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 | D8 |
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 |