The purpose of this assignment is to help you learn about computer architecture, assembly language programming, and testing strategies. It also will give you the opportunity to learn more about the GNU/Unix programming tools, especially Bash, Emacs, GCC, and GDB for assembly language programs.
The assignment consists of two parts, each of which has subparts.
We strongly encourage you to complete Part 1 by the end of the first
week of the two-week assignment period.
The Unix operating system has a command named wc (word
count). In its simplest form, wc reads characters from stdin
until end-of-file, and prints to stdout a count of how many lines, words, and
characters it has read. A word is a sequence of characters that is delimited
by one or more white space characters.
Consider some examples. In the following, a space is shown as "s" and a newline character as "n".
If the file named proverb contains these characters:
Learningsissan treasureswhichn accompaniessitsn ownerseverywhere.n --sChinesesproverbn
then the command:
$ wc < proverb
prints this line to standard output:
5 12 82
If the file proverb2 contains these characters:
Learningsissan treasureswhichn accompaniessitsn ownerseverywhere.n --sssChinesesproverb
(note that the last "line" does not end with a newline character) then the command:
$ wc < proverb2
prints this line to standard output:
4 12 83
The file mywc.c in the /u/cos217/Assignment4 directory contains a C program that implements the subset of the wc command described above. Translate that program into assembly language, thus creating a file named mywc.s. It is acceptable to use global (i.e. bss section or data section) variables in mywc.s, but we encourage you to use local (i.e. stack) variables instead. Your assembly language program should have exactly the same behavior (i.e. should write exactly the same characters to stdout) as the given C program.
Design a test plan for your mywc program. Your test plan should include tests in three categories: (1) boundary testing, (2) statement testing, and (3) stress testing.
Create text files to test your programs. Name each such file such that its prefix is "mywc" and its suffix is ".txt". The command "ls mywc*.txt" should display the names of all mywc test files, and only those files.
Describe your mywc test plan in your readme file. Your description should have this structure:
mywc boundary tests:
mywcXXX.txt: Description of the
characteristics of that file, and how it tests boundary conditions of your
mywcYYY.txt: Description of the characteristics of that file, and how it tests boundary conditions of your mywc program.
mywc statement tests:
mywcXXX.txt: Description of the
characteristics of that file, and which statements of your mywc program it
mywcYYY.txt: Description of the characteristics of that file, and which statements of your mywc program it tests.
Your descriptions of the test files should be of the form "This file contains such-and-such characteristics, and so tests lines such-and-such of the program." You may identify the lines of code tested either by description (e.g. "the second 'if' statement") or by line number (e.g. lines 20 though 25). Line numbers may refer to either your assembly language code or the corresponding C code.
mywc stress tests:
mywcXXX.txt: Description of the
characteristics of that file, and how it stress tests your mywc
mywcYYY.txt: Description of the characteristics of that file, and how it stress tests your mywc program.
Submit test files that contain no more than approximately 50000 characters. Submitting very large files could exhaust the course's allotted disk space on hats, and so could prohibit other students from submitting any files at all.
Submit test files that contain only printable ASCII characters. Specifically, make sure your computer-generated test files contain only characters having ASCII codes (in hexadecimal) 09, 0A, and 20 through 7E. Submitting test files that contain other characters would make it difficult for your grader to examine those files.
Finally, create a Bash shell script named testmywc to automate your mywc test plan. A Bash shell script is simply a text file that contains commands, and that has been made executable via the chmod command, for example, "chmod 700 testmywc".
The testmywc script should build a mywc program from the given C code, build a mywc program from your assembly language code, execute both programs, and compare the output.
It is acceptable for your testmywc script to call other scripts that you create. Each such script should have a name that is prefixed with "testmywc". The command "ls testmywc*" should display the names of all mywc test scripts, and only those files.
Feel free to use the testdecomment and testdecommentdiff Bash shell scripts from Assignment 1 as models.
Many programming environments contain modules to handle high-precision integer arithmetic. For example, the Java Development Kit (JDK) contains the BigDecimal and BigInteger classes.
The Fibonacci numbers are used often in computer science. See http://en.wikipedia.org/wiki/Fibonacci_numbers for some background information on Fibonacci numbers. Note in particular that Fibonacci numbers can be very large integers.
This part of the assignment asks you to write a minimal high-precision integer arithmetic module, and use it to compute large Fibonacci numbers.
Suppose you must compute Fibonacci number 500000, that is, fib(500000)...
The /u/cos217/Assignment4 directory contains a C program that computes Fibonacci numbers. It consists of two modules: a client module and a BigInt ADT.
The client consists of the file fib.c. The client accepts an integer n as a command-line argument, validates it, and computes and prints fib(n) to stdout as a hexadecimal number. It prints to stderr the amount of CPU time consumed while performing the computation. The client module delegates most of the work to BigInt objects.
The BigInt ADT performs high precision integer arithmetic. It is a minimal ADT; essentially it implements only an "add" operation. The BigInt ADT consists of four files:
Study the given code. Then build the program with no optimization. Run the program to compute fib(500000). In your readme file note the amount of CPU time consumed.
