The aim of this assignment is to introduce you to writing simple programs in MIPS
assembly and C languages. The assignment asks you to write three MIPS programs
and test them using the MARS IDE. For details on MARS, see Lab Exercise 1. The
assignment also asks you to write C code and use C code versions of the programs as
models for the MIPS versions.
1 Preliminaries
One of the most common and simplest tools that can assist the user with writing is
a spell checker. A spell checker is a program or function of a program which determines
the correctness of the spelling of a given word based on the language set being used.
The main focus of this exercise is to implement a spell checker for a subset of the
English language in C and MIPS.
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1.1 Task A: Tokenizer
The first task is a warm-up exercise. It helps you get familiar with the basic structure
of MIPS and C programs, and with using the MARS IDE.
In this task, an input file is used. The input file is a text document, consisting of
alphabetic characters (a-z and A-Z), punctuation marks ,.!? (i.e., comma, period,
exclamation, question mark) and spaces in any order and combination (without any
newlines). The input file is terminated by End-of-File (EOF). A valid input file does
not include any other characters.
You are given a MIPS file named tokenizer.s which contains a skeleton of your
program. This skeleton reads an input file named input.txt and stores its content into
a null-terminated string named content. Your task is to break the content string into
a list of tokens and output them, one token per line.
A token consists of one or more consecutive characters of exactly one of the following
three different character types:
1. Alphabetic characters, which includes only alphabetic characters (a-z and A-Z)
e.g. ’CompSys’
2. Punctuation marks, which includes only punctuation mark characters (,.!?)
e.g. ’?!?!?.?’
3. Spaces, which includes only space characters (’ ’)
e.g. ’ ’
If the content of the input file is:
Let go your earthly tether. Enter the void… Empty,and become wind.
then the output of your program should look like:
Let
go
your
earthly
tether
.
Enter
the
2
void
…
Empty
,
and
become
wind
.
A good way to go about writing a MIPS program is to first write an equivalent C
program. It is much easier and quicker to get all the control flow and data manipulations
correct in C than in MIPS. Once the C code for a program is correct, one can translate
it to an equivalent MIPS program statement-by-statement. To this end, a C version of
the desired MIPS program is provided in the file tokenizer.c.
For convenience, the C program includes definitions of functions such as read string
and print char, which mimic the behaviour of MARS system calls with the same names.
Derive your MIPS program from this tokenizer.c C program.
Do not try optimizing your MIPS code. Aim to keep the correspondence with the C
code as clear as possible. Use your C code as comments in the MIPS code to explain
the structure. Then put additional comments to document the details of the MIPS
implementation.
As a model for creating and commenting your MIPS code, have a look at the supplied
file hex.s and the corresponding C program hex.c. These are versions of the hexOut.s
program from the MIPS lab which converts an entered decimal number into hexadecimal.
1.2 Task B: Spell Checker
The goal of this task is to write a program that detects misspelled words in a string
and marks them in the output. A word is not misspelled if it exists in the dictionary.
All characters in the dictionary are lowercase. For the purposes of this exercise, spell
checking will be case insensitive. Thus, ’tHen’ is a correct spelling of a dictionary word
’then’.
This task requires an input file similar to the one in Task A. In addition to this
input file, we also use a dictionary file. The dictionary file consists of English words in
lowercase alphabetic characters (a-z) delimited by a newline (\n) and terminated by
End-of-File (EOF). A valid dictionary does not include any other characters.
You are given a MIPS file named spell checker.s that contains a skeleton of your
program. This skeleton reads an input file named input.txt and stores its content
into a null-terminated string named content. It also reads a dictionary file named
dictionary.txt and stores its content into a null-terminated string named dictionary.
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In this task, you will write a program in both C and MIPS, that in the first step,
tokenizes the content string and in the second step, checks whether each Alphabetic
token is correct; if not, the program marks it by putting underscores ( ) at the beginning
and end of the token. In the final step, the program outputs all of tokens.
If the content of a given input file is:
Everybody is speciall.Everybody.. Everybody is a HeRo, a luver, a fool,
a villain!! Everybody… Everybody has thier story to tell.
then the output of your program should look like:
Everybody is _speciall_.Everybody.. Everybody is a HeRo, a _luver_, a
fool, a villain!! Everybody… Everybody has _thier_ story to tell.
Approach this task by first writing an equivalent C program spell checker.c and
then translating this C program into a MIPS program spell checker.s. Before trans-
lating, you should compile and test your C program. Ensure it is working correctly
before starting on the MIPS code. To help you get started with the C program, an
outline spell checker.c is supplied. Furthermore, you can reuse the Tokenizer of Task
A to extract the tokens from the content string. We recommend structuring the code
into functions for ease of development and readability. See section 2.2 for further advice
and requirements on MIPS code.
As in Task A, use the C code to comment your MIPS code, and put additional
comments for MIPS-specific details. In your C program, your comments should focus
on explaining the higher-level features of your program, so your comments in the MIPS
code can mainly just concern themselves with the MIPS implementation details.
