Homework Assignment #2 Solution

Assignment Objectives




The primary objectives of this homework assignment are to continue mastering array processing in MIPS, to be able to write functions in MIPS, and to apply MIPS register conventions.




Register Conventions




You must follow the register conventions taught in lecture and reviewed in recitation. Failure to follow them will result in loss of credit when we grade your work. Here is a brief summary of the register conventions and how your use of them will impact grading:




It is the callee’s responsibility to save any $s registers it overwrites by saving copies of those registers on the stack and restoring them before returning.




If a function calls a secondary function, the caller must save $ra before calling the callee. In addition, if the caller wants a particular $a, $t or $v register’s value to be preserved across the secondary function call, the best practice would be to place a copy of that register in an $s register before making the function call.




A function which allocates stack space by adjusting $sp must restore $sp to its original value before returning.




Registers $fp and $gp are treated as preserved registers for the purposes of this course. If a function modifies one or both, the function must restore them before returning to the caller. There is no reason for your code to touch the $gp register, so leave it alone.




The following practices will result in loss of credit:




“Brute-force” saving of all $s registers in a function or otherwise saving $s registers that are not overwrit-ten by a function.




Callee-saving of $a, $t or $v registers as a means of “helping” the caller.




Getting Started




Visit Piazza and download the file hw2.zip. Decompress the file and then open hw2.zip. Fill in the following information at the top of hw2.asm:




your first and last name as they appear in Blackboard



your Net ID (e.g., jsmith)












CSE 220 – Homework Assignment #2 2
3. your Stony Brook ID # (e.g., 111999999)




Having this information at the top of the file helps us locate your work. If you forget to include this information but don’t remember until after the deadline has passed, don’t worry about it – we will track down your submission.




Inside hw2.asm you will find several function stubs that consist simply of jr $ra instructions. Your job in this assignment is implement all the functions as specified below. Do not change the function names, as the grading scripts will be looking for functions of the given names. However, you may implement additional helper functions of your own, but they must be saved in hw2.asm.




If you are having difficulty implementing these functions, write out pseudocode or implement the functions in a higher-level language first. Once you understand the algorithm and what steps to perform, then translate the logic to MIPS assembly code.




Be sure to initialize all of your values (e.g., registers) within your functions. Never assume registers or memory will hold any particular values (e.g., zero). MARS initializes all of the registers and bytes of main memory to zeroes. The grading scripts will fill the registers and/or main memory with random values before calling your functions.




Finally, do not define a .data section in your hw2.asm file. A submission that contains a .data section will receive a score of zero.




Preliminaries




In this assignment, you will familiarize yourself with a feature of the C programming language known as structs. A struct is a composite data type that is used to group variables under one name in a contiguous block of memory. You will be reading and writing arrays of structs that represents cars and car repairs. The * notation used in the struct definitions below is known as a pointer in C. A pointer is simply a memory address. The data type associated with the pointer identifies how the data should be accessed at the memory location that is specified by the pointer. For example, int * would signify the address of an integer located somewhere in memory.




The car Data Type




The car struct stores information about a particular car, identified by its 17-character VIN (vehicle identification number):




struct car {




string vin;
// 4 bytes
string make;
// 4
bytes
string model;
// 4
bytes
short year;
// 2
bytes
byte features;
// 1-byte bit vector
byte padding;
// 1
byte of zero-padding
};







The meaning of each field:




vin: a pointer to the null-terminated string containing the car’s 17-character VIN.




make: a pointer to the null-terminated string containing the car’s make.










CSE 220 – Homework Assignment #2 3
model: a pointer to the null-terminated string containing the car’s model.




year: a 16-bit integer (half-word) containing the car’s year of manufacture.




features: a bit vector that gives additional information about the car bit 0: is it a convertible (1) or not (0)?




bit 1: is it hybrid (1) or not (0)




bit 2: are the windows tinted (1) or not (0)?




bit 3: is GPS built-in (1) or not (0)? bits 4-7 are unused




Note that the car struct contains 16 bytes, including 1 byte of zero-padding.




