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AY2018/19 Semester 2 - 1 of 5 - TIC2401 Lab Assignment
TIC2401 Introduction to Computer Systems
Lab Assignment 1
(Due 8th March 23:59)
Name: ________________________________ Student No.: ____________________
SUBMISSION: Zip this document together with the TWO .asm files (task1.asm &
arrayCount.asm). Rename the file in the format: “Lab_Assignment1_Axxxxxxxx.zip”
and upload to the LumiNUS Submission folder. Please note that Axxxxxxx is your student
number that starts with the letter ‘A’.
This document and its associated files (messages.asm and arrayCount.asm) can be
downloaded from LumiNUS.
Reading and Writing Message to Console Window: messages.asm
To request a service, a program loads the system call code into register $v0 and arguments
into registers $a0 – $a3 (see Figure A.9.1 below). System calls that return values put their
results in register $v0. For this lab, we are interested in only the following system calls:
print_string, print_int, read_int and exit.
AY2018/19 Semester 2 - 2 of 5 - TIC2401 Lab Assignment
For example, the following code in messages.asm prints “the answer = 5”.
# messages.asm
str: .asciiz "the answer = "
li $v0, 4 # system call code for print_string
la $a0, str # address of string to print
syscall # print the string
li $v0, 1 # system call code for print_int
li $a0, 5 # integer to print
syscall # print the integer
li $v0, 10 # system call code for exit
syscall # terminate program
The print_string system call (system call code 4) is passed a pointer (memory address)
to a null-terminated string, which it writes to the console. The print_int system call
(system call code 1) is passed an integer and it prints the integer on the console. The exit
system call (system call code 10) indicates the end of the program.
The li (load immediate) and la (load address) are pseudo-instructions (refer to Lab #3).
Run the above program to verify your understanding.
Task 1: Modify messages.asm [2 marks]
Modify messages.asm and call the new program task1.asm. The modified program
should read the value to be printed from the console before printing the value. The system call
read_int reads an entire line of input up to and including the newline. Characters following
the number are ignored. Note that read_int modifies the register $v0 (where you put the
code for system call) as it returns the integer value in register $v0.
The following screen capture shows a run of the program. The first line is your input, and the
second line is the output of your program.
Ensure that your new program is working as expected before proceeding.
AY2018/19 Semester 2 - 3 of 5 - TIC2401 Lab Assignment
Task 2: Getting Real (arrayCount.asm) [18 marks]
When we discuss MIPS code in the lecture, it is common to see the “variable mappings” list.
The list indicates how certain program variables are “mapped” to their respective registers. In
this task, we are going to actually perform these mappings.
First, let us learn about allocating memory space for variables in a program. The assembler
directive ".data" allows us to reserve memory space in the data segment. These reserved
locations are used to store the values of various program variables during program execution.
Key idea: Values of program variables are stored in the memory. We load them into
registers (perform a mapping) only when we want to manipulate or access them during
This is because register is a fast storage in the processor, while memory is a much slower
storage outside the processor. As the access speed is not simulated in the QtSpim, the
separation and mapping between memory and register may seem strange to you. In real
processor, the difference in access speed of register versus memory can be much more than 10
The problem statement now reads:
Count the number of multiples of X in a given array of 8 non-negative numbers,
where X is a user chosen power-of-two value, e.g. 1, 2, 4, 8, ….
Download arrayCount.asm from LumiNUS. The initial content of the file is:
# arrayCount.asm
arrayA: .word 1, 0, 2, 0, 3 # arrayA has 5 values
count: .word 999 # dummy value
# code to setup the variable mappings
add $zero, $zero, $zero # dummy instructions, can be removed
# code for reading in the user value X
# code for counting multiples of X in arrayA
# code for printing result
# code for terminating the program
li $v0, 10
The main routine contains several dummy instructions (instructions with no real effect) so that
you can step through the program to observe the content in the data segment.
AY2018/19 Semester 2 - 4 of 5 - TIC2401 Lab Assignment
Where is the array arrayA located in the data segment? Give the base address (starting
address) of the array:
arrayA is at 0x______________________
Where is the program variable count in the data segment?
count is at 0x_______________________
(Hint: Don’t forget that 999 is in decimal…)
The given code only allocates 5 elements for arrayA. Enlarge the array to size 8. You can
place any valid integer values for the new locations. Fill in the assembler directive below:
arrayA: ____________________________________
Now, let us perform the following mappings:
Base address of arrayA  $t0 (similar to notation used in lectures)
count  $t8
You may use the "la" (load address) instruction here to help. Give the instruction sequence
(which may consist of 1 or 2 instructions) below:
To map arrayA: _____________________
To map count : _____________________
We are almost ready to tackle the task. One last obstacle is to figure out how to check for
“Multiples of X, where X is a power-of-two”. Recall that in Tutorial #2, we learned that
“andi” instruction can be used to find the remainder of division by 16. For the following
questions, give the correct mask for the andi instruction to compute “$t4 = $t3 % X”.
If X is 32, andi $t4, $t3, 0x______
If X is 8, andi $t4, $t3, 0x______
Observe that we can easily generate the mask from X. If X is stored in register $t8, complete
the following instruction to generate the mask in register $t5. (Hint: look at the mask as a
______ $t5, $t8, ______ (fill in the operation and the last operand)
AY2018/19 Semester 2 - 5 of 5 - TIC2401 Lab Assignment
We are now ready to finish off the task. Write the necessary code to:
a. Read user input value, X. You can assume X is always a power-of-two integer, i.e.
there is no need to check for invalid user input.
b. Count the number of multiples of X in arrayA and print the result on screen.
You should use loop wherever appropriate, or full credit will not be given. Sample code can
be found in Lecture (slides 32-34). Here’s a sample output screenshot for a
predefined array {11, 0, 31, 22, 9, 17, 6, 9} and user input of X = 2. The
output is 3 as there are three multiples of 2 in the array: 0, 22 and 6.
Try to use different values in your code to test. Also, please make sure the “count” value is
properly recorded in the data segment at the end of execution.
Total marks: 20.
User input X = 2
Three multiples of 2: 0, 22 and 6