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Department of Computer Science
COMPX203 Computer Systems
Exercise 2 – C and WRAMP Programming
Due Date: 13 April 2022
Objectives
This exercise explores the relationship between high level programming languages, assembly code,
and the system stack. You’ll be writing programs using C and WRAMP assembly code, and interacting
between the two using the system stack.
It is highly recommended that you read this entire specification before beginning to write any code.
Assessment
This exercise contributes 10% towards your internal grade, to be assessed in two parts:
● The correctness of your source code, to be submitted via Moodle (7%)
● Your completion of an online quiz about the exercise, also on Moodle (3%)
Both of these parts must be completed on or before the due date.
This is an individual exercise; while you may discuss the assignment with others in general terms,
you must write your own code, and complete the quiz by yourself.
The name of the file must match the following format:
ex[exercise number]_q[question number].srec
For example, the third question in this exercise would be called ex2_q3.srec
Questions
- Passing Parameters using the Stack
Create a new WRAMP assembly file called ex2_q1.s, and in that file create a subroutine called
print. You don’t need to declare a main label in this file, because we’ll link against
another file later on that contains the main subroutine of the program.
print is a function that takes a single integer parameter and prints it on the first serial port,
then returns. The caller will read the switches and call print with the integer as argument.
Your code should only print the value received by the caller on the first serial port. The
maximum decimal value represented by the switches is 65535 (when all switches are on or
‘up’). Since the Basys board has only four SSDs, we will send the value of the switches to the
first serial port instead. This will allow showing the full decimal number represented by the
switches (four SSDs will show only up to 9999 decimal) on the serial port. The value printed
should be padded to the left with 0s if it is shorter than five characters. For example, if the
number given is 1234, your code should print“01234”.
For testing, you should be able to change the switches positions and type go to see the decimal
value of the switches on the screen (you need to enter go whenever you change the
switches to see the new value on the screen, as the main object file reads the switches and
calls print only once, then returns). Hint: a five digit decimal number cannot be sent to the
serial port as is, you have to send each digit separately.
To form a complete program, your ex2_q1.s file needs to be assembled and linked with two
other object files: one containing the main subroutine, and the other containing the
collection of library functions listed at the end of this document. The library file (lib_ex2.o)
and main_q1.o can be found at /home/compx203/ex2/.
Figure 1: Layout of the program for question 1. Grey boxes are the files we provide.
Assemble and link your code using the commands:
wasm ex2_q1.s
wlink -o ex2_q1.srec
ex2_q1.o
/home/compx203/ex2/main_q1.o
/home/compx203/ex2/lib_ex2.o
(wlink’s arguments should be on a single line, separated by spaces)
Once you’ve created your S-Record, load and run your program. If it works correctly, you
should see a decimal number appearing in either the“Serial Port 1”form if you’re using
wsim, or in the“remote”window if you’re using a physical Basys board.
QUESTION 1 MUST BE WRITTEN IN WRAMP ASSEMBLY – DO NOT USE C
To ensure your program works correctly, be sure to follow the conventions described in the
WRAMP manual (Chapter 2: Stack Guide) and lecture notes. Failing to do so will result in
grade deduction(s), even if the program is working as required. - A Counter Implementation using C
C is a high-level language that offers many advantages over assembly code, enabling the use of
familiar programming structures like for-loops, functions (“methods”), and if-statements.
Start by creating a C file called ex2_q2.c, which will contain a single function called count. This
function takes two integer parameters, called start and end. It should look something
like:
/** - Counts from “start” to “end” (inclusive),
- showing progress on the seven segment displays.
**/
void count(int start, int end) {…}
When called, this function begins counting from the value start, and continues until it
reaches the value end (both inclusive). As it counts, each number is shown on the seven
segment displays as a four-digit base-10 number. If start is larger than end, it counts
down; otherwise, it counts up.
To ensure that start and end are sensible values, check that they are both within the
range 0 <= n < 10000. If either number falls outside this range, you should return without
doing any counting. If start and end are equal, you can choose to either return without
doing any counting, or set the SSDs to the value provided as though counting from start
to the same number.
For example, if we call count(6, 12), the function should output 06, 07, 08, 09, 10, 11, to the seven segment displays, with a short delay between each number.
Hints:
● For the compiler to know how to call the library functions, you need to put the lineinclude “/home/compx203/ex2/lib_ex2.h”
at the top of your C file, hard against the left margin. Don’t copy/paste, because
PDFs often contain non-ASCII characters that the compiler doesn’t understand.
● See function descriptions at the end of the assignment.
● All variables need to be declared at the top of a block, e.g., the top of a function.
● Unlike in the previous lab exercise, the writessd() function in the lib_ex2.o
library formats the value as a base-10 (decimal) number.
● Calling the delay() function after each call to writessd() will slow down the
counting so that it is visible.
● You don’t have to worry about the stack for this question, because the compiler will
take care of it for you. Just call the necessary functions like you would in C# or Java.
● The WRAMP compiler is wcc. To compile your code, use the command:
wcc -S ex2_q2.c
The“-S”tells the compiler to generate an assembly file as its output, which will be
called ex2_q2.s in this case. Open up this file with a text editor, and see how it
corresponds to your original C code.
