# Week 03 Laboratory Exercises

### Objectives

• understanding how array indices are calculated
• practicing using MIPS control instructions (branch)
• learning how MIPS memory access works (lw/sw)
• practicing running MIPS programs with mipsy and mipsy-web

### Preparation

Before the lab you should re-read the relevant lecture slides and their accompanying examples.

### Getting Started

Set up for the lab by creating a new directory called lab03 and changing to this directory.
mkdir lab03
cd lab03


There are some provided files for this lab which you can fetch with this command:

1521 fetch lab03


If you're not working at CSE, you can download the provided files as a zip file or a tar file.

### Exercise — individual: Bigger MIPS

In the files for this lab, you have been given print_bigger.s, a MIPS assembler program that reads 10 numbers and then prints them:

cat numbers1.txt
12086
24363
47363
64268
34001
6800
60742
48867
26002
54999
1521 mipsy print_bigger.s <numbers1.txt
12086
24363
47363
64268
34001
6800
60742
48867
26002
54999


Add code to print_bigger.s to make it equivalent to this C program:

// Read 10 numbers into an array then print the numbers which are
// larger than the final number read.

#include <stdio.h>

#define ARRAY_LEN 10

int main(void) {
int i, final_number;
int numbers[ARRAY_LEN] = { 0 };

i = 0;
while (i < ARRAY_LEN) {
scanf("%d", &numbers[i]);
final_number = numbers[i];
i++;
}
i = 0;
while (i < ARRAY_LEN) {
if (numbers[i] >= final_number) {
printf("%d\n", numbers[i]);
}
i++;
}
}


For example:

1521 mipsy print_bigger.s <numbers1.txt
64268
60742
54999
cat numbers2.txt
53906
9064
40906
4504
4774
7892
15334
45515
55387
5681
1521 mipsy print_bigger.s <numbers2.txt
53906
9064
40906
7892
15334
45515
55387
5681


When you think your program is working, you can use autotest to run some simple automated tests:

1521 autotest print_bigger


When you are finished working on this exercise, you must submit your work by running give:

give cs1521 lab03_print_bigger print_bigger.s


You must run give before Monday 17 June 12:00 (midday) (2024-06-17 12:00:00) to obtain the marks for this lab exercise. Note that this is an individual exercise, the work you submit with give must be entirely your own.

### Exercise — individual: MIPS Order Checking

In the files for this lab, you have been given unordered.s, a MIPS assembler program that reads 10 numbers and then prints 42:

Add code to unordered.s to make it equivalent to this C program:

// Read 10 numbers into an array
// print 0 if they are in non-decreasing order
// print 1 otherwise

#include <stdio.h>

#define ARRAY_LEN 10

int main(void) {
int i;
int numbers[ARRAY_LEN] = { 0 };

i = 0;
while (i < ARRAY_LEN) {
scanf("%d", &numbers[i]);
i++;
}

int swapped = 0;
i = 1;
while (i < ARRAY_LEN) {
int x = numbers[i];
int y = numbers[i - 1];
if (x < y) {
swapped = 1;
}
i++;
}

printf("%d\n", swapped);
}


For example:

cat numbers1.txt
12086
24363
47363
64268
34001
6800
60742
48867
26002
54999
1521 mipsy unordered.s <numbers1.txt
1
cat sorted.txt
1
2
3
4
5
6
7
8
9
10
1521 mipsy unordered.s <sorted.txt
0


When you think your program is working, you can use autotest to run some simple automated tests:

1521 autotest unordered


When you are finished working on this exercise, you must submit your work by running give:

give cs1521 lab03_unordered unordered.s


You must run give before Monday 17 June 12:00 (midday) (2024-06-17 12:00:00) to obtain the marks for this lab exercise. Note that this is an individual exercise, the work you submit with give must be entirely your own.

### Exercise — individual: MIPS Swapping

In the files for this lab, you have been given swap_numbers.s, a MIPS assembler program that reads 10 numbers and then prints them:

Add code to swap_numbers.s to make it equivalent to this C program:

// Read 10 numbers into an array
// swap any pair of numbers which are out of order
// then print the array

#include <stdio.h>

#define ARRAY_LEN 10

int main(void) {
int i;
int numbers[ARRAY_LEN] = { 0 };

i = 0;
while (i < ARRAY_LEN) {
scanf("%d", &numbers[i]);
i++;
}

i = 1;
while (i < ARRAY_LEN) {
int x = numbers[i];
int y = numbers[i - 1];
if (x < y) {
numbers[i] = y;
numbers[i - 1] = x;
}
i++;
}

i = 0;
while (i < ARRAY_LEN) {
printf("%d\n", numbers[i]);
i++;
}
}


For example:

1521 mipsy swap_numbers.s <numbers1.txt
12086
24363
47363
34001
6800
60742
48867
26002
54999
64268
1521 mipsy swap_numbers.s <numbers2.txt
9064
40906
4504
4774
7892
15334
45515
53906
5681
55387


When you think your program is working, you can use autotest to run some simple automated tests:

1521 autotest swap_numbers


When you are finished working on this exercise, you must submit your work by running give:

give cs1521 lab03_swap_numbers swap_numbers.s


You must run give before Monday 17 June 12:00 (midday) (2024-06-17 12:00:00) to obtain the marks for this lab exercise. Note that this is an individual exercise, the work you submit with give must be entirely your own.

