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

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
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 −
Following are the conditional jump instructions used on unsigned data used for logical operations −
The following conditional jump instructions have special uses and check the value of flags −
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: ...
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, 22 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


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