Description
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 overwritten 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:
1. your first and last name as they appear in Blackboard
2. your Net ID (e.g., jsmith)
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.
• 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 – Fall 2018 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 implementation 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 – Fall 2018 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 – Fall 2018 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 – Fall 2018 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 – Fall 2018 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 manipulated, 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 – Fall 2018 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 – Fall 2018 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 easilyreadable 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 – Fall 2018 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 – Fall 2018 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 – Fall 2018 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 recommended 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:
1. I understand that representing another person’s work as my own is academically dishonest.
2. 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.
3. 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.
4. I understand that protecting my work from possible plagiarism is my responsibility. I understand the imCSE 220 –
portance of saving my work such that it is visible only to me.
5. 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.
6. I understand that academic dishonesty is treated very seriously in this course. I understand that the instructor
will report any incident of academic dishonesty to the College of Engineering and Applied Sciences.
7. 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.
8. I understand that buying or paying another entity for any code, partial or in its entirety, and submitting it as
my own work is considered academic dishonesty.
9. 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:
1. Login to Blackboard and locate the course account for CSE 220.
2. Click on “Assignments” in the left-hand menu and click on the link for this assignment.
3. Click the “Browse My Computer” button and locate the hw2.asm file. Submit only that one .asm file.
4. 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.
