Loop Instructions 8086 Microprocessor 8086 Microprocessor

Loop using DEC and JMP Instructions

We are familiar with the conditional branch instructions which as shown below could be used for repetitive tasks. Looping using these technique involve more than one instruction as shown in following format

MOV CX, N

Begin : do some operation

operations

DEC CX JNZ Begin

Loop Instructions in 8086:

loop instructions are used to simplify the decrementing, testing and branching portion of the loop. In the above case this portion required two instructions, but in more complicated situation may require more than two instructions.
The loop instruction for 8086 all have the form:

OPCODE

where D8 is byte displacement from current IP. The loop instruction is simplified to:
With loop instruction the format for branch become:

MOV CX, N

Begin: — —

LOOP Begin

The Loop Instructions supported by 8086 are given below:

name	Mnemonic and format	Alternate Form	Test Condition
LOOP	LOOP Opr		CX not equal to ‘0’
Loop while zero, or equal	LOOPZ Opr	LOOPE Opr	ZF=1 and CX not zero
Loop while non zero or not equal	LOOPNZ OPR	LOOPNE OPR	ZF=0 and CX not 0
Branch on CX	JCXZ OPR		CX = 0

Programming Examples

Program for adding an array of binary numbers

Address	Hex Code	Mnemonics	Comment
		Data SEGMENT
0000		NUM DB 12h,23h,44h,67h,32h,89h,38h,98h,65h,55h
000A		N DB 5
000B		Total DB 10 dup(?)
		CODE SEGMENT
		ASSUME CS: CODE, DS:Data
0000		MOV AX, data
		MOV DS, AX
		MOV CX, N	;set counter
		MOV AX, 0
		MOV SI, 0
		START: ADD AX, NUM[SI]
		ADD SI, 2
		LOOP START
		MOV Total, AX
		CODE ENDS
		END

Program to add two 16 digit packed BCD numbers
The flow chart below shows the procedure to add packed BCD numbers

Address	Hex Code	Mnemonics	Comment
		Data SEGMENT
0000		Augend DB 12,23,44,67,32,89,38,98,65,55,22,14,43,56,66,77
0010h		Addend DB 22,23,43,61,12,19,48,68,33,55,26,28,82,92,88,19
0020h		Sum DB 16 dup(?)
		CODE SEGMENT
		ASSUME CS: CODE, DS:Data
0000		MOV AX, data
		MOV DS, AX
		MOV CX, 8	;set counter
		MOV AX, 0
		MOV SI, 0
		START: MOV AL, Augend[SI]
		ADC AL, Addend[SI]
		DAA
		MOV Sum[SI], AL
		INC SI
		LOOP START
		CODE ENDS
		END

Program to search space character in a string
The following flowchart shows the method of searching a character (space) in an array of characters(string)

Address	Hex Code	Mnemonics	Comment
		Data SEGMENT
0000		my_name DB “Ravinder Nath”
000Dh		msg DB “Space Found”
0019h		msg1 DB “Space Not Found”
		CODE SEGMENT
		ASSUME CS: CODE, DS:Data
0000		MOV AX, data
		MOV DS, AX
		MOV CX, LENGTH my_name	;set counter
		MOV SI, -1
		MOV AL, 20h
		Next:
		INC SI
		CMP AL, my_name[SI]
		LOOPNE Next
		JNZ Not_found
		LEA BX, msg
		MOV AH, 09h
		INT 21h
		JMP Last
		Not_found:
		LEA BX, msg1
		MOV AH, 09h
		INT 21h
		Last:
		CODE ENDS
		END

Sort an array of numbers in descending order using Bubble sort
The flow chart below shows the sorting technique using bubble sort.

Address	Hex Code	Mnemonics	Comment
		Data SEGMENT
0000		num DB 12,34,76,1,45,34,76,56,78,23,21,31
000Bh		sorted_num DB 10 dup(?)
		CODE SEGMENT
		ASSUME CS: CODE, DS:Data
0000		MOV AX, Data
		MOV DS, AX
		MOV CX, 06h	;set counter
		DEC CX
		for_Loop1:
		MOV DI, CX
		MOV BX, 0
		for_Loop2:
		MOV AX, num[BX]
		CMP AX, num[BX+2]
		JGE CONTINUE
		XCHG AX, num[BX+2]
		MOV sorted_array[BX], AX
		CONTINUE:
		ADD BX, 2
		LOOP for_Loop2
		MOV CX, DI
		LOOP for_Loop1
		CODE ENDS
		END

Programming Examples

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