1.3 Task C: Punctuation Checker
The goal of this task is to extend Task B by adding basic punctuation checking
to your program. In this task, in addition to spell checking (Task B), your program
(punctuation checker.s) also has to detect punctuation errors and inconsistencies in
the content string and mark them in the output. The rules of correctness of Punctuation
tokens are shown in Table 1. The rules are sorted according to their priority, with higher
priority rules on top. These rules must be evaluated and executed sequentially against
each token.
If the content of a given input file is:
Everybody is speciall.. Everybody. Everybody is a HeRo ,a luver, a fool,a
villain!! Everybody… Everybody has thier story to tell….
then the output of your program should look like:
Everybody is _speciall__.._ Everybody. Everybody is a HeRo _,_a _luver_,
a fool_,_a villain_!!_ Everybody… Everybody has _thier_ story
to tell_…._
While you are encouraged to write a C program to facilitate the MIPS implementa-
tion, you should not submit the C program for this task.
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Punctuation Token Correct Incorrect Example
has a Space token before it Hello .Is
has an Alphabetic token after it Hello.Is
has ellipsis (”…”) now…
has only one punctuation mark yours?
has more than one punctuation marks Really?!
other
Table 1: Grammar checker rules
1.4 Assumptions and Restrictions on Program Inputs
Your programs in both tasks will be tested against dictionary and input files that
adhere to the following rules:
• A dictionary file will contain a maximum of 10,000 words.
• A single word in the dictionary file will contain a maximum of 20 lowercase alpha-
betic characters.
• An input file will be a maximum of 2048 characters including spaces and without
any newline character.
• An input file may contain alphabetic (a-z, A-Z), punctuation marks (,.!?) and
space (’ ’) characters.
1.5 Restrictions on the use of C Library Functions
The only C library functions that can be used are fgets and printf, which are called
within the functions print char, print int, print string and read string provided
with the C files.
1.6 Output
For your convenience, the output function is provided in both C files. You can use
it to output a string. Make sure your program does not print anything other than what
is printed by the output function as it may interfere with automated marking. In MIPS
programs, you should output exactly the same way as done in C programs.
2 Program Development and Testing
2.1 C
You can compile a C program written in a file called, for instance, prog.c at the
command prompt on the DICE machines with the following command:
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gcc -o prog prog.c
If compilation succeeded without any errors, it creates an executable prog which can
then run by entering:
./prog
2.2 MIPS
You will need to choose what kind of storage to use in MIPS for all variables from
the C code. In all tasks, we recommend you to store all tokens in the data segment. Use
the .space directive to reserve space in the data segment for arrays, preceding it with
an appropriate .align directive if the start of the space needs to be aligned to a word
or other boundary.
In the MIPS implementation of Tasks B and C, you must call at least one function
written by you. Function calls are optional in the implementation of Task A, and you are
allowed to use the same function(s) in all three tasks, if you wish. You are also allowed
to have multiple functions and nested function calls. Which functionality to abstract
into a separate function(s) is entirely up to you. To reiterate, the only requirement is
having at least one call to a user-defined function in Tasks B and C.
You must abide by the MIPS register convention. Remember that values of $t* reg-
isters are not guaranteed to be preserved across function call invocations (including both
syscall and user-defined functions), whereas values of $s* registers will be preserved.
However, do not use only $s* registers in your code; make use of $t* registers when
appropriate.
Ultimately, you must ensure that your MIPS programs assemble and run without
errors when using MARS. When testing your programs, we will run MARS from the
command line. Please check that your MIPS programs run properly in this way before
submitting them. For example, if the MARS JAR file is saved as mars.jar in the same
directory as a MIPS program prog.s, running the following command at the command-
line, assembles and then runs prog.s.
java -jar mars.jar sm prog.s
Notes:
1) The sm option tells MARS to start running at the main label rather than with
the first instruction in the code in prog.s. When running MARS with its IDE, marking
the check-box for Initialize Program Counter to global ’main’ if defined on the Settings
menu achieves the same effect.
2) MARS supports a variety of pseudo-instructions, more than the ones that are de-
scribed in the MIPS appendix of the Hennessy and Patterson book. In the past, we have
often found errors and misunderstandings in student code relating to the inadvertent use
of pseudo-instructions that are not documented in this appendix. For this reason, make
sure you only use pseudo-instructions that are explicitly mentioned in the appendix.
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3 Submission
Submit your work using the command
4 Assessment
Your programs will be primarily judged according to correctness, completeness, and
correct use of MIPS instructions, registers and function call convention.
In both C and MIPS programming, commenting a program and keeping it tidy is
very important. Make sure that you comment the code throughout and format it neatly.
A proportion of the marks will be allocated to these aspects of your code.
When editing your code, please make sure you do not use tab characters for indenta-
tion. Different editors and printing routines treat tab characters differently, and, if you
use tabs, it is likely that your code will not look pretty when we come to mark it. If you
use emacs, the command (m-x)untabify will remove all tab characters from the file in
a buffer.
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submit tokenizer.s spell checker.c spell checker.s punctuation checker.s
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Preliminaries
Task A: Tokenizer
Task B: Spell Checker
Task C: Punctuation Checker
Assumptions and Restrictions on Program Inputs
Restrictions on the use of C Library Functions
Output
Program Development and Testing
C
MIPS
Submission
Assessment
Similarity Checking and Academic Misconduct
Questions