Assembly - Conditions

Conditional execution in assembly language is accomplished by several looping and branching instructions. These instructions can change the flow of control in a program. Conditional execution is observed in two scenarios −

SN	Conditional Instructions
1	Unconditional jump This is performed by the JMP instruction. Conditional execution often involves a transfer of control to the address of an instruction that does not follow the currently executing instruction. Transfer of control may be forward, to execute a new set of instructions or backward, to re-execute the same steps.
2	Conditional jump This is performed by a set of jump instructions j<condition> depending upon the condition. The conditional instructions transfer the control by breaking the sequential flow and they do it by changing the offset value in IP.

Let us discuss the CMP instruction before discussing the conditional instructions.

CMP Instruction

The CMP instruction compares two operands. It is generally used in conditional execution. This instruction basically subtracts one operand from the other for comparing whether the operands are equal or not. It does not disturb the destination or source operands. It is used along with the conditional jump instruction for decision making.

Syntax

CMP destination, source

CMP compares two numeric data fields. The destination operand could be either in register or in memory. The source operand could be a constant (immediate) data, register or memory.

Example

CMP DX, 00  ; Compare the DX value with zero
JE  L7      ; If yes, then jump to label L7
.
.
L7: ...  

CMP is often used for comparing whether a counter value has reached the number of times a loop needs to be run. Consider the following typical condition −

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 LP1

Unconditional Jump

As mentioned earlier, this is performed by the JMP instruction. Conditional execution often involves a transfer of control to the address of an instruction that does not follow the currently executing instruction. Transfer of control may be forward, to execute a new set of instructions or backward, to re-execute the same steps.

Syntax

The JMP instruction provides a label name where the flow of control is transferred immediately. The syntax of the JMP instruction is −

JMP label

Example

The following code snippet illustrates the JMP instruction −

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 statements

Conditional Jump

If some specified condition is satisfied in conditional jump, the control flow is transferred to a target instruction. There are numerous conditional jump instructions depending upon the condition and data.

Following are the conditional jump instructions used on signed data used for arithmetic operations −

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

Following are the conditional jump instructions used on unsigned data used for logical operations −

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

The following conditional jump instructions have special uses and check the value of 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

The syntax for the J<condition> set of instructions −

Example,

CMP AL, BL
JE EQUAL
CMP AL, BH
JE EQUAL
CMP AL, CL
JE EQUAL
NON_EQUAL: ...
EQUAL: ...

Example

The following program displays the largest of three variables. The variables are double-digit variables. The three variables num1, num2 and num3 have values 47, 72 and 31, 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 '47'
   num2 dd '22'
   num3 dd '31'

segment .bss
   largest resb 2  

When the above code is compiled and executed, it produces the following result −

The largest digit is: 
47