Suppose you decide that the amount of CPU time consumed is unacceptably large. You decide to command the compiler to optimize the code that it produces...
Build the program using optimization. Specifically, specify the -DNDEBUG option so the preprocessor disables the assert() macro, and the -O3 option so the compiler generates optimized code. Run the resulting program to compute fib(500000). In your readme file note the amount of CPU time consumed.
Suppose you decide that the amount of CPU time consumed still is too large. In an attempt to gain speed, you decide manually to code the BigInt_add() function in assembly language...
Manually translate the C code in the bigintadd.c file into assembly language, thus creating the file bigintadd.s. You need not translate the code in other files into assembly language.
Your assembly language code should store all variables in memory. It should contain definitions of the BigInt_add() and BigInt_larger() functions; the former should call the latter, just as the C code does.
Note that assert() is a parameterized macro, not a function. (See Section 14.3 of the King book for a description of parameterized macros.) So you cannot call assert() from assembly language code. When translating bigintadd.c to assembly language, simply pretend that the calls of assert() are not in the C code.
Build the program consisting of the files fib.c, bigint.c, and bigintadd.s using the -DNDEBUG and -O3 options. Run the program to compute fib(x) for various values of x, and make sure it writes the same output as the program built from C code does. Finally, run the program to compute fib(500000). In your readme file note the amount of CPU time consumed.
Suppose, to your horror, you discover that you have taken a step backward: the CPU time consumed by your assembly language code is approximately the same as that of the non-optimized compiler-generated code. So you decide to optimize your assembly language code...
Manually optimize your assembly language code in bigintadd.s, thus creating the file bigintaddopt.s. Specifically, perform these optimizations:
Build the program consisting of the files fib.c, bigint.c, and bigintaddopt.s using the -DNDEBUG and -O3 options. Run the program to compute fib(x) for various values of x, and make sure it writes the same output as the program built from C code does. Finally, run the program to compute fib(500000). In your readme file note the amount of CPU time consumed.
Can you write assembly language code that is approximately as fast as the optimized code that the compiler generates? That is, can you approximately tie the compiler?
Finally, suppose you decide to optimize your assembly language code even further, moving away from a statement-by-statement translation of C code into assembly language...
Further optimize your assembly language code in bigintaddopt.s, thus creating the file bigintaddoptopt.s. Specifically, perform these optimizations:
Feel free to implement any additional optimizations you want. However, your BigInt_add() function must be a general-purpose function for adding two given BigInt objects; the function cannot be specific to the task of adding two Fibonacci numbers to generate a third Fibonacci number.
The extra credit optimizations are difficult. To do them you will need to learn about the "adcl" instruction, and about which instructions affect and do not affect the C ("carry") flag in the EFLAGS register. We encourage you to do the extra credit optimizations, but will not think unkindly of you if you decide not to.
Hint: When writing bigintaddoptopt.s, the problem is this: How can you preserve the value of the C flag between executions of the adcl instruction? One approach to solving that problem is to determine how to save and then restore the value of the EFLAGS register. Another approach is to determine how to express the logic such that only instructions that do not affect the C flag are executed between each execution of the adcl instruction.
Build the program consisting of the files fib.c, bigint.c, and bigintaddoptopt.s using the -DNDEBUG and -O3 options. Run the program to compute fib(x) for various values of x, and make sure it writes the same output as the program built from C code does. Finally, run the program to compute fib(500000). In your readme file note the amount of CPU time consumed.
Can you beat the compiler?
Develop on hats. Use Emacs to create source code. Use GDB to debug.
Do not use a C compiler to produce any of your assembly language code. Doing so would be considered an instance of academic dishonesty. Instead produce your assembly language code manually.
We encourage you to develop "flattened" C code (as described in precepts) to bridge the gap between the given "normal" C code and your assembly language code. Using flattened C code as a bridge can eliminate logic errors from your assembly language code, leaving only the possibility of translation errors.
We also encourage you to use your flattened C code as comments in your assembly language code. Such comments can clarify your assembly language code substantially.
You should submit:
Your readme file should contain:
Submit your work electronically using the commands:
submit 4 mywc.s submit 4 testmywc submit 4 anyBashScriptsCalledByTestmywc submit 4 mywc*.txt submit 4 bigintadd.s submit 4 bigintaddopt.s submit 4 bigintaddoptopt.s (for extra credit) submit 4 readme
Please do not submit Emacs backup files, that is, files that end with '~'.
As always, we will grade your work on quality from the user's and programmer's points of view. To encourage good coding practices, we will deduct points if gcc217 generates warning messages.
Comments in your assembly language programs are especially important. Each assembly language function -- especially the main() function -- should have a comment that describes what the function does. Local comments within your assembly language functions are equally important. Comments copied from corresponding "flattened" C code are particularly helpful.
Your assembly language code should use .equ directives to avoid "magic numbers." In particular, you should use .equ directives to give meaningful names to:
Testing is a substantial aspect of the assignment. Approximately 10% of the grade will be based upon your mywc test plan as described in your readme file, and as implemented by your test scripts and data files.
Your grade for the extra credit option will be based upon raw performance (How quickly does your code compute fib(500000)?) and style (Does your code contain optimizations that logically should improve performance?).