Conditional instructions are used in assembly language to control the flow of a program. Conditional statements in assembly language are used through the program to direct the computer to make decisions when certain conditions are given. These decisions are made only when pre-stated conditions are true or false, depending on what the programmer has in mind.
Conditional instructions in assembly language - cms jmp lable
Conditional statements are used in assembly language to control the flow of the program. i.e. which code to run next if the pre-stated conditions are true or false. Like - if, elas, jmp, loop etc.
jump Conditions
There are 2 types of jump statements in assembly language. - Unconditional jump, Conditional jump.
- Unconditional jump - There is no condition before this instruction. When this instruction is run, the program runs the code block specified by this instruction first, skipping the further code. This is done by JMP instruction.
- Conditional jump - This instruction is preceded by a condition. When this instruction is run, it checks the condition, then depending on whether the condition is true or false, the program runs the next code or directed code block. This is done by JMP instruction. This is done by a set of jump instructions j < condition>.
CMP Instruction
The CMP instruction compares two operands or numeric data fields. It is commonly used in conditional execution. This instruction basically compares one operand by subtracting another to see if the operands are equal or not. It does not disturb the destination or source operands. It is used with conditional jump instructions to make decisions.
Syntax CMP destination, sourceFor example
INC EDX
CMP EDX, 10 ; Compares whether the counter has reached 10
JLE LP1 ; If it is less than or equal to 10, then jump to LP1CMP is often used to compare whether the counter value has reached the number of times the loop needs to be run.
What is a label in assembly language code?
What is a label in assembly language code? A label is a symbol that represents the memory address of an instruction or data. Addresses can be PC-relative, register-relative, or absolute. Labels are local to the source file unless you make them global using the export directive. You can think of it like an "id" in markup language.
Unconditional jump
Unconditional jump is done by JMP keyword. This JMP instruction jumps from one code block to another without any condition.
Syntax JMP labelFor example
MOV AX, 00 ; Initializing AX to 0
MOV BX, 00 ; Initializing BX to 0
MOV CX, 01 ; Initializing CX to 1
L20:
ADD AX, 01 ; Increment AX
ADD BX, AX ; Add AX to BX
SHL CX, 1 ; shift left CX, this in turn doubles the CX value
JMP L20 ; repeats the statementsConditional jump
Conditional execution often involves transfer of control to an instruction address. If certain specified conditions are met in a conditional jump, the control flow is transferred to the target instruction. There are many conditional jump instructions based on condition and data.
In nasm assembly language, we can divide conditional jump instructions into three categories on the basis of usage. Arithmetic, logical, special uses.
Arithmetic Jump Instruction
| Instruction | Description | Flags tested |
|---|---|---|
| JE/JZ | Jump Equal or Jump Zero | ZF |
| JNE/JNZ | Jump not Equal or Jump Not Zero | ZF |
| JG/JNLE | Jump Greater or Jump Not Less/Equal | OF, SF, ZF |
| JGE/JNL | Jump Greater/Equal or Jump Not Less | OF, SF |
| JL/JNGE | Jump Less or Jump Not Greater/Equal | OF, SF |
| JLE/JNG | Jump Less/Equal or Jump Not Greater | OF, SF, ZF |
Logical Jump Instructions
| Instruction | Description | Flags tested |
|---|---|---|
| JE/JZ | Jump Equal or Jump Zero | ZF |
| JNE/JNZ | Jump not Equal or Jump Not Zero | ZF |
| JA/JNBE | Jump Above or Jump Not Below/Equal | CF, ZF |
| JAE/JNB | Jump Above/Equal or Jump Not Below | CF |
| JB/JNAE | Jump Below or Jump Not Above/Equal | CF |
| JBE/JNA | Jump Below/Equal or Jump Not Above | AF, CF |
Special uses and check the value of the flags
| Instruction | Description | Flags tested |
|---|---|---|
| JXCZ | Jump if CX is Zero | none |
| JC | Jump If Carry | CF |
| JNC | Jump If No Carry | CF |
| JO | Jump If Overflow | OF |
| JNO | Jump If No Overflow | OF |
| JP/JPE | Jump Parity or Jump Parity Even | PF |
| JNP/JPO | Jump No Parity or Jump Parity Odd | PF |
| JS | Jump Sign (negative value) | SF |
| JNS | Jump No Sign (positive value) | SF |
j<condition>For example
CMP AL, BL
JE EQUAL
CMP AL, BH
JE EQUAL
CMP AL, CL
JE EQUAL
NON_EQUAL: ...
EQUAL: ...Exampal
The following program displays the largest of three variables. The variables are double-digit variables. The three variables num1, num2 and num3 have values 37, 22 and 41, respectively −
section .text
global _start ;must be declared for using gcc
_start: ;tell linker entry point
mov ecx, [num1]
cmp ecx, [num2]
jg check_third_num
mov ecx, [num2]
check_third_num:
cmp ecx, [num3]
jg _exit
mov ecx, [num3]
_exit:
mov [largest], ecx
mov ecx,msg
mov edx, len
mov ebx,1 ;file descriptor (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
mov ecx,largest
mov edx, 2
mov ebx,1 ;file descriptor (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
mov eax, 1
int 80h
section .data
msg db "The largest digit is: ", 0xA,0xD
len equ $- msg
num1 dd '37'
num2 dd '22'
num3 dd '41'
segment .bss
largest resb 2 When the above code is compiled and executed, it produces the following result −
41
