Assembly Language

Lecture 7(Looping Structures)

Looping Structures 1

Lecture Outline

• Introduction• Looping Structures

• FOR Loop• WHILE Loop• REPEAT Loop

Introduction

2Looping Structures

• A loop is a sequence of instructions that is repeated.

• The number of times to repeat may:• Be known in advance, or • Depend on conditions.

• Looping structures:• FOR loop.• WHILE loop.• REPEAT loop.

3Looping Structures

FOR Loop

• This is a loop structure in which the loop statements are repeated a known number of times.

• Pseudocode: FOR loop_count times DO

statements END_FOR

Count=0

Initialize count

Statements

Count = count -1

FalseTrue

4Looping Structures

FOR Loop

• The LOOP instruction can be used to implement a for loop.

• Syntax: LOOP destination_label

• Destination_label must precede the LOOP instruction by no more than 126 bytes.

• The counter for the loop is the register CX, which is initialized to loop_count.

• Execution of the LOOP instruction causes CX to be decremented automatically.

• If (CX < > 0) control transfers to destination_label else the next instruction after LOOP is done.

5Looping Structures

FOR Loop

• Using the instruction LOOP, a FOR loop can be implemented as follows:

; initialize CX to loop_count TOP:

; body of the loopLOOP TOP

6Looping Structures

FOR Loop

• Example: Write some code to display a row of 80 stars.

• Solution: Pseudocode:

FOR 80 times DO display '*'END_IF

It can be coded as follows:MOV CX, 80MOV AH, 2MOV DL, '*'

TOP:INT 21hLOOP TOP

; what if CX =0?

MOV CX, 80MOV AH, 2MOV DL, '*'JCXZ SKIP ;jump if CX=0

TOP:INT 21hLOOP TOP

SKIP:

7Looping Structures

WHILE Loop

• This loop depends on a condition.

• Pseudocode: WHILE condition DO

statements END_WHILE Condition

Statements

False True

8Looping Structures

WHILE Loop

• Example: Write some code to count the number of characters in an input line.

• Solution: Pseudocode:

initialize count to 0read a characterWHILE character <> carriage_return DO count = count + 1

read characterEND_WHILE

continue

9Looping Structures

WHILE Loop

It can be coded as follows:

WHILE_:

END_WHILE:

MOV DX, 0 ; DX counts charactersMOV AH, 1 ; prepare to readINT 21h ; character in AL

CMP AL, 0Dh ; CR?JE END_WHILE ; yes, exitINC DX ; not CR, increment countINT 21h ; read a characterJMP WHILE_ ; loop back

9Looping Structures

REPEAT Loop

• This loop depends on a condition.

• Pseudocode: REPEATStatementsUNTIL conditions

9Looping Structures

REPEAT Loop

• Example: write code to read characters until a blank is read

• Pseudocode: REPEATRead characterUNTIL character is blank The code is:

MOV AH,1REAPEAT: INT 21H CMP AL,’ ‘ JNE REAPEAT:

Looping Structures 12

WHILE Versus REPEAT

• Use of a WHILE loop or a REPEAT loop is a matter of personal preference.

• A WHILE loop can be bypasses if the terminating condition is initially false. (a REPEAT loop must be done at least once)

• The code for a REPEAT loop is likely to be a little shorter because there is only one jump. (WHILE loops has two jumps)