Here is what a car struct might look like in MIPS:




Data declarations from elsewhere in data.asm: vin_03: .asciiz "1G1AK15F967719757"



make_E: .asciiz "Mersaydeez"




model_D: .asciiz "Road Hog"




The car struct itself starts at label car_03:



car_03: .word vin_03 # vin_03 is the starting address of "1G1AK15F967719757"




car_03_make_addr: .word make_E # make_E is the starting address of "Mersaydeez"




car_03_model_addr: .word model_D # model_D is the starting address of "Road Hog"

car_03_year: .byte 175, 7
# the year
of
manufacture:
175 + 7*256 = 1967
car_03_features: .byte 10
# a bit-vector of features
(binary 1010_2)
.byte 0
# one byte
of
zero-padding





See the included data.asm file for sample car structs.




While working on the homework you might find it helpful to write a function that prints the contents of a car struct to give you a human-readable form of the data structure.




The repair Data Type




struct repair {




car* car_ptr;
// 4
bytes
string desc;
// 4
bytes
short cost;
// 2
bytes
short padding; // 2
bytes of zero-padding
}







The notation * indicates that the field is a pointer, i.e., the address of a car struct.




The meaning of each field:




car ptr: a pointer to the struct for the car that was repaired.




desc: a pointer to the null-terminated string containing the description of the repair.










CSE 220 – Homework Assignment #2 4
cost: a 16-bit unsigned integer containing the cost of the repair.




Note that the repair struct contains 12 bytes of data, including 2 bytes of zero-padding.




Here is what a repair struct might look like in MIPS:




A data declaration from elsewhere in data.asm: repair_desc_A: .asciiz "fix cracked windshield"



The car struct itself starts at label repair_05_car:



repair_05_car: .word car_03


#
starting addr. of
repaired car
struct
repair_05_desc_addr: .word
repair_desc_A #
address of repair
string


repair_05_cost: .byte 156,
1
# cost
to repair the car: 156 + 1*256
= 412
.byte 0, 0


# two bytes of zero-padding





See the included data.asm file for sample repair structs.




How to Test Your Functions




Numerous .asm files have been provided to you for this assignment to help you test your work. However, note the following:




These tests are not exhaustive. You will need to modify the provided “main” files to test your work more thoroughly.




These test cases will not be used to grade your work.




The grading test cases will use different labels than the ones you will find in the data.asm file and similar files.




Do not, under any circumstances, add a .data section to your hw2.asm file. Doing this will render your submission ungradeable!




Using the provided main files is pretty simple. For example, suppose you wanted to test your index of car function from Part I of the assignment. First, make sure that in the Settings menu in MARS the option Initialize Program Counter to global ‘main’ if defined is checked. Then, open index of car main.asm, assemble it, and then run it. The main file will load the contents of your hw2.asm file and call your implemen-tation of the index of car function.







The main files will provide some basic output, but it is your responsibility to dig deeper into the function outputs to make sure your implementations are correct. These main files will not be used in grading, and you should not submit them for grading. Submit only your hw2.asm file to Blackboard.




Finally, make sure that all code required for implementing your functions is included only in the hw2.asm file. To make sure that your code is self-contained, try assembling your hw2.asm file by itself in Mars. If you get any errors (such as a missing label), this means that you need to refactor (reorganize) your code, possibly by moving labels you inadvertently defined in a main file (e.g., a helper function) to hw2.asm.







Part I: Search for a Car by Year




int index of car(car[] cars, int length, int start index, int year)







CSE 220 – Homework Assignment #2 5
This function searches an array of car structs starting at index start index and returns the index of the first car it finds that was manufactured in the given year. (That is, when multiple cars of the given year are found, the lowest index start index is returned.) Recall that each car struct is 16 bytes in size. Therefore, the starting addresses of consecutive structs are 16 bytes apart. As an example, suppose that cars[] starts at address 0x00100000. cars[0] begins at 0x00100000, cars[1] begins at 0x00100010, cars[2] begins at 0x00100020, etc.







The function takes the following arguments, in this order:




cars: an array of car structs




length: the length of the cars array (i.e., how many structs are stored in the array)




start index: the index at which to start the search




year: the year of manufacture







Returns in $v0:




the index of the located car, as described earlier Returns -1 in $v0 for error in any of the following cases:

length 0




start index < 0




start index length year < 1885




No car with the given year was found.







