CSE341 Lab 05 Flow control instructions and Looping structures Solution

I. Topic Overview:
Alongside the capability of making decisions, any functional program should also have a mechanism to repeat sections of code. In this lab, students will familiarize themselves with the loop instructions to achieve that. Alongside jump, the loop instruction is also used to transfer control to another part of the program. The students will then learn to implement different looping structures. This application will make it much easier to convert a pseudo code algorithm to assembly code.

II. Lesson Fit:
There is prerequisite to this lab. Students must have a basic idea on the following concepts:
a. Jump instruction
b. Some basic operations such as MOV, ADD, SUB, MUL and DIV
c. Basic I/O operations
d. Character encoding using ASCII

III. Learning Outcome:
a. Control flow of the program
b. Use loops to avoid repetition



IV. Anticipated Challenges and Possible Solutions
i. Step by step simulation

V. Acceptance and Evaluation
Code: 50%
Viva: 50% VI. Activity Detail
a. Hour: 1
Discussion: Looping Structure
Just like for and while loops in high level programming languages, loops can also be implemented in assembly. There are 2 ways of implementing a loop: explicit and implicit.
• Explicit: By using compare and jump instructions to decide whether to enter the loop or not and by using the inc /add /sub instruction for increments.



Java int x = 0;
while (x < 5){
System.out.println(x);
x++;
}

Assembly

mov ah,2
mov dl,30h ;the emu8086 uses hexadecimal. 30h is 0


• Implicit: In this case we do not have to check whether the counter has reached the limit or not. This will be done automatically. The instructions will be loop. loop destination_line is the syntax. CX will be used as the counter always in order for the loop instruction to execute.

JAVA
int x = 0; while (x < 5){
System.out.println(x); x++;
}


Assembly
mov cx,5 ;the bound will be in cx. (the number of times the loop will run) mov dl,30h mov ah,2 start: int 21h inc dl
loop start
NB. CX will always start from the specified count and will always decrement by
1.

Problem Task: Task 01-Task03 (Page 5)

b. Hour: 2
Discussion: Discuss the properties of a repeat-until loop structure.
• Repeat Until loop: Repeat the loop until a condition is satisfied. For example you have been asked to take in characters from the user and print them until a space is pressed.
.code repeat: mov ah,1 int 21h mov ah,2 mov dl,al int 21h cmp al,' ' jne repeat
Problem Task: Task 04 – 06 (Page 5-6)

c. Hour: 3
Discussion:
Check progress while the students carry on with the rest of the tasks.
Problem Task: Task 07 (Page 6-7)

VII. Home Tasks: All the unfinished lab tasks.



Lab 5 Activity List
Task 01
Write a count-controlled loop to display a row of 80 stars.
Task 02
Write a sequence of instructions to do each of the following:
a. Put the sum of 1 + 4 + 7+ .. + 148 in AX.
b. Put the sum 100 + 95 + 90 + ... + 5 in AX.
Task 03
Read a five character password and overprint it by executing a carriage return and displaying five X's. You need not store the input characters anywhere.
Task 04
Initialize product to 0
REPEAT
add M to product decrement N
UNTIL N equals 0
Task 05
Write a program to display the extended ASCII characters (ASCII codes 80h to FFh). Display 10 characters per line, separated by blanks. Stop after the extended characters have been displayed once.
Task 06
Write a program that will prompt the user to enter a hex digit character ("0" ... "9" or "A" ... "F"), display it on the next line in decimal, and ask the user if he or she wants to do it again. If the user types "y" or "Y", the program repeats; If the user types anything else, the program terminates. If the user enters an illegal character, prompt the user to try again
Sample execution:
ENTER A HEX DIGIT: 9
IN DECIMAL IT IS 9
DO YOU WANT TO DO IT AGAIN? y
ENTER A HEX DIGIT: c
ILLEGAL CHARACTER - ENTER 0 .. 9 OR A .. F: C
IN DECIMAL IT IS 12
DO YOU WANT TO DO IT AGAIN? N
Task 07
[Hard] Write a program that reads a string of capital letters, ending with a carriage return, and displays the longest sequence of consecutive alphabetically increasing capital letters read. Sample execution:
ENTER A STRING OF CAPITAL LETTERS:
FGHADEFGHC
THE LONGEST CONSECUTIVELY INCREASING STRING IS:
DEFGH