1. Jump & Loop instructions
Lecture 4
Jump & Loop instructions

2. Chapter Outline Jump Instruction IF-THEN-ELSE Loop instruction
For Loop
While Loop
REPEAT Loop

3. IF-THEN-ELSE
Example: Suppose AL and BL contain extended ASCII characters.
Display the one that comes first in the character sequence.
Solution:
Pseudocode:
IF AL <= BL
THEN
display the character in AL
ELSE
display the character in BL
END_IF
continue

4. IF-THEN-ELSE It can be coded as follows: ; if AL <= BL
CMP AL, BL ; AL <= BL?
JNBE ELSE_ ; no, display char in BL
; AL <= BL
MOV DL, AL ; move char to be displayed
JMP DISPLAY ; go to display
ELSE_: ; BL < AL
MOV DL, BL
DISPLAY:
MOV AH, 2 ; prepare to display
INT 21h ; display it

5. Branches with compound Conditions
Sometimes the branching condition in an IF or CASE takes the
form:
condition_1 AND condition_2 or
condition_1 OR condition_2
where condition_1 and condition_2 are either true or false.
AND condition
OR condition

6. AND Condition
An AND condition is true if and only if all conditions are true.
Example: Read a character, and if it’s an uppercase letter, display it.
Solution:
Pseudocode:
Read a character (into AL)
IF ('A' <= character) and (character <= 'Z')
THEN
display character
END_IF
continue

7. AND Condition It can be coded as follows: ; read a character
; if ('A' <= char) and (char <='Z')
; then display char
MOV AH,1 ; prepare to read
INT 21h ; char in AL
CMP AL, 'A' ; char >= 'A'?
JNGE END_IF ; no, exit
CMP AL, 'Z' ; char <= 'Z'?
JNLE END_IF ; no, exit
MOV DL, AL ; get char
MOV AH, 2 ; prepare to display
INT 21h ; display char
END_IF:

8. OR Condition
An OR condition is true if at least one of the conditions is true.
Example: Read a character. If it’s 'y' or 'Y', display it; otherwise,
terminate the program.
Solution:
Pseudocode:
Read a character (into AL)
IF (character = 'y') or (character = 'Y')
THEN
display character
ELSE
terminate the program
END_IF
continue

9. OR Condition It can be coded as follows: ; read a character
; if (char = 'y') or (char = 'Y')
MOV AH,1 ; prepare to read
INT 21h ; char in AL
CMP AL, 'y' ; char = 'y'?
JE THEN ; yes, go to display it
CMP AL, 'Y' ; char = 'Y'?
JMP ELSE_ ; no, terminate
THEN:
MOV DL, AL ; get char
MOV AH, 2 ; prepare to display
INT 21h ; display char
JMP END_IF ; and exit
ELSE_:
MOV AH, 4Ch
INT 21h ; DOS exit
END_IF:

10. Loop Instruction
The LOOP instruction can be used to implement a for loop.
Syntax:
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.
SHORT address
LOOP

11. Loop Instruction
Using the instruction LOOP, a FOR loop can be implemented as
follows:
; initialize CX to loop_count
TOP:
; body of the loop
LOOP TOP
In for loop the loop statements are repeated a known number of times.

12. FOR Loop Example: Write some code to display a row of 80 stars.
Solution:
Pseudocode:
FOR 80 times DO
display '*'
END_IF
; what if CX =0?
MOV CX, 80
MOV AH, 2
MOV DL, '*'
JCXZ SKIP ;jump if CX=0
TOP:
INT 21h
LOOP TOP
SKIP:
It can be coded as follows:
MOV CX, 80
MOV AH, 2
MOV DL, '*'
TOP:
INT 21h
LOOP TOP

13. WHILE Loop This loop depends on a condition.
The condition is checked at the top; if true the statements are executed; if false program goes into whatever follow.
Pseudocode:
WHILE condition DO
statements
END_WHILE

14. WHILE Loop
Example: Write some code to count the number of characters in an
input line.
Solution:
Pseudocode:
initialize count to 0
read a character
WHILE character <> carriage_return DO
count = count + 1
read character
END_WHILE
continue

15. WHILE Loop It can be coded as follows:
END_WHILE:
MOV DX, 0 ; DX counts characters
MOV AH, 1 ; prepare to read
INT 21h ; character in AL
CMP AL, 0Dh ; CR?
JE END_WHILE ; yes, exit
INC DX ; not CR, increment count
INT 21h ; read a character
JMP WHILE_ ; loop back

16. REPEAT Loop This loop depends on a condition.
the statements are executed then the condition is checked. If true the loop terminates; if false, control branches to the top of the loop.
Pseudocode:
REPEAT
Statements
UNTIL conditions

17. REPEAT Loop
Example: write code to read characters until a blank is read
Pseudocode:
REPEAT
Read character
UNTIL character is blank
The code is:
MOV AH,1
REAPEAT:
INT 21H
CMP AL,’ ‘
JNE REAPEAT

18. 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)