Additional requirements:




Do not modify the contents of the cars array in memory.




Examples:




For the examples below, the cars argument is the all cars array provided in the data.asm file distributed with the homework. Refer to that file for the particular contents of the array. Be sure to test your function with different arguments ($a1, $a2, $a3) by modifying the index of car main.asm file.







Function Call
Return Value
Explanation
index


of


car(all


cars, 6, 0, 2017)
2
car found
index


of


car(all


cars, 6, 2, 2017)
2
car found at/after start


index
index


of


car(all


cars, 6, 3, 2017)
-1
car not found at/after start


index
index


of


car(all


cars, 6, 0, 1950)
-1
no car found for given year






index


of


car(all


cars, 6, -2, 2017)
-1
invalid start


index
index


of


car(all


cars, 0, 1, 2017)
-1
invalid length
index


of


car(all


cars, 6, 0, 1800)
-1
invalid year
















































CSE 220 – Homework Assignment #2 6
Part II: Compare Two Strings




int strcmp(string str1, string str2)




This function takes two null-terminated strings as arguments and returns an integer that indicates the lexicographic ordering of the strings. The function begins by comparing the first character of each string. If they are equal to each other, it continues with the following pairs until the characters differ or until a terminating null-character is reached. Note that the strings can be of different lengths.




The function takes the following arguments, in this order:




str1: a pointer to the first string




str2: a pointer to the second string




Returns in $v0:




the difference between the ASCII values of the first mismatch, if any: str1[n] - str2[n], where n is the index of the first mismatch




0: the contents of both strings are identical (including the case where they are both empty strings)




length of str1: str2 is an empty string but str1 is non-empty




negated length of s2: str1 is an empty string but str2 is non-empty




Additional requirements:




Do not modify the contents of the argument strings in memory.




Examples:




Function Call
Return Value
Explanation
strcmp("ABCD", "ABCGG")
-3
First string is smaller; mismatch in middle
strcmp("WHOOP!", "WHOA")
14
First string is larger; mismatch in middle
strcmp("Intel", "pentium")
-39
First string is smaller; mismatch at start
strcmp("STONY", "BROOK")
17
First string is larger; mismatch at start
strcmp("", "mouse")
-5
First string is empty
strcmp("lonely guy", "")
10
Second string is empty
strcmp("Wolfie", "Wolfie")
0
Identical non-empty strings
strcmp("", "")
0
Two empty strings
strcmp("happy", "Z")
14
One argument is very short
strcmp("WOLF", "WOLFIE")
-73
First string is substring of second string
strcmp("StonyBrook", "Stony")
66
Second string is substring of first string






Part III: Copy a Memory Buffer




int memcpy(byte *src, byte *dest, int n)




The memcpy() function copies n bytes of data from address src to address dest. We may assume that the dest buffer is at least n bytes in size.







CSE 220 – Homework Assignment #2 7
The function takes the following arguments, in this order:




src: the address to copy bytes from




dest: the address to copy bytes to




n: the number of bytes to copy (must be greater than 0)




Returns:




$v0: 0 for success, -1 if an error occurs.




Returns -1 in $v0 for error in any of the following cases:




n 0




Additional requirements:




Do not modify the bytes starting at src.




Only the first n bytes memory starting at dest may be changed. All other bytes in dest must be left unchanged.




The src and dest arguments are guaranteed to be valid.




Examples:




Function Call
Return Value
Updated dest
memcpy("ABCDEFGHIJ", "XXXXXXX", 3)
0
"ABCXXXX"
memcpy("ABCDEFGHIJ", "XXXXXXX", 7)
0
"ABCDEFG"
memcpy("ABCDEFGHIJ", "XXXXXXX", -3)
-1
"XXXXXXX"
memcpy("ABCDEFGHIJ", "XXXXXXX", 0)
-1
"XXXXXXX"












Part IV: Insert a car Struct into an Array




int insert car(car[] cars, int length, car new car, int index)







The insert car function inserts a new 16-byte car struct at index index of array cars, shifting the existing car at index index and all other cars 16 bytes to the right.







