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Systems Fundamentals I Homework #2

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CSE 220: Systems Fundamentals I
Homework #2

Ÿ Download the Stony Brook version of MARS posted on Piazza. DO NOT USE the MARS available on the
official webpage. The Stony Brook version has a reduced instruction set, added tools, and additional system
calls you will need to complete the homework assignments.
o You personally must implement the assignment in MIPS Assembly language by yourself. You may not use a
code generator or other tools that write any MIPS code for you. You must manually write all MIPS Assembly
code you submit as part of the assignment. You may also not write a code generator in MIPS Assembly that
generates MIPS Assembly.
o All test cases MUST execute in 10,000 instructions or less. Efficiency is an important aspect of programming.
o Any excess output from your program (debugging notes, etc) WILL impact your grading. Do not leave erroneous printouts in your code!
o Do not submit a file with the functions/labels main or _start defined. You are also not permitted to start
your label names with two underscores ( __ ). You will obtain a ZERO for the assignment if you do this.
Introduction
In this assignment you will be creating functions and working with 1D and 2D arrays. The goal is to understand
function calls, returning values, and the role of register conventions. We will implement functions to gather
information about the students in a class (1D array) and then based on a 2D seating chart of students write a
function to identify potential student cheaters.
You MUST implement all the functions in the assignment as defined. It is OK to implement additional helper
functions of your own in hw2.asm .
Ÿ If you are having difficulties implementing these functions, write out the pseudo code 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.
Ÿ When writing your program, try to comment as much as possible. Try to stay consistent with your formatting.
It is much easier for your TA and the professor to help you if we can figure out what your code does quickly.
For this assignment, there is a new cse220_exam struct that will be used in addition to the old cse220_student
struct.
CSE 220 – Spring 2018 Homework #2 Page 1
1 struct cse220_student {
2 int id; // size: 4b (b == bytes)
3 String netid; // size: 4b; starting address of the string
4 float percentile; // size: 4b; single precision IEEE-754
5 short grade; // size: 2b; byte[0] is letter and byte[1] is sign
6 nibble recitation; // size: 4 bits; NOTE: nibble isn’t a true C datatype
7 nibble favtopics; // size: 4 bits; bit vector
8 };
9
10 struct cse220_exam {
11 cse220_student* student; // size: 4b; starting address of cse220_student struct
12 int grade; // size: 4b; exam grade of the student
13 };
The * notation used in the cse220_exam struct is known as a pointer in C. A pointer is a space in memory
which holds a memory address. The data type associated with the pointer identifies the how the data should be
accessed at the memory location that is specified by the pointer.
Visually, the structs are defined in memory as:
CSE 220 – Spring 2018 Homework #2 Page 2
Getting started
Download hw2.zip from Piazza in the Homework section of Resources. This file contains hw2.asm and multiple hw2_main files, which you can use to test for the assignment. At the top of your hw2.asm program in
comments put your name and SBU ID number.
# Homework #2
# name: MY_NAME
# sbuid: MY_SBU_ID
The hw2_main files include a gradeshelpers.asm file which has two provided functions getGradeIndex
and getGrade . These functions map between a grade and an assigned index value. You MUST treat these functions as black boxes, meaning do not rely on their implementations in any way other than their specification.
o We will modify the implementations of these functions during grading!
o Do NOT include the gradeshelpers.asm file in your hw2.asm file! Your program will not compile with
our test cases and you will get a ZERO.
• int getGradeIndex(short grade)
The function takes the 2 characters for the grade as an argument and returns the assigned index associated
with the grade. It is best to treat this function as a blackbox, as its implementation should not matter to your
function.
Function parameter and return value summary:
◦ grade : the grade in the set {“A “, “A-“, “B+”, “B “, “B-“, “C+”, “C “, “C-“, “D+”, “D “, “D-“, “F ” }
◦ returns: index value associated with specified grade, -1 if invalid grade
• short getGrade(int index)
The function takes a given index and returns the associated grade. It is best to treat this function as a
blackbox, as its implementation should not matter to your function.
Function parameter and return value summary:
◦ index : value to look up associated grade
◦ returns: 2 characters for the grade associated with the index, -1 if index is outside of range [0,11]
How to test your functions
To test your functions, simply open the provided hw2_main file which tests your function in MARS. Next, assemble the main file and run. Mars will take the contents of the file referenced with the .include at the end
of the file and add the contents of your hw2.asm file before assembling it.
