Assembly Jump Instructions

date 15.11.2020

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. ```asm JMP label ;syntax of JMP instruction ```asm

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

Now we can try what we learned on example: ```asm .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 ```asm

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. ```asm .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 ```asm

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.