The function takes the following arguments, in this order:




cars: The array of cars to insert into.




length: The number of cars in the array.




new car: The new car to insert into the array.




index: The index at which the new car should be inserted. If index equals length, the new car is inserted immediately after the last element of the array.







Returns:













CSE 220 – Homework Assignment #2 8
$v0: 0 on success, -1 if an error occurs. Returns -1 for error in any of the following cases:

length < 0 index < 0




index length




Additional requirements:




You may assume that at least 16 bytes of memory has been set aside after the cars array to accommodate the new car.




Do not modify the order of the cars in the array before and after the newly inserted car. insert car() must call memcpy() to shift the cars in the array.







Examples:




Providing extensive examples in this document is not practical due to the nature of the array being manipu-lated, so only return values are given in the table below. You are strongly urged to use (and modify!) the insert car main.asm file provided with the homework materials to test your work. You will note that one complete test case is given in that file. The array after insertion is called expected all cars. Use your strcmp function from earlier in the assignment to compare the contents of all cars (from data.asm) with expected all cars after the insert car() function has been called.







Function Call
Return Value
Explanation
insert


car(cars


array, 6, new


car, 3)
0
new car inserted at index 3






insert


car(cars


array, 6, new


car, 0)
0
new car inserted at start of array
insert


car(cars


array, 6, new


car, 6)
0
new car appended to array
insert


car(cars


array, -1, new


car, 3)
-1
invalid array length
insert


car(cars


array, 6, new


car, -1)
-1
invalid insertion index
insert


car(cars


array, 6, new


car, 8)
-1
insertion index too large


































Part V: Find the Most Damaged Car




(int, int) most damaged(car[] cars, repair[] repairs, int cars length, int repairs length)










The most damaged() function finds the car in cars that has the highest total repair cost, returning both the index of that car in cars and its total repair cost. If there are multiple cars with the same total repair cost, the function returns the lowest of the indices. A particular VIN can appear in multiple repair structs in the repairs array. (clarification added 10/7/2018)







The function takes the following arguments, in this order:




cars: an array of car structs




repairs: an array of repair structs










CSE 220 – Homework Assignment #2 9
cars length: the number of elements in cars




repairs length: the number of elements in repairs







Returns:




$v0: the index of the car with the highest total repair cost, or -1 for an error case




$v1: the total repair cost for the car, or -1 for an error case




The following are error cases:




cars length 0




repairs length 0







Additional requirements:




Do not modify the contents of the cars or repairs arrays in memory. This function must call strcmp() to compare VINs. (updated 10/3/2018)




Examples:




For the contents of the all cars array from data.asm, the returned index in $v0 should be 4 and the returned total repair cost in $v1 should be 587. You are strongly encouraged to modify the all repairs array to make additional examples yourself. This is very easy to do – simply change the cost of particular repair objects. Some examples to consider making: what if the array of repairs has only one item? what if two or more cars have the same total repair cost? what if all the cars have the same repair cost?










Part VI: Sort an Array of car Structs




int sort(car[] cars, int length)




The sort() function sorts an array of cars by year. The function must implement odd-even sort (which is a stable sorting algorithm) according to the following pseudocode:




def sort(cars):




sorted = False




while !sorted:




sorted = True




for(i = 1; i < cars.length - 1; i += 2):




if cars[i] cars[i + 1]:




swap cars[i] and cars[i + 1]




sorted = False




for(i = 0; i < cars.length - 1; i += 2):




if cars[i] cars[i + 1]:




swap cars[i] and cars[i + 1]




sorted = False













CSE 220 – Homework Assignment #2 10
The function must use the stack for temporary space when swapping cars in the array. Again, you must use the stack for temporary space – do not add a .data section to your hw2.asm file. The following pseudocode can be used to swap cars at indices i and j using the stack:




$sp -=
16
// make
space on the
stack for 1 car
memcpy(cars[i], $sp, 16)
// copy
cars[i] onto
the stack
memcpy(cars[j], cars[i], 16) // copy
cars[j] to cars[i]
memcpy($sp, cars[j], 16)
// copy
the temp car
on the stack to cars[j]
$sp +=
16









The function takes the following arguments, in this order:




cars: an array of cars




length: the number of elements in cars




Returns:




$v0: 0 if success, -1 if an error occurs. Returns -1 for error in any of the following cases:

length 0




Additional requirements:




sort() must call memcpy() to swap cars in the array.




