CS2100 Lab #5: Making Function Calls Solution

[ This document is available on LumiNUS and module website http://www.comp.nus.edu.sg/~cs2100 ]


Name: ________________________________ Student No.: ____________________
Lab Group: _____
Note:
Objective
In this lab, you will use QtSpim to explore the idea of function calls in MIPS Assembly Code. This document and its associated files (sayHi.asm and arrayFunction.asm) can be downloaded from the CS2100 module website or found in the ZIP file.
Just like any high level programming language, modularization (separating code into welldefined procedures/functions) is an important idea for assembly programming. Conceptually, making function call is actually simple: we need to "jump" to another portion of code (the function body) then start executing the instructions in the function body. When we reach the end of that function, another "jump" is needed to go back to the caller.
0x3024 #Body of Function F
0x3028 #Body of Function F
0x302C #Done! Jump back to caller


So, the simplest kind of function call can be accomplished by just two "jump" instructions! To facilitate function calls, MIPS gives us two variants of the "j" instructions, the "jal" (jumpand-link) and the "jr" (jump by register). Don’t worry, they are much easier than the name suggested.
First, download and load the assembly program "sayHi.asm" in QtSpim. The original content of the file is given on the next page.
# sayHi.asm .data
str1: .asciiz "Before function " str2: .asciiz "After function "
str3: .asciiz "Inside function: Say Hi! "
.text main: li $v0, 4 # system call code for print_string la $a0, str1 # address of string to print syscall # print the string

jal sayHi # Make a function call to sayHi()

li $v0, 4 # system call code for print_string la $a0, str2 # address of string to print syscall # print the string

# End of main, make a syscall to "exit" li $v0, 10 # system call code for exit syscall # terminate program

# start of function sayHi() sayHi: li $v0, 4 # system call code for print_string la $a0, str3 # address of string to print syscall # print the string
# Use "jr" to go back to caller

The intention of the program is to print 3 messages in the following order:


The first and third messages are printed in the "main" function while the second message is printed by the "sayHi" function. The given program is almost complete, with only one missing instruction. The purpose of this code is to demonstrate the necessary instructions needed for a making a function call.
Now, let us step through the program to make several observations. Use the “Single Step” button or press F10 to go through the program line by line. Stop when you reach the instruction "jal sayHi".

Answer: The instruction address of "jal sayHi" is at 0x_______________

Press F10 one more time to execute the "jal" instruction. Answer the following:

Answer: The PC is now at 0x_______________
Answer: The register $31 now contains 0x_______________
At this point, you should be able to see why the name of register $31 is $ra (return address). Express the content of register $31 with respect to the instruction address of the corresponding "jal" instruction. Use the notation Addr(jal) to indicate the instruction address of "jal" instruction.

Answer: $31 = _______________

If you continue stepping through, we will reach the end of the "sayHi" function and get ‘stuck’. We need a way to go back to the main function and continue from where we left off. We can do this easily by the "jr" (jump by register) instruction which is missing in the program. This "jr" instruction takes a register number as operand. It will jump to the address stored in the specified register. For example,
jr $15
The content of register $15 will be used as the target address. This is known as direct addressing (the address is directly specified in full).
What is the correct register number to be used in the "jr" instruction so that we can jump back to main?
Answer: jr ______

Now, edit your code and insert the "jr" instruction accordingly. Run your program, you should see the 3 messages in the same order as shown in the earlier output screenshot.




We can now turn to other aspects of function call, namely function parameters (arguments) and function return value. Actually, we have encountered this idea in previous labs. Take a look at this very familiar sequence of using the system call read_int:
li $v0, 5 # System call code for read_int syscall
sw $v0, 0($t1) # "return result" is in $v0

You can see that there is an agreement to use the register $v0 to store the system call code for the system call (a special kind of function call). Additionally, the return result (an integer read from user) is placed in register $v0 when the system call is completed.
Key idea: we can pass information to the function by placing values in registers and retrieve the return result in the same way.

Let us first attempt to pass information to a function. Download and load the arrayFunction.asm in QtSpim. The main function code is given below:
.data
array: .word 8, 2, 1, 6, 9, 7, 3, 5, 0, 4 newl: .asciiz " "

.text main:
# Print the original content of array
# setup the parameter(s)
# call the printArray function

# Ask the user for two indices
li $v0, 5 # System call code for read_int syscall
addi $t0, $v0, 0 # first user input in $t0

li $v0, 5 # System call code for read_int syscall
addi $t1, $v0, 0 # second user input in $t1

# Call the findMin function
# setup the parameter(s)
# call the function

# Print the min item
<code not shown>

# Calculate and print the index of min item
<code not shown>

# End of main, make a syscall to "exit" li $v0, 10 # system call code for exit syscall # terminate program


The basic flow of the program is as follows:
1. Print the original content of array.
2. Ask the user for two indices X and Y, where X  Y.
3. Find the minimum item between A[X] and A[Y] (inclusive).
4. Print the minimum item and the index of the minimum item.
You’ll need to code for parts 1, 3 and 4. Again, don’t panic as most of the code are already written! For part 1, the following function is already given in the program:
### Function printArray ###
# Input: Array Address in $a0, Number of elements in $a1
# Output: None
# Purpose: Print array elements
# Registers used: $t0, $t1, $t2
# Assumption: Array element is word size (4-byte) printArray: addi $t1, $a0, 0 # $t1 is the pointer to the item sll $t2, $a1, 2 # $t2 is the offset beyond the last item add $t2, $a0, $t2 # $t2 is pointing beyond the last item loop: beq $t1, $t2, end
lw $t3, 0($t1) # $t3 is the current item li $v0, 1 # system call code for print_int addi $a0, $t3, 0 # integer to print syscall # print it addi $t1, $t1, 4
j loop # Another iteration end: li $v0, 4 # system call code for print_string la $a0, newl #
syscall # print newline
jr $ra # return from this function
The comments at the beginning of the function give you a good idea of how to make use of this function. Pay special attention to the “input” information, which tells you where to place the expected parameters. Without changing this function, complete the first part of the main program. You only need to place the correct information in the registers $a0 and $a1 then make a function call. Test your program, and you should see the original content of array printed on screen. (Hint: Don’t forget the use of "li" and "la" instructions).
Now, let’s tackle something slightly more challenging. Let us now write a function to find the minimum element. The function header is given in the program as follows:
################################################################ ##
### Function findMin ###
# Input: Lower Array Pointer in $a0, Higher Array Pointer in $a1
# Output: $v0 contains the address of minimum item
# Purpose: Find and return the minimum item
# between $a0 and $a1 (inclusive)
# Registers used: <Fill in with your register usage> # Assumption: Array element is word size (4-byte), $a0 <= $a1 findMin:
# Your implementation here
jr $ra # return from this function
Once you have written the findMin function, you are left with the last piece of puzzle to solve. How do you find out the index of an item from the address of the item? (Hint: think about how we calculate the address of an item given the index of the item.)
Complete the main function by calling the findMin function. Then print both the minimum item and the index of the minimum item.
Below are two separate test runs (user input circled):