Assembly language CMP instructions

The CMP (Compare) instruction performs an implicit subtraction operation of subtracting the source operand from the destination operand without modifying any operands.

Instruction format:

CMP destination operand, source operand

Flags When the actual subtraction occurs, the CMP instruction modifies the overflow, sign, zero, carry, auxiliary carry, and parity flags according to the calculation result. If two unsigned numbers are compared, the relationship between the two operands represented by the zero flag and the carry flag is as shown in the following table:

If two signed numbers are compared, the relationship between the two operands represented by the sign flag, zero flag and overflow flag is as shown in the following table:

The CMP instruction is an important tool for creating conditional logic structures. When using CMP in a conditional jump instruction, the execution result of the assembly language is the same as that of the IF statement.

The following is an example of destination operand < source operand:

; ZF CF

mov ax, 5

cmp ax, 10 ; 0 1

The following is an example of destination operand = source operand:

                        ; ZF CF

mov ax, 1000

mov cx, 1000

cmp cx, ax ; 1 0

The following is an example of destination operand > source operand:

   ;ZF CF

mov ax, 105

cmp ax, 0 ;0 0

Use the CMP instruction to count the number of bytes with a value of 8 in the data segment, and use ax to save the statistical results.

; 1. Program to count the number of bytes with a value of 8 in the data segment, and use ax to save the statistical results

assume cs:code, ds:data, ss:stack

data segment

db 8, 11, 8, 1, 8, 5, 63, 38

data ends

stack segment stack

db 128 dup(0)

stack ends

code segment

start: mov ax, stack

mov ss,ax

mov sp, 128

call init_reg

call get_eight

mov ax, 4C00H

int 21H

;============================================

get_eight:

movsi,0

mov cx, 8

mov ax, 0

getEight: cmp byte ptr ds:[si], 8

jne nextNumber

inc ax

nextNumber: inc si

loop getEight

ret

;================================================

init_reg:

mov bx, data

mov ds, bx

ret

code ends

end start

Use the CMP instruction to count the number of bytes in the data segment with a value greater than 8, and use ax to save the statistical results.

; 2. Program to count the number of bytes in the data segment with a value greater than 8, and use ax to save the statistical results.

assume cs:code, ds:data, ss:stack

data segment

db 8, 11, 8, 12, 8, 5, 63, 38

data ends

stack segment stack

db 128 dup(0)

stack ends

code segment

start: mov ax, stack

mov ss,ax

mov sp, 128

call init_reg

call get_eight

mov ax, 4C00H

int 21H

;============================================

get_eight:

movsi,0

mov cx, 8

mov ax, 0

getEight: cmp byte ptr ds:[si], 8

jna nextNumber; ja > na <=

inc ax

nextNumber: inc si

loop getEight

ret

;================================================

init_reg:

mov bx, data

mov ds, bx

ret

code ends

end start

Based on signed examples:

mov al, 1

movbl, 2

cmp al, bl → infer al < bl

sub al, bl → influence flag 1 - 2 = -1 SF symbol flag position 1

mov al, 22H → 34 34 - (-96) = 140 -128 ~ 127

mov bl, A0H -96

sub al, bl → OF flag bit and SF flag bit, overflow flag position 1, symbol flag position 1

mov al, 8AH -118 - 112 = - 230 overflow 1A positive number

movbl, 70H

cmp al, bl OF = 1 SF = 0 Overflow flag position 1 Symbol flag position 0

Summary: based on signed comparison

Let cmp al, bl

if SF = 1 OF = 0

Then al < bl

If SF = 1 OF =1

Then al > bl

if SF = 0 OF = 1     

Because SF = 0

The inequality should be al - bl > 0 al > bl        

When OF = 1 holds, then

al < bl

if SF = 0 OF = 0    

Then al - bl > 0 

get al > bl

If the actual result is negative due to overflow, then logically the real result must be positive.

If the actual result is positive due to overflow, then logically the real result must be negative.

Jump based on unsigned comparison

Jump based on equality

Jump based on signed comparison

Jumps based on carry and zero flags