Lab 4: Functions and Memory Allocations Solution

Memory Allocations




In general, if we want to use the main memory to store some data, we need to allocate it in advance usually in the data segment (.data). For example, if we want to allocate 100 bytes:







.data




buffer: .space 100







In the above case, .space directive allocates a section of the main memory (in data segment) for 100 bytes where the location of the section of memory can be referred to by the label buffer. For example, we you want to store a value 12 (as a byte), this can be done by







addi $s0, $zero, 12




la $s1, buffer




sb $s0, 0($s1)







Often time, we have no idea how many bytes we need to allocate in advance. As a result, we trend to allocate way to much space in advance which wastes a lot of memory. In case of MIPS, we can allocate a region of memory on the y while the program is running. The memory region will be allocated in heap instead of the data segment. This can be done by using the system call number




Simply set the number of bytes that you want to allocate in $a0, set $v0 to 9, and execute the syscall instruction. The address of newly allocate section of memory will be in the register $v0. For example, suppose you have a string of length 20 and you want to allocate memory to store this string:






addi $v0, $zero, 9




addi $a0, $zero, 21 # Need an extra byte for the null character syscall




add $s0, $zero, $v0 # $s0 is the address of the memory




Copy the string



:










Syscall 8 (Read String)




Note that a null-terminated string in MIPS is simply a sequence of characters in a contiguous region of the main memory. Recall that a null-terminated string can be created in the data segment as follows:







.data




aString: .asciiz "Hello World!!!"










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Lab 4: Functions and Memory Allocations










MIPS also provides a system call to receive a string from the keyboard input. This is system call number 8 which requires two arguments, $a0 | address of input bu er, and $a1 | the maximum number of characters to read. The following is an example of a code that read string from keyboard input and print it on the console screen:







.data










buffer:
.space
100




.text










addi $v0, $zero, 8
# Syscall
8: Read string


la
$a0, buffer
# Set the
buffer


addi $a1, $zero, 100
# Set the
maximum to 100
(size of the buffer)
syscall








addi $v0, $zero, 4
# Syscall
4: Print string


la
$a0, buffer
# Address
of the string to print



syscall







Unfortunately, the system call number 8 has a strange behavior. If a user types Hello and presses




Enter, the data in the bu er (e.g., from the above code) will be:




0x48 0x65 0x6C 0x6C 0x6F 0x0A 0x00 0x00 ...




Note that 0x48 is the ASCII character ’H’, 0x65 is the ASCII character ’e’, and so on. Note that before the rst null-character (0x00), the bu er contains the character 0x0A which is the line feed character (Enter). In general, it should not be a part of an input string from a keyboard.




Functions




For this lab, you are going to implements four small functions related to null-terminated strings.




All functions must follow the calling conventions discussed in class as follows:




Use jal instruction to call a function



Use jr $ra instruction to return to the caller



Arguments should be stored in registers $a0 to $a3



Return values should be stored in register $v0 and $v1



Caller cannot expect that values stored in registers $t0 to $t9 will be the same after making a function call



Callee must maintain values stored in registers $s0 to $s7



Callee must use stack to backup registers



Stack should be adjusted according to the size of the memory required to backup registers and restore before going back to a caller



For this lab, a starter code is given. Download the le lab04.asm from the CourseWeb under this lab. In that le, you will see the main program on the top and four functions, including their descriptions, that you have to implement right after the main program. Simply add your implementation between a function name and the instruction jr $ra. You must implement the following functions:







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Lab 4: Functions and Memory Allocations










strLength: This function takes exactly one argument in $a0 which is the address of a null-terminated string. It returns the length of the given string (number of characters excluding the null-character) in $v0.



strCopy: This function takes two arguments; (1) an address of a destination in $a0, and



an address of a source null-terminates string. This function simply copy (character-by-character) from source to destination. Note that after this function is done, the destination should contain a null-terminated string identical to the given source string.



strCompare: This function takes two arguments in $a0 and $a1 which are addresses of two null-terminated strings. This function compares whether two strings are identical. If they are identical, a 0 is returned in the register $v0. Otherwise, -1 is returned.



readString: The purpose of this function are as follows:



read a string from the keyboard input using the system call number 8,



get rid of the linefeed character at the end,



allocate a region in heap for storing this input string,



copy string from temporary bu er to the memory allocated in previous step



return the memory address of the input string (in heap) using the register $v0






Since the system call number 8 requires a bu er to store an input string temporary, your function can simply use a given bu er named buffer located in the data segment (provided by the starter code) as shown below:







.data




:




buffer: .space 100







Note that if this function needs to know the length of the input string to allocate the correct amount of memory. You must use your implementation of the function strLength. During the copy process from buffer to a new allocated memory, it must uses the function strCopy. Use system call number 9 as discussed earlier to allocate memory in the heap section. Do not forget to allocate one extra byte for the null character.







Test Code




For this lab, a starter code is given. This code will perform the following:




Ask user for a number of strings that user wants to enter



Allocate memory in heap to store addresses of input string (not string themselves)



Ask user to enter strings by calling your function readString. Every time your function






readString returns, it will store the address returned by the readString function into the memory allocated earlier in step 2.







Once all strings have been entered, it will display all string starting from index 0.



Then it will ask user to enter another string






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Lab 4: Functions and Memory Allocations










6. Once user enter a new string, it will search for the index of a given string using your strCompare function. If a string is found, it will display the index of the string. Other-wise, it simply says string not found.







The following is an example of the output of the program when a string is found.







How many strings do you have?: 5




Please enter a string: Hello




Please enter a string: World




Please enter a string: Computer




Please enter a string: Data




Please enter a string: Assembly




The string at index 0 is "Hello"




The string at index 1 is "World"




The string at index 2 is "Computer"




The string at index 3 is "Data"




The string at index 4 is "Assembly"




Please enter a string: Computer




The index of the string "Computer" is 2.







The following is an example of the output of the program when a string is not found.







How many strings do you have?: 5




Please enter a string: Hello




Please enter a string: World




Please enter a string: Computer




Please enter a string: Data




Please enter a string: Assembly




The string at index 0 is "Hello"




The string at index 1 is "World"




The string at index 2 is "Computer"




The string at index 3 is "Data"




The string at index 4 is "Assembly"




Please enter a string: Dog




Could not find the string "Dog".







Note that if the program does not work properly, most likely there is something wrong with one of your functions. So, make sure your functions work correctly and follow all calling convention discussed in class.




Submission




Submit your lab04.asm le via CourseWeb before the due date stated on the CourseWeb.




























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