Figure 2: Layout of the program for question 2. Grey boxes are files we provide.
Next, complete the compile/assemble/link process according to the layout shown in Figure- The main subroutine is implemented in the file /home/compx203/ex2/main_q2.o. This
main subroutine reads the value of the switches as two 8-bit numbers; the leftmost eight
bits are passed to your C function as the parameter start, and the rightmost eight bits are
passed as the parameter end.
When you type go, your program should count from the 8-bit number on the leftmost
switches, to the number on the rightmost switches, in base 10. For example, if the leftmost
switches were all up, and the rightmost switches were all down, your program should count
from 255 to 0. You’ll notice the leftmost seven-segment display will always be 0 – this is fine!
You can type go as many times as you like without resetting the board, which makes it easy
to try other switch combinations. - Manually Calling count() from Assembly Code
Replicate the functionality of the program in Question 2, but with the values of the switches
reversed: that is, the rightmost eight bits are now passed to your C function as start, while
the leftmost eight bits are now passed as end. For example, if the leftmost switches were all
down, and the rightmost switches were all up, your program should now count from 255
to 0.
To do this, create a new WRAMP assembly file called ex2_q3.s, and in that file create the main
subroutine for the program. When assembled, this will serve as a drop-in replacement for
the main_q2.o file provided and used in question 2. Do not modify any existing code;
instead, just link against the unmodified ex2_2.o file as shown in Figure 3.
This means that the code in your ex2_q3.s file will need to call readswitches and count by
following all the relevant subroutine conventions.
Hints:
● Make sure that you can run your program (i.e. by typing go) multiple times without
resetting the board. This requires halting your program either by returning from
main, or using the syscall instruction.
● The subroutine main is like any other: if you want to return from it after calling any
other subroutines, you first have to save $ra.
QUESTION 3 MUST BE WRITTEN IN WRAMP ASSEMBLY, NOT C!
Figure 3: Layout of the program for Question 3. Grey boxes are files we provide, while light orange boxes are the files
you created for Question 2.
Tips
This lab exercise is often seen as one of the most difficult in the course. Do yourself a favour and
start early, make use of the supervised lab sessions, and ask for help sooner rather than later if you
get stuck.
That being said, the programs are not huge. If you find yourself writing lots of code, stop and think
about the problem again. Draw diagrams. Explore the debugging features of wsim, especially the
“Memory (RAM)”form that shows the stack and $sp. Ask questions.
For reference, model solutions are (for each question): - 39 lines, only 14 of which are assembly instructions that don’t just back up registers onto the
stack - 30 lines, including 14 that are either whitespace or a single curly bracket
- 19 lines, only 13 of which are assembly instructions
If you’re writing significantly more than this, chances are that you’re doing something wrong.
Submission
You are required to submit all source files that you have written; you do not need to submit any files
that we have provided, nor any files that have been generated by a tool, e.g., wcc, wasm or wlink.
Each file must follow the naming convention indicated below.
For this exercise, the required files are:
● ex2_q1.s (Question 1)
● ex2_q2.c (Question 2)
● ex2_q3.s (Question 3)
These files must be compressed into a single“tar gz”archive called firstName_lastName.tgz
(replace firstName_lastName with your own name) before being submitted to Moodle. You can
create this archive from the terminal, using the command:
tar -cvzf firstName_lastName.tgz ex2_q1.s ex2_q2.c ex2_q3.s
Be especially careful about file extensions with this one – we’re not interested in your ex2_q2.s file,
because we can easily generate that from your ex2_q2.c file (which we are very interested in).
Please be aware that a grade penalty applies if you do not submit your code in the correct format,
including using the correct file names and file extensions. If you are unsure if what you’ve done is
correct, please ask and we’ll help you out.
Library Functions (/home/compx203/ex2/lib_ex2.o)
void putc(int c)
Parameters: c The ASCII-encoded character to transmit
Returns: none
Transmits a single ASCII character through the first serial port. This character will appear in remote if using the
physical REX boards, or the Serial Port 1 form if using the simulator. If the serial port is not ready to transmit,
this function will block.
int getc()
Parameters: none
Returns: The ASCII-encoded character that was read from the serial port
Reads a single ASCII character from the first serial port. This is a character that was sent via remote if using the
physical REX boards, or via the Serial Port 1 form if using the simulator. This function will block until a character
is received.
void putstr(const char *s)
Parameters: *s A pointer to the string to be written to the serial port
Returns: none
Transmits a null-terminated ASCII string through the first serial port. This string will appear in remote if using
the physical REX boards, or in the Serial Port 1 form if using the simulator.
int readswitches()
Parameters: none
Returns: The value represented by the switches
Reads the current value represented by the switches into register. The lowest 16 bits (i.e. bits 0 to 15) will have
the value of the corresponding switch (on or off), while the remaining bits (15 to 31) will be set to zero. The up
(or‘on’) position is represented as a binary‘1’.
void writessd(int n)
Parameters: n The number to write to the seven-segment displays. 0 <= n < 10000.
Returns: none
Writes a number to the seven-segment displays, formatted as a four-digit decimal number.
void delay()
Parameters: none
Returns: none
Delays execution for approximately 135 ms.