Example:




The all cars array given in sort data.asm contains the following cars, shown here in a more easily-readable form:







JTDKN3DU0D5614628 Fjord Wolfie-Z 2017 1000 <----




2FMDK4JC5DBC37904 Hunday X27 2018 1000




1B4HR28N51F502695 Fjord Escapade 2017 1100 <----




1G1AK15F967719757 Mersaydeez Road Hog 2018 1010 1HGEM1159YL037618 Fjord Elantris 2019 0101 5N1AR2MM2EC648945 Toyoter Raoden 2019 1010 1NKDX90X1WR777109 Honder Sazed 2019 0000




3FAHP0HA5CR371712 Fjord Scadrial 2017 1100 <----




5J6TF2H55AL005521 Fjord Metalborn 2019 1010 1G4GC5G34FF231147 Fjord Terris Roadster 2018 0001 1B7HF16Z0XS322729 Honder Allomancer 2018 0100 1G1PC5SHXC7276485 Toyoter Stormlight 2019 1101




After sorting with the even-odd sorting algorithm, the cars will be in this order (given as sorted all cars in data.asm):







JTDKN3DU0D5614628 Fjord Wolfie-Z 2017 1000 <----










CSE 220 – Homework Assignment #2 11
1B4HR28N51F502695 Fjord Escapade 2017 1100 <----




3FAHP0HA5CR371712 Fjord Scadrial 2017 1100 <----




2FMDK4JC5DBC37904 Hunday X27 2018 1000 1G1AK15F967719757 Mersaydeez Road Hog 2018 1010 1G4GC5G34FF231147 Fjord Terris Roadster 2018 0001 1B7HF16Z0XS322729 Honder Allomancer 2018 0100 1HGEM1159YL037618 Fjord Elantris 2019 0101 5N1AR2MM2EC648945 Toyoter Raoden 2019 1010 1NKDX90X1WR777109 Honder Sazed 2019 0000 5J6TF2H55AL005521 Fjord Metalborn 2019 1010 1G1PC5SHXC7276485 Toyoter Stormlight 2019 1101




Since the even-odd sorting algorithm is stable, cars with equal years in the original array appear in the same order in the sorted array. We have put an arrow next to the cars manufactured in 2017 to help illustrate the idea.







Part VII: Find the Most Popular Car Feature




int most popular feature(car[] cars, int length, nibble features)







The most popular feature() function determines the most popular feature amongst the ones defined in features. features is a bit vector specifying which of the features to consider when calculating the most popular feature (1 means consider, 0 means do not consider). The bit positions in the features argument are assigned these meanings, which correspond with those of the car structs:







Bit Position
What a 1 in This Bit Position Means
0
Count the # cars that are convertibles
1
Count the # cars that are hybrids
2
Count the # of car that have tinted windows
3
Count the # of cars that have GPS built-in







The function takes the following arguments, in this order:




cars: an array of car structs




length: the number of elements in cars




features: the nibble (half-byte) of features that should be considered




Returns:




$v0: a 1 in the bit position corresponding to the most popular of the features we are considering. In the case of a tie, return the higher-valued bit number (i.e. GPS Tinted windows Hybrid Convertible). The examples given below will help to further clarify what this return value means.




Returns -1 for error in any of the following cases:




length 0




features not in range [1, 15].













CSE 220 – Homework Assignment #2 12
All cars have no features from features (i.e. there is no favorite because the count is 0 for all features that are being considered).




Additional requirements:




Do not modify the contents of the cars array in memory.




Examples:




Consider the following function call: most popular feature(all cars, 6, 7). The third argument, 01112 in binary, indicates that we will count how many cars are convertibles (bit 0 is 1), how many cars are hybrid (bit 1 is 1), and how many cars have tinted windows (bit 2 is 1). We will not count cars that have a GPS because bit 3 is 0. When we call the function with these arguments for the cars in data.asm, the return value is 4, 01002 in binary. Bit 2 is associated with tinted windows. This means either that (1) of the three features we considered, tinted windows was the most popular feature, or, (2) there was a tie in the counts for tinted windows and some other feature, but we reported tinted windows as the “winner” because it has higher precedence than hybrid engines or a convertible roof.