The main files only call each function with ONE of the sample test cases and print the return value(s) to the
output. You will need to modify the arguments passed to the functions to test with the other cases.
CSE 220 – Spring 2018 Homework #2 Page 3
o It is highly advised to write your own main programs (new individual files) and create your own data arrays (in
the main files) to test each of your functions thoroughly. Your assignment will not be graded using the examples
provided!
o Make sure to initialize all of your values within your functions! Never assume registers or memory will hold
any particular values!
Any modifications to the main files will not be graded. You will only submit your hw2.asm file via Sparky.
Make sure that all code required for implementing your functions are included in the hw2.asm file!
o There is no need to define a .data section within your homework. Your functions should not refer to any
labels directly.
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 to hw2.asm .
Part 1: Basic Leaf Functions
In this section you will create “leaf” functions (a function which does not call any other functions).
a. int recitationCount(cse220_student[] class, int classSize, int rnum)
The function counts the number of students in class which are in recitation rnum.
Ÿ We WILL NOT test the function with an invalid class argument address.
Function parameter and return value summary:
• class : starting address of the 1D array of students in the class.
• classSize : number of entries in class array.
• rnum : recitation number.
• returns: the number of students in the class who are in the specified recitation.
Return -1 for error in any of the following cases:
• classSize ≤ 0.
• rnum is not in the set {8, 9, 10, 12, 13, 14}.
V Do not modify class in memory.
Examples:
Code Return Value
recitationCount(class1, 0, 8) -1
recitationCount(class1, 5, 11) -1
recitationCount(class1, 5, 8) 2
recitationCount(class2, 5, 12) 3
recitationCount(class2, 10, 12) 5
Ÿ The length specified for an array does not have to match the actual length of the array. By setting
the value smaller, we can check subsets of the array entries.
CSE 220 – Spring 2018 Homework #2 Page 4
b. float aveGradePercentage(int[] histogram, float[] gradepoints)
The function calculates the average grade percentage based on the specified histogram counts in hist .
The hist array contains the number of students who obtained a given grade.
Ÿ We WILL NOT test the function with an invalid histogram or gradepoints argument address, and each will always refer to 12 entries.
Function parameter and return value summary:
• histogram : address of 1D array of integers specifying the number of students with each grade.
• gradepoints : address of 1D array of floats specifying the gradepoint values.
• returns: average grade percentage, -1 if error.
Return (float -1) for error in any of the following cases:
• any entry of histogram or gradepoints contains a negative value.
• histogram contains a total count of 0 (ie. there are no grades specified).
Ÿ The bit representation for single precision floating point -1 is 0xBF800000.
Ÿ Reminder, even though you are returning a float, place the return value in $v0.
Examples:
Code Return Value
aveGradePercentage(histErr1, gradepoints1) -1
aveGradePercentage(histErr2, gradepoints1) -1
aveGradePercentage(hist1, gradepointsErr) -1
aveGradePercentage(hist1, gradepoints1) 2.4
aveGradePercentage(hist1, gradepoints2) 2.6029413
aveGradePercentage(hist2, gradepoints1) 3.5066667
aveGradePercentage(hist2, gradepoints2) 4.4.266667
c. float favtopicPercentage(cse220_student[] class, int classSize, nibble topics)
The function calculates the percentage of students in the class who like ANY of the topics specified by
topics (1 means consider, 0 means do not consider).
Function parameter and return value summary:
• class : starting address of the 1D array of students in the class.
• classSize : number of entries in class array.
• topics : bit vector specifying topics to consider.
• returns: percentage of students who like topics, -1 if error.
Ÿ You will be returning the float in $v0 even though it is not a floating point register.
Return (float -1) for error in any of the following cases:
• classSize ≤ 0.
• topics is not in range [1,15].
Ÿ The bit representation for single precision floating point -1 is 0xBF800000.
Examples:
CSE 220 – Spring 2018 Homework #2 Page 5
Code Return Value
favtopicPercentage(class1, 0, 2) -1
favtopicPercentage(class1, 5, 0) -1
favtopicPercentage(class1, 5, 1) 0.2
favtopicPercentage(class1, 5, 12) 0.6
favtopicPercentage(class2, 5, 15) 1
favtopicPercentage(class2, 10, 2) 0.6
d. int findFavtopic(cse220_student[] class, int classSize, nibble topics)
The function determines the favtopic of the class amongst the ones specified in topics . topics is a bit
vector specifying which of the topics to consider when calculating the favorite topic (1 means consider, 0
means do not consider). In the case of a tie, return the higher valued bit topic (eg. MIPS > Boolean Logic >
Digital Logic > Datapaths).
