![INCLUDE Irvine32.inc
.data
intarray DD 10000h,20000h,30000h,40000h
.code
main PROC
mov edi,OFFSET intarray ; 1: EDI = address of intarray
mov ecx,LENGTHOF intarray ; 2: initialize loop counter
mov eax,0 ; 3: sum = 0
L1: ; 4: mark beginning of loop
add eax,[edi] ; 5: add an integer
add edi,TYPE intarray ; 6: point to next element
loop L1
Call writedec ; 7: repeat until ECX = 0
exit
main ENDP
END main](https://image.slidesharecdn.com/loopupdated-241216141548-adef49ce/85/Loop-Updated-pptxLoop-Updated-pptxLoop-Updated-pptxLoop-Updated-pptxLoop-Updated-pptxLoop-Updated-pptx-11-320.jpg)
![Example contd.
L1:
mov al,source[esi] ; get a character from
source
mov target[esi],al ; store it in the target
inc esi ; move to next character
loop L1
exit
main ENDP
END main](https://image.slidesharecdn.com/loopupdated-241216141548-adef49ce/85/Loop-Updated-pptxLoop-Updated-pptxLoop-Updated-pptxLoop-Updated-pptxLoop-Updated-pptxLoop-Updated-pptx-18-320.jpg)
Loop Updated.pptxLoop Updated.pptxLoop Updated.pptxLoop Updated.pptxLoop Updated.pptxLoop Updated.pptx
- 1. Computer Organization and Assembly Language Loop Introduction
- 2. The LOOP instruction, formally known as Loop According to ECX Counter, repeats a block of statements a specific number of times. ECX is automatically used as a counter and is decremented Its syntax is LOOP destination
- 3. The execution of the LOOP instruction involves two steps: First, it subtracts 1 from ECX. Next, it compares ECX to zero. If ECX is not equal to zero, a jump is taken to the label identified by destination. Otherwise, if ECX equals zero, no jump takes place, and control passes to the instruction following the loop.
- 4. Rarely should you explicitly modify ECX inside a loop. If you do, the LOOP instruction may not work as expected. In the following example, ECX is incremented within the loop. It never reaches zero, so the loop never stops: mov ecx,5 top: Inc ecx loop top
- 5. In the following example, we add 1 to AX each time the loop repeats.When the loop ends, EAX 5 and ECX 0: mov eax,0 mov ecx,5 L1: inc eax loop L1
- 6. A common programming error is to inadvertently initialize ECX to zero before beginning a loop. If this happens, the LOOP instruction decrements ECX to FFFFFFFFh, and the loop repeats 4,294,967,296 times!
- 7. while( eax < ebx) eax = eax + 1; Consider the following example: top: cmp eax,ebx ; check loop condition jae next ; false? exit loop inc eax ; body of loop jmp top ; repeat the loop next:
- 8. If you need to modify ECX inside a loop, you can save it in a variable at the beginning of the loop and restore it just before the LOOP instruction: .data count DWORD ? .code mov ecx,100 ; set loop count top: mov count,ecx ; save the count mov ecx,20 ; modify ECX mov ecx,count ; restore loop count loop top
- 9. When creating a loop inside another loop, special consideration must be given to the outer loop counter in ECX.You can save it in a variable:
- 10. .data count DWORD ? .code mov ecx,100 ; set outer loop count L1: mov count,ecx ; save outer loop count mov ecx,20 ; set inner loop count L2: loop L2 ; repeat the inner loop mov ecx,count ; restore outer loop count loop L1
- 11. INCLUDE Irvine32.inc .data intarray DD 10000h,20000h,30000h,40000h .code main PROC mov edi,OFFSET intarray ; 1: EDI = address of intarray mov ecx,LENGTHOF intarray ; 2: initialize loop counter mov eax,0 ; 3: sum = 0 L1: ; 4: mark beginning of loop add eax,[edi] ; 5: add an integer add edi,TYPE intarray ; 6: point to next element loop L1 Call writedec ; 7: repeat until ECX = 0 exit main ENDP END main
- 12. Character Print INCLUDE Irvine32.inc .code main PROC continue: CALL ReadChar ;char in AL cmp al,0 ;extended key? je stop ;yes, then exit CALL WriteChar ;no, print char jmp continue stop: exit main ENDP END main
- 13. LOOPZ and LOOPE Instructions The LOOPZ (loop if zero) instruction works just like the LOOP instruction except that it has one additional condition: the Zero flag must be set in order for control to transfer to the destination label.The syntax is LOOPZ destination
- 14. LOOPZ and LOOPE Instructions cont. The LOOPE (loop if equal) instruction is equivalent to LOOPZ, and they share the same opcode.They perform the following tasks: ECX = ECX - 1 if ECX > 0 and ZF = 1 ; jump to destination
- 15. LOOPNZ and LOOPNE Instructions The LOOPNZ (loop if not zero) instruction is the counterpart of LOOPZ. The loop continues while the unsigned value of ECX is greater than zero (after being decremented) and the Zero flag is clear.The syntax is LOOPNZ destination
- 16. LOOPNZ and LOOPNE Instructions contd. The LOOPNE (loop if not equal) instruction is equivalent to LOOPNZ, and they share the same opcode.They perform the following tasks: ECX = ECX - 1 if ECX > 0 and ZF = 0 , jump to destination Otherwise, nothing happens, and control passes to the next instruction.
- 17. Example code INCLUDE Irvine32.inc .data source BYTE "This is the source string",0 target BYTE SIZEOF source DUP(0) .code main PROC mov esi,0 ; index register mov ecx,SIZEOF source ; loop counter
- 18. Example contd. L1: mov al,source[esi] ; get a character from source mov target[esi],al ; store it in the target inc esi ; move to next character loop L1 exit main ENDP END main
- 19. Enter a no. 7 49 36 25 16 9 4 1
- 20. Code of previous slide Task include irvine32.inc .data s1 byte "Enter a number:",0 .code main Proc mov edx,offset s1 call writestring call readint mov ecx,eax L1: mov eax,ecx mul eax call writedec call crlf loop L1 exit main endP end
- 21. Enter a number: 9 You have entered odd number Are you want to play again press any number except 0? 1 Enter a number: 8 You have entered even number Are you want to play again press any number except 0? 0