Lab 1: Instructions

When you’re ready to submit your solution, go to the assignments list.

Lab 1: Manipulating Bits Using C

Overview

The purpose of this assignment is to become more familiar with data at the bit-level representation. You’ll do this by solving a series of programming “puzzles”. Many of these puzzles may seem artificial, but in fact bit manipulations are very useful in cryptography, data encoding, implementing file formats (e.g., MP3), etc. By working your way through these problems, you will get very familiar with bit representations and hopefully will have some fun. You will also be doing some very basic pointer manipulations and arithmetic. Again, the purpose is to get you familiar with data representations and pointers.

Instructions

Perform an update to your course-materials directory on the by running the update-course command from a terminal window. On the script’s success, you should find the provided code for lab1 in your course-materials directory. As a convenience, here is an archive of the course-materials directory as of this lab assignment: lab1.tar.gz.

The lab1 folder contains a number of tools, described later, a bits.c file, and a pointer.c file. Both files contain skeletons for the programming puzzles, along with a comment block that describes exactly what the function must do and what restrictions there are on its implementation. Your assignment is to complete each function skeleton using:

• only straightline code (i.e., no loops or conditionals);
• a limited number of C arithmetic and logical operators; and
• no constants larger than 8 bits (i.e., 0 – 255 inclusive).

The intent of the restrictions is to require you to think about the data as bits – because of the restrictions, your solutions won’t be the most efficient way to accomplish the function’s goal, but the process of working out the solution should make the notion of data as bits completely clear.

Similarly, you will start working with basic pointers and use them to compute the size of different data items in memory and to modify the contents of an array

The Bit Puzzles

This section describes the puzzles that you will be solving in bits.c. More complete (and definitive, should there be any inconsistencies) documentation is found in the bits.c file itself.

Bit Manipulations

The table below describes a set of functions that manipulate and test sets of bits. The Rating column gives the difficulty rating (the number of points) for each puzzle and the Description column states the desired output for each puzzle along with the constraints. See the comments in bits.c for more details on the desired behavior of the functions. You may also refer to the test functions in tests.c. These are used as reference functions to express the correct behavior of your functions, although they don’t satisfy the coding rules for your functions.

Two’s Complement Arithmetic

The following table describes a set of functions that make use of the two’s complement representation of integers. Again, refer to the comments in bits.c and the reference versions in tests.c for more information.

Checking Your Work

We have included two tools to help you check the correctness of your work.

dlc is a modified version of an ANSI C compiler from the MIT CILK group that you can use to check for compliance with the coding rules for each puzzle. The typical usage is:

$ ./dlc bits.c

The program runs silently unless it detects a problem, such as an illegal operator, too many operators, or non-straightline code in the integer puzzles. Running with the -e switch:

$ ./dlc -e bits.c

causes dlc to print counts of the number of operators used by each function. Type ./dlc -help for a list of command line options.

btest is a program that checks the functional correctness of the code in bits.c. To build and use it, type the following two commands:

$ make
$ ./btest

Notice that you must rebuild btest each time you modify your bits.c file. (You rebuild it by typing make.) You’ll find it helpful to work through the functions one at a time, testing each one as you go. You can use the -f flag to instruct btest to test only a single function:

$ ./btest -f bitXor

You can feed it specific function arguments using the option flags -1, -2, and -3:

$ ./btest -f bitXor -1 7 -2 0xf

Check the file README for documentation on running the btest program.

Advice

Start early on bits.c, if you get stuck on one problem move on. You may find you suddenly realize the solution the next day. Puzzle over the problems yourself, it is much more rewarding to find the solution yourself than stumble upon someone else’s solution. If you do not quite meet the operator limit don’t worry there will be partial credit, but often times working with a suboptimal solution will allow you to see how to improve it.

Do not include the <stdio.h> header file in your bits.c file, as it confuses dlc and results in some non-intuitive error messages. You will still be able to use printf in your bits.c file for debugging without including the <stdio.h> header, although gcc will print a warning that you can ignore.

You should be able to use the debugger on your code. For example:

$ make
gcc -O -Wall -m32 -g -lm -o btest bits.c btest.c decl.c tests.c
gcc -O -Wall -m32 -g -o fshow fshow.c
gcc -O -Wall -m32 -g -o ishow ishow.c
$ gdb ./btest
GNU gdb (GDB) Fedora (7.1-34.fc13)
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-redhat-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Reading symbols from /homes/iws/dvhc/cse351/lab1/src/btest...done.
(gdb) b bitXor
Breakpoint 1 at 0x8048717: file bits.c, line 144.
(gdb) r
Starting program: /homes/iws/dvhc/cse351/lab1/src/btest
ScoreRatingErrorsFunction

Breakpoint 1, bitXor (x=-2147483648, y=-2147483648) at bits.c:144
144}
(gdb) p x
$1 = -2147483648
(gdb) p/x x
$2 = 0x80000000
(gdb) q
A debugging session is active.

Inferior 1 [process 12728] will be killed.