Function parameter and return value summary:
• class : starting address of the 1D array of students in the class.
• classSize : number of entries in class array.
• topics : bit vector specifying topics to consider for favorite topic.
• returns: a 1 in the bit position corresponding to the favtopic, -1 if error.
Return -1 for error in any of the following cases:
• classSize ≤ 0.
• topics is not in range [1,15].
• All students have no favtopics from the topics (eg. there is no favorite as the count is 0 for all topics)
Examples:
Code Return Value
findFavtopic(class1, -2, 6) -1
findFavtopic(class1, 5, 0) -1
findFavtopic(class1, 5, 5) 4
findFavtopic(class1, 5, 15 2
findFavtopic(class1, 5, 12) 8
findFavtopic(class2, 8, 11) 8
findFavtopic(class2, 10, 15) 1
Part 2: Non-Leaf functions
In this part of the assignment you will create more complex functions which make function calls. These
functions should follow MIPS register conventions using prologue and epiloge blocks.
e. (int,int) twoFavtopics(cse220_student[] class, int classSize)
The function returns the top 2 favorite topics amongst the students in the course. In the case of a tie, the
function return the higher valued bit topic (eg. MIPS > Boolean Logic > Digital Logic > Datapaths).
V Your function MUST CALL findFavtopic .
• class : starting address of the 1D array of students in the class.
• classSize : number of entries in class array.
CSE 220 – Spring 2018 Homework #2 Page 6
• returns: (favorite, second favorite) or (-1,-1) on error. Each favorite is specified by a 1 in the bit position
corresponding to the favtopic
Return (-1,-1) for error in any of the following cases:
• classSize ≤ 0.
• All students have no favtopics from the topics (eg. there is no favorite as the count is 0 for all topics)
Examples:
Code Return Value
twoFavtopics(class1, -2) (-1, -1)
twoFavtopics(class1, 5) (2,8)
twoFavtopics(class2, 7) (8, 2)
twoFavtopics(class2, 10) (1, 2)
f. float calcAveClassGrade(cse220_student[] class, int classSize, int[] histogram,
int[] gradepoints)
The function creates a histogram count of the cse220_student grade fields in the class and calculates the average grade percentage for the course.
You MUST use the int getGradeIndex(short grade) function provided to determine which index
in histogram to place each grade count. Treat this function as a blackbox!
Ÿ We WILL NOT test the function with an invalid histogram or gradepoints argument address, and each will always refer to 12 entries.
Ÿ Remember, the grade stored in the struct has a space character (ASCII 32), if it does not have a ‘+’
or ‘-’ modifier.
Function parameter and return value summary:
• class : starting address of the 1D array of students in the class.
• classSize : number of entries in class array.
• histogram : starting address of a 1D array in memory to hold the histogram counts.
• gradepoints : 1D array of integers specifying the gradepoint values.
• returns: average grade percentage or -1 if error.
Return (float -1) for error in any of the following cases. When an error occurs, the values stored to histogram are considered invalid.
• classSize ≤ 0.
• class contains an invalid grade (eg. getGradeIndex returns -1).
Ÿ The bit representation for single precision floating point -1 is 0xBF800000.
Ÿ Reminder, even though you are returning a float, place the return value in $v0.
V The function must modify histogram in memory. histogram may already contain values.
Do not assume the memory is initialized to 0.
V Your function MUST CALL aveGradePercentage and getGradeIndex .
Examples:
CSE 220 – Spring 2018 Homework #2 Page 7
Code Return Value
calcAveClassGrade(class1, -12, hist, gradepoints1) -1
calcAveClassGrade(class1, 5, hist1, gradepoints1) 2.28
hist after function call contains [1,0,0,1,1,0,0,1,0,0,0,1]
calcAveClassGrade(class2, 10, hist, gradepoints1) 2.2599998
hist after function call contains [0,2,1,1,0,3,0,0,1,0,1,1]
g. int updateGrades(cse220_student[] class, int classSize, float[] cutoffs)
The function compares each student’s percentile to the cutoffs values and reassigns the student a
new grade . The cutoffs array has the same index mapping as specified in the aveGradePercentage
function. Use the getGrade function to get the grades using the index used for cutoffs . Each cutoff
value is inclusive, ie. if cutoffs[0] is 92.5 then any student with a percentile ≥ 92.5 is assigned an A.
