Assembly Jump Instructions

With jump instructions we can control the program flow. There are different types of jumps:

Unconditional jumps

With JMP command we can move to another point in the program.

JMP label   ;syntax of JMP instruction

In order to run jump instruction we should have a label. Labels are simple checkpoints to describe it to computer. We can simply declare label with adding ”:” to the end. Like other languages we cannot start with a number. Here are examples for labels:

label1: MOV AX, 1
label2: MOV AX, 2

Now we can try what we learned on example:

.MODEL SMALL

.STACK 64

.CODE
  MOV AX, 5               ;set AX to 5
  MOV BX, 2               ;set BX to 2

  JMP label1              ;go to label1

  label2: JMP label3      ;go to label3

  label1:
  ADD AX, BX              ;add BX to AX and store it in AX
  JMP label2              ;go to label2

  label3:
  MOV AX, 4C00H           ;terminate
  INT 21H

END                       ;exit

Short Conditional Jumps

With JMP instruction we can do unconditional jump but sometimes we only want to jump when some conditions are in act. To make it happen we have 3 different groups of other instructions.

Jump instructions that test single flag

InstructionDescriptionConditionJZ, JEJump if zero (equal)ZF = 1JC, JB, JNAEJump if carry (below, not above equal)CF = 1JSJump if signSF =1JOJump if overflowOF = 1JPE, JPJump if parity evenPF = 1 All instructions have their opposites. Like JZ (Jump if zero) with JNZ (Jump if not zero). Also some instructions do the same thing like JZ (Jump if zero) and JE (Jump if equal).

Jump instructions for signed numbers

InstructionDescriptionConditionJE, JZJump if equal (zero)ZF = 1JG, JNLEJump if greater (not less or equal)ZF = 0 and SF = OFJL, JNGEJump if less (not greater or equal)SF <> OFJGE, JNLJump if greater or eqaul (not less)SF = OFJLE, JNGJump if less or equal (not greater)ZF = 1 or SF <> OF <> is means not equal, in other words !=

Jump instructions for unsigned numbers

InstructionDescriptionConditionJE, JZJump if equal (zero)ZF = 1JA, JNBEJump if above (not below or equal)CF = 0 and ZF = 0JB, JNAE, JCJump if below (not above or equal, carry)CF = 1JAE, JNB, JNCJump if above or equal (not below, not carry)CF = 0JBE, JNAJump if below or equal (not above)CF = 1 or ZF = 1 Ok, that’s look complicated. I want to give you an example to make it understandable.

.MODEL SMALL

.STACK 64

.CODE
  MOV AL, 25
  MOV BL, 10
  CMP AL, BL      ;compare AL with BL (I'll explain it later...)
  JE equal        ;if AL = BL, go to equal label
  MOV CL, "N"     ;if it's run here that means AL <> BL
  JMP stop        ;go to stop and move 'N' to CL

  equal:          ;if it's come here
  MOV CL, "Y"     ;move 'Y' to CL

  stop:
  MOV AH, 02H     ;set interrupt
  MOV DL, CL      ;set interrupt message
  INT 21H

  MOV AX, 4C00H   ;terminate
  INT 21H

END               ;exit

With this program we can compare 2 integers and tell the user are they equal or not. If you look carefully you’ll see CMP command, it’s comparing 2 integers. If they are equal we move the flow to equal label, if not we are not changing the flow.