Assembly - Conditions

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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:

SNConditional Instructions
1Unconditional 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.
2Conditional 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.

The 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:

InstructionDescriptionFlags tested
JE/JZJump Equal or Jump ZeroZF
JNE/JNZJump not Equal or Jump Not ZeroZF
JG/JNLEJump Greater or Jump Not Less/EqualOF, SF, ZF
JGE/JNLJump Greater or Jump Not LessOF, SF
JL/JNGEJump Less or Jump Not Greater/EqualOF, SF
JLE/JNGJump Less/Equal or Jump Not GreaterOF, SF, ZF

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

InstructionDescriptionFlags tested
JE/JZJump Equal or Jump ZeroZF
JNE/JNZJump not Equal or Jump Not ZeroZF
JA/JNBEJump Above or Jump Not Below/EqualCF, ZF
JAE/JNBJump Above/Equal or Jump Not BelowCF
JB/JNAEJump Below or Jump Not Above/EqualCF
JBE/JNAJump Below/Equal or Jump Not AboveAF, CF

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

InstructionDescriptionFlags tested
JXCZJump if CX is Zeronone
JCJump If CarryCF
JNCJump If No CarryCF
JOJump If OverflowOF
JNOJump If No OverflowOF
JP/JPEJump Parity or Jump Parity EvenPF
JNP/JPOJump No Parity or Jump Parity OddPF
JSJump Sign (negative value)SF
JNSJump 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, [num3]
   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


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