### Challenge Exercise — individual: MIPS Bubbles

In the files for this lab, you have been given bubblesort.s, a MIPS assembler program that reads 10 numbers and then prints them:

Add code to bubblesort.s to make it equivalent to this C program:

// Reads 10 numbers into an array, bubblesorts them
// and then prints the 10 numbers
// then print them

#include <stdio.h>

#define ARRAY_LEN 10

int main(void) {
int i;
int numbers[ARRAY_LEN] = { 0 };

i = 0;
while (i < ARRAY_LEN) {
scanf("%d", &numbers[i]);
i++;
}

int swapped = 1;
while (swapped) {
swapped = 0;
i = 1;
while (i < ARRAY_LEN) {
int x = numbers[i];
int y = numbers[i - 1];
if (x < y) {
numbers[i] = y;
numbers[i - 1] = x;
swapped = 1;
}
i++;
}
}

i = 0;
while (i < ARRAY_LEN) {
printf("%d\n", numbers[i]);
i++;
}
}


For example:

1521 mipsy bubblesort.s <numbers1.txt
6800
12086
24363
26002
34001
47363
48867
54999
60742
64268
1521 mipsy bubblesort.s <numbers2.txt
4504
4774
5681
7892
9064
15334
40906
45515
53906
55387


When you think your program is working, you can use autotest to run some simple automated tests:

1521 autotest bubblesort


When you are finished working on this exercise, you must submit your work by running give:

give cs1521 lab03_bubblesort bubblesort.s


You must run give before Monday 17 June 12:00 (midday) (2024-06-17 12:00:00) to obtain the marks for this lab exercise. Note that this is an individual exercise, the work you submit with give must be entirely your own.

### Challenge Exercise — individual: Reversing MIPS

In most of the previous lab exercises involving MIPS assembly you have been given a C program and your task has been to write an equivalent MIPS assembly program. In this challenge exercise you will do the opposite: you have been given a MIPS program unmips.s, and you must write an equivalent C program in unmips.c.

Deciphering the behaviour of an assembly program can be a very useful skill in a variety of situations, such as during reverse engineering, when examining compiler output, or when attempting to exploit certain security vulnerabilities.

unmips.s does not have any comments, and its label names are extremely bad. This is typical of compiler output and the output of disassemblers, and hopefully also demonstrates the importance of good style in human written assembly.

You may assume that the input to unmips.c will consist of three lines (with each line followed by a newline character):

• The first line will consist of 70 characters, each either a 0 or 1.
• The second line will be a positive integer less than 1000.
• The third line will be an integer greater than or equal to 0 and less than 1000.

For example:

make unmips
./unmips
0111011110000111111100001101001111111101110011101100101000011001101011
22
22
@        @  @       @  @   @@@            @@      @@@ @@  @  @@   @
@@      @@@@@@     @@@@@@ @   @          @  @    @      @@@@@  @ @@@
@    @      @   @       @@ @@@        @@@@@@  @@@    @     @@@    @
@@@  @@@    @@@ @@@     @      @      @      @@   @  @@@   @   @  @@@
@   @@   @  @       @   @@@    @@@    @@@    @  @ @@@@   @ @@@ @@@@
@@ @  @ @@@@@@     @@@ @   @  @   @  @   @  @@@@@     @ @@         @
@@@@       @   @    @@ @@@@@@ @@@@@@ @@@@     @   @@   @       @@@
@    @     @@@ @@@  @                     @   @@@ @  @ @@@     @   @
@@@  @@@   @       @@@@                   @@@ @    @@@@    @   @@@ @@
@   @@   @ @@@     @    @                 @    @@  @    @  @@@ @
@@ @  @ @@    @   @@@  @@@               @@@  @  @@@@  @@@@    @@
@@@@   @  @@@ @   @@   @             @   @@@@@    @@    @  @  @
@    @ @@@@    @@ @  @ @@@           @@@ @     @  @  @  @@@@@@@@@
@@@  @@     @  @   @@@@    @         @    @@   @@@@@@@@@@         @
@   @@  @   @@@@@@ @    @  @@@       @@@  @  @ @          @       @@@
@@ @  @@@@ @       @@  @@@@   @     @   @@@@@@ @@        @@@     @   @
@@@     @@     @  @@    @ @@@   @@@ @         @      @   @   @@@ @@
@   @   @  @   @@@@  @  @@    @ @    @@       @@@    @@@ @@@ @
@@@ @@@ @@@@@@ @    @@@@@  @  @@ @@  @  @     @   @  @        @@
@               @@  @     @@@@@     @@@@@@@   @@@ @@@@@@      @  @
@@             @  @@@@   @     @   @       @ @          @    @@@@@@
@           @@@@    @ @@@   @@@ @@@     @@ @@        @@@  @      @


You can find more examples in the provided unmips_examples.txt.

When you think your program is working, you can use autotest to run some simple automated tests:

1521 autotest unmips


When you are finished working on this exercise, you must submit your work by running give:

give cs1521 lab03_unmips unmips.c


You must run give before Monday 17 June 12:00 (midday) (2024-06-17 12:00:00) to obtain the marks for this lab exercise. Note that this is an individual exercise, the work you submit with give must be entirely your own.

### Submission

When you are finished each exercises make sure you submit your work by running give.

You can run give multiple times. Only your last submission will be marked.

Don't submit any exercises you haven't attempted.

If you are working at home, you may find it more convenient to upload your work via give's web interface.

Remember you have until Week 4 Monday 12:00:00 (midday) to submit your work without receiving a late penalty.

You cannot obtain marks by e-mailing your code to tutors or lecturers.

You check the files you have submitted here.

Automarking will be run by the lecturer several days after the submission deadline, using test cases different to those autotest runs for you. (Hint: do your own testing as well as running autotest.)

After automarking is run by the lecturer you can view your results here. The resulting mark will also be available via give's web interface.

#### Lab Marks

When all components of a lab are automarked you should be able to view the the marks via give's web interface or by running this command on a CSE machine:

1521 classrun -sturec