You MUST use the short getGrade(int index) function provided. Treat this function as a blackbox.
Ÿ We WILL NOT test the function with an invalid cutoffs argument address, and cutoffs will
contain 12 entries.
Function parameter and return value summary:
• class : starting address of the 1D array of students in the class.
• classSize : number of entries in class array.
• cutoffs : starting address of the 1D array of float values for each grade.
• returns: 0 if success, -1 if error
Return -1 for error in any of the following cases:
• classSize ≤ 0.
• cutoffs contains non-decreasing values, ie. index 0 has value 85 and index 1 has value 90.
• cutoffs[11] does not contain 0.
V The function must modify class in memory.
Examples:
Code Return Value
updateGrades(class1, -2, cutoffs1) -1
updateGrades(class1, 5, cutoffsErr) -1
updateGrades(class1, 5, cutoffs1) 0
class1 grades stored in structs after function call are A, B+, F, B+, C+
updateGrades(class1, 5, cutoffs2) 0
class1 grades stored in structs after function call are A, A-, F, A-, C+
updateGrades(class2, 10, cutoffs1) 0
class2 grades stored in structs after function call are D+, B, B-, B+, B-, B-, C, B+, C, A
updateGrades(class2, 10, cutoffs2) 0
class2 grades stored in structs after function call are D+, B+, B, A-, B, B-, C-, A-, C-, A
CSE 220 – Spring 2018 Homework #2 Page 8
Part 3: 2D Arrays
h. (int, int) find_cheaters(cse220_exam[][] seats, int rows, int cols,
String[] cheaters)
This function will use a “seating chart” to figure out which students cheated in the exam. The seating chart
will be a rows x cols 2D array of cse220_exam structs.
Ÿ Take a look at the introduction to see the details about the cse220_exam struct.
Ÿ We guarantee that the seats array will only contain valid data. You do not need to check for valid
values.
Iterate through the 2D array in row-major order from (0, 0) to (rows-1, cols-1), checking if a student has the same exam score as any student in the adjacent (including diagonals) seats.
o Watch out for boundaries of the room!
If you find a potential cheater, add the ADDRESS of the netid of the student you are currently checking to
the list of strings specified by cheaters . You can access a student’s information in thecse220_student
struct using the address available in the cse220_exam struct.
Ÿ It is guaranteed there is enough space allocated in memory to store the addresses of all strings in
the cheaters argument.
All seats do not have to be filled. An empty seat will be denoted by a zero-ed out cse220_exam struct. All
students may not be present for the exam and therefore may not appear in seats.
Function parameter and return value summary:
• seats : starting address of a 2D array of cse220_exam structs.
• rows : the number of rows in seats.
• cols : the number of cols in seats.
• cheaters : starting address of the array of strings where all the cheaters’ netid will be stored.
• returns: (The number of cheaters, the number of students who took the exam) or (-1,-1) if error
Returns (-1, -1) for error in any of the following cases:
• rows ≤ 0
• cols ≤ 0
Examples:
Use hw2_main_cheaters to test this function. cheaters.asm contains the .data section. The
CSE 220 – Spring 2018 Homework #2 Page 9
Room1.png and Room2.png images provide a visual of each test case and the resultant cheaters array
(in terms of strings, not addresses).
Code Return Value
find_cheaters(room1, -3, 4, cheaters) (-1, -1)
find_cheaters(room1, 3, -4, cheaters) (-1, -1)
find_cheaters(room1, 3, 4, cheaters) (6, 12)
find_cheaters(room1, 4, 3, cheaters) (6, 12)
find_cheaters(room2, 3, 10, cheaters) (7, 14)
find_cheaters(room2, 5, 6, cheaters) (4, 7)
Hand-in instructions
Do not add any miscellaneous printouts, as this will probably make the grading script give you a zero. Please print
out the text exactly as it is displayed in the examples, one output line ONLY.
See Sparky Submission Instructions on Piazza for hand-in instructions. There is no tolerance for homework submission via email. They must be submitted through Sparky. Please do not wait until the last minute to submit
your homework. If you are struggling, stop by office hours.
When writing your program try to comment as much as possible. Try to stay consistent with your formatting. It
is much easier for your TA and the professor to help you if we can figure out what your code does quickly.
CSE 220 – Spring 2018 Homework #2 Page 10

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