AX, BX means AX = AX + BX.

The flags

Every result also sets single bits in the FLAGS register. Later instructions read them: the jumps of lesson 6 decide by them.

; What ADD and SUB do to the flags.
.MODEL SMALL
.STACK 100H
.CODE
MAIN PROC
    MOV AL, 200
    ADD AL, 100       ; 300 does not fit in a byte: AL = 44, CF = 1
    MOV AL, 127
    ADD AL, 1         ; 128 is too big for a signed byte: OF = 1, SF = 1
    MOV AL, 5
    SUB AL, 5         ; 0: ZF = 1
    MOV AL, 3
    SUB AL, 5         ; below 0: AL = FEh (-2), CF = 1, SF = 1
    INC AL            ; FFh (-1); INC leaves CF alone
    NEG AL            ; 0 - (-1) = 1
    MOV AH, 4CH
    INT 21H
MAIN ENDP
END MAIN

Step through it and watch the flags panel. 200 + 100 = 300 does not fit in a byte (0 to 255), so AL keeps 300 − 256 = 44 and CF = 1. 127 + 1 = 128 fits unsigned, but as a signed byte (−128 to 127) it overflows, so OF = 1 and SF = 1: the bits now read as −128.

One pattern, two meanings

The bits 11111111b are 255 as an unsigned number and −1 as a signed number (two's complement: to negate, flip every bit and add 1). The CPU does not know which you mean; it sets CF for the unsigned reading and OF for the signed one, and you choose which flag to look at. NEG AX with AX = 5 gives FFFBh, which is −5.

INC and DEC leave CF unchanged, which loops use to keep a carry across steps.

Numbers bigger than 16 bits

ADD a pair of registers at a time: add the low words with ADD, then the high words with ADC, which also adds the carry from the low words:

; Add two 32-bit numbers: 0001FFFFh + 00000001h = 00020000h.
.MODEL SMALL
.STACK 100H
.CODE
MAIN PROC
    MOV DX, 0001H     ; first number, high word
    MOV AX, 0FFFFH    ; first number, low word
    MOV CX, 0000H     ; second number, high word
    MOV BX, 0001H     ; second number, low word
    ADD AX, BX        ; low words: FFFFh + 1 = 0000h, CF = 1
    ADC DX, CX        ; high words plus the carry: 1 + 0 + 1 = 2, so DX:AX = 0002:0000h
    MOV AH, 4CH
    INT 21H
MAIN ENDP
END MAIN

Mark lesson complete