As another example, suppose we performed a search with 00112 as the feature vector, but no cars are hybrids or have convertible roofs. In this case, both of those counts are zeroes, and so the function returns -1 to indicate an error.




So to summarize, there are only 5 valid return values: -1, 1, 2, 4 or 8. Do you see why?




Function Call
Return Value
most


popular


feature(all


cars, 6, 15)
8






most


popular


feature(all


cars, 6, 7)
4
most


popular


feature(all


cars, 6, 3)
2
most


popular


feature(all


cars, 6, 5)
4
most


popular


feature(all


cars, 6, 1)
1
most


popular


feature(all


cars, 6, 0)
-1






most


popular


feature(all


cars, -1, 14)
-1
























Part VIII: Compute a VIN’s Check Digit




char compute check digit(string vin, string map, string weights, string transliterate str)










The compute check digit() function computes the check digit for a valid VIN according to the following pseudocode:







def transliterate(ch, transliterate_str):




return transliterate_str.index_of(ch) % 10




def compute_check_digit(vin, map, weights, transliterate_str):




sum = 0




for (i = 0; i < 17; i++):

sum += transliterate(vin.charAt(i), transliterate_str) *










CSE 220 – Homework Assignment #2 13
map.index_of(weights.char_at(i))




return map.char_at(sum % 11)




Implementing separate functions for transliterate, char at, and index of is not required, but is rec-ommended to make your code easier to debug.







The function takes the following arguments, in this order:




vin: a VIN




map: the string "0123456789X", whose address is passed to the function via $a1




weights: the string "8765432X098765432", whose address is passed to the function via $a2




transliterate str: the string "0123456789.ABCDEFGH..JKLMN.P.R..STUVWXYZ", , whose address is passed to the function via $a3







Returns:




$v0: The check digit of the VIN given as an ASCII character. You do not need to do any error-checking for this function.




Additional requirements:




Do not modify vin in memory.




Examples:




In the table below, transliterate str has been abbreviated trs to save space.







Function Call
Return Value
compute


check


digit("JTDKN3DU0D5614628", map, weights, trs)
0






compute


check


digit("1B4HR28N01F502695", map, weights, trs)
5






compute


check


digit("1HGEM1150YL037618", map, weights, trs)
9










compute


check


digit("1FTDF15N0KNB73611", map, weights, trs)
3






compute


check


digit("1M2P198C0JW002996", map, weights, trs)
5




















Academic Honesty Policy




Academic honesty is taken very seriously in this course. By submitting your work to Blackboard you indicate your understanding of, and agreement with, the following Academic Honesty Statement:




I understand that representing another person’s work as my own is academically dishonest.



I understand that copying, even with modifications, a solution from another source (such as the web or another person) as a part of my answer constitutes plagiarism.



I understand that sharing parts of my homework solutions (text write-up, schematics, code, electronic or hard-copy) is academic dishonesty and helps others plagiarize my work.



I understand that protecting my work from possible plagiarism is my responsibility. I understand the im-









CSE 220 – Homework Assignment #2 14
portance of saving my work such that it is visible only to me.




I understand that passing information that is relevant to a homework/exam to others in the course (either lecture or even in the future!) for their private use constitutes academic dishonesty. I will only discuss material that I am willing to openly post on the discussion board.



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I understand that the penalty for academic dishonesty might not be immediately administered. For instance, cheating in a homework may be discovered and penalized after the grades for that homework have been recorded.



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I understand that there are no extenuating circumstances for academic dishonesty.



How to Submit Your Work for Grading




To submit your hw2.asm file for grading:




Login to Blackboard and locate the course account for CSE 220.



Click on “Assignments” in the left-hand menu and click on the link for this assignment.



Click the “Browse My Computer” button and locate the hw2.asm file. Submit only that one .asm file.



Click the “Submit” button to submit your work for grading.



Oops, I messed up and I need to resubmit a file!




No worries! Just follow steps 4–6 again. We will grade only your last submission.



























































































CSE 220 – Homework Assignment #2 15