Quit anyway? (y or n) y

The dlc program enforces a stricter form of C declarations than is the case for C++ or that is enforced by gcc. In particular, in a block (what you enclose in curly braces) all your variable declarations must appear before any statement that is not a declaration. For example, dlc will complain about the following code:

int foo(int x)
{
    int a = x;
    a *= 3;     /* Statement that is not a declaration */
    int b = a;  /* ERROR: Declaration not allowed here */
}

Instead, you must declare all your variables first, like this:

int foo(int x)
{
    int a = x;
    int b;
    a *= 3;
    b = a;
}

Using Pointers

This section describes the four functions you will be completing in pointer.c that is also in the lab1 folder you downloaded. Refer to the file pointer.c itself for more complete details.

Pointer Arithmetic

The first three functions in pointer.c ask you to compute the size (in bytes) of various data elements (ints, doubles, and pointers). You will accomplish this by noting that arrays of these data elements allocate contiguous space in memory so that one element follows the next. You are permitted to use casts for these functions.

Manipulating Data Using Pointers

The fourth function in pointer.c asks you to change the value of an element of an array using only the starting address of the array. You will add the appropriate value to the pointer to create a new pointer to the data element to be modified.

The next two functions deal with boundings checking of pointers and the final function deals with more bit manipulation. You

Checking your work

For pointer.c, we have included a simple test harness: ptest.c. You can test your solutions like this:

$ make ptest
$ ./ptest

This test harness only checks if your solutions return the expected result, not if they meet the specific criteria of each problem. We will review your solutions to ensure they meet the restrictions of the assignment.

Submitting Your Work

You will be able to submit your assignment with the submit-hw script that is bundled with the lab1 course-materials update. Using the script should be straight-forward, but it does expect you to not move/rename any files from the ”course-materials” directory. Open a terminal and type the following commands:

  submit-hw lab1.bits
  submit-hw lab1.pointer

This will submit both bits.c and pointer.c (one file per command). However, you may just want to submit all parts for Lab1 together. You may use the following command to do so. It will save you the trouble of calling both commands, but it will still count as a submission for each part.

  submit-hw lab1

After calling the submit-hw script, you will be prompted for your Coursera username and then your submission password. Your submission password is NOT the same as your regular Coursera account password!!!! You may locate your submission password at the top of the assignments list page.

After providing your credentials, the submit-hw script will run some basic checks on your code. For bits.c, it will run the driver.pl script and warn you if you get below a score of 41 (full-credit without the extra-credit). For pointer.c, it will simply check to see if ptest reports any failures to the tests.

Once confirming that you wish to submit, with a working Internet connection, the script should submit your code properly. You can go to the assignments list page to double-check that it has been submitted and check your score as well as any feedback the auto-grader gives you. You may also download your submission code from this page, if you wish.

// Rating: 1
int bitAnd(int, int);
int test_bitAnd(int, int);
int bitXor(int, int);
int test_bitXor(int, int);
int thirdBits();
int test_thirdBits();
// Rating: 2
int fitsBits(int, int);
int test_fitsBits(int, int);
int sign(int);
int test_sign(int);
int getByte(int, int);
int test_getByte(int, int);
// Rating: 3
int logicalShift(int, int);
int test_logicalShift(int, int);
int addOK(int, int);
int test_addOK(int, int);
// Rating: 4
int bang(int);
int test_bang(int);
// Extra Credit: Rating: 3
int conditional(int, int, int);
int test_conditional(int, int, int);
// Extra Credit: Rating: 4
int isPower2(int);
int test_isPower2(int);

/*
 * CSE 351 HW1 (Data Lab )
 *
 * *
 * bits.c - Source file with your solutions to the Lab.
 * This is the file you will hand in to your instructor.
 *
 * WARNING: Do not include the header; it confuses the dlc
 * compiler. You can still use printf for debugging without including
 * , although you might get a compiler warning. In general,
 * it's not good practice to ignore compiler warnings, but in this
 * case it's OK.
 */

#if 0
/*
 * Instructions to Students:
 *
 * STEP 1: Read the following instructions carefully.
 */

You will provide your solution to this homework by
editing the collection of functions in this source file.

INTEGER CODING RULES:

 Replace the "return" statement in each function with one
 or more lines of C code that implements the function. Your code
 must conform to the following style:

 int Funct(arg1, arg2, ...) {
 /* brief description of how your implementation works */
 int var1 = Expr1;
 ...
 int varM = ExprM;

 varJ = ExprJ;
 ...
 varN = ExprN;
 return ExprR;
 }

 Each "Expr" is an expression using ONLY the following:
 1. Integer constants 0 through 255 (0xFF), inclusive. You are
 not allowed to use big constants such as 0xffffffff.
 2. Function arguments and local variables (no global variables).
 3. Unary integer operations ! ~
 4. Binary integer operations & ^ | + << >>

 Some of the problems restrict the set of allowed operators even further.
 Each "Expr" may consist of multiple operators. You are not restricted to
 one operator per line.

 You are expressly forbidden to:
 1. Use any control constructs such as if, do, while, for, switch, etc.
 2. Define or use any macros.
 3. Define any additional functions in this file.
 4. Call any functions.
 5. Use any other operations, such as &&, ||, -, or ?:
 6. Use any form of casting.
 7. Use any data type other than int. This implies that you
 cannot use arrays, structs, or unions.

 You may assume that your machine:
 1. Uses 2s complement, 32-bit representations of integers.
 2. Performs right shifts arithmetically.
 3. Has unpredictable behavior when shifting an integer by more
 than the word size.

EXAMPLES OF ACCEPTABLE CODING STYLE:
 /*
 * pow2plus1 - returns 2^x + 1, where 0 <= x <= 31
 */
 int pow2plus1(int x) {
 /* exploit ability of shifts to compute powers of 2 */
 return (1 << x) + 1;
 }

 /*
 * pow2plus4 - returns 2^x + 4, where 0 <= x <= 31
 */
 int pow2plus4(int x) {
 /* exploit ability of shifts to compute powers of 2 */
 int result = (1 << x);
 result += 4;
 return result;
 }

NOTES:
 1. Use the dlc (data lab checker) compiler (described in the handout) to
 check the legality of your solutions.
 2. Each function has a maximum number of operators (! ~ & ^ | + << >>)
 that you are allowed to use for your implementation of the function.
 The max operator count is checked by dlc. Note that '=' is not
 counted; you may use as many of these as you want without penalty.
 3. Use the btest test harness to check your functions for correctness.
 4. The maximum number of ops for each function is given in the
 header comment for each function. If there are any inconsistencies
 between the maximum ops in the writeup and in this file, consider
 this file the authoritative source.

/*
 * STEP 2: Modify the following functions according the coding rules.
 *
 * IMPORTANT. TO AVOID GRADING SURPRISES:
 * Use the dlc compiler to check that your solutions conform
 * to the coding rules.
 */

#endif
// Rating: 1
/*
 * bitAnd - x&y using only ~ and |
 * Example: bitAnd(6, 5) = 4
 * Legal ops: ~ |
 * Max ops: 8
 * Rating: 1
 */
int bitAnd(int x, int y) {
 return ~(~x | ~y);
}
/*
 * bitXor - x^y using only ~ and &
 * Example: bitXor(4, 5) = 1
 * Legal ops: ~ &
 * Max ops: 14
 * Rating: 1
 */
int bitXor(int x, int y) {
 return ~(~x & ~y);
}
/*
 * thirdBits - return word with every third bit (starting from the LSB) set to 1
 * and the rest set to 0
 * Legal ops: ! ~ & ^ | + << >>
 * Max ops: 8
 * Rating: 1
 */
int thirdBits(void) {
 return 2;
}
// Rating: 2
/*
 * fitsBits - return 1 if x can be represented as an
 * n-bit, two's complement integer.
 * 1 <= n <= 32
 * Examples: fitsBits(5,3) = 0, fitsBits(-4,3) = 1
 * Legal ops: ! ~ & ^ | + << >>
 * Max ops: 15
 * Rating: 2
 */
int fitsBits(int x, int n) {
 return 2;
}
/*
 * sign - return 1 if positive, 0 if zero, and -1 if negative
 * Examples: sign(130) = 1
 * sign(-23) = -1
 * Legal ops: ! ~ & ^ | + << >>
 * Max ops: 10
 * Rating: 2
 */
int sign(int x) {
 return 2;
}
/*
 * getByte - Extract byte n from word x
 * Bytes numbered from 0 (LSB) to 3 (MSB)
 * Examples: getByte(0x12345678,1) = 0x56
 * Legal ops: ! ~ & ^ | + << >>
 * Max ops: 6
 * Rating: 2
 */
int getByte(int x, int n) {
 return 2;
}
// Rating: 3
/*
 * logicalShift - shift x to the right by n, using a logical shift
 * Can assume that 0 <= n <= 31
 * Examples: logicalShift(0x87654321,4) = 0x08765432
 * Legal ops: ~ & ^ | + << >>
 * Max ops: 20
 * Rating: 3
 */
int logicalShift(int x, int n) {
 return 2;
}
/*
 * addOK - Determine if can compute x+y without overflow
 * Example: addOK(0x80000000,0x80000000) = 0,
 * addOK(0x80000000,0x70000000) = 1,
 * Legal ops: ! ~ & ^ | + << >>
 * Max ops: 20
 * Rating: 3
 */
int addOK(int x, int y) {
 return 2;
}
// Rating: 4
/*
 * bang - Compute !x without using !
 * Examples: bang(3) = 0, bang(0) = 1
 * Legal ops: ~ & ^ | + << >>
 * Max ops: 12
 * Rating: 4
 */
int bang(int x) {
 return 2;
}
// Extra Credit: Rating: 3
/*
 * conditional - same as x ? y : z
 * Example: conditional(2,4,5) = 4
 * Legal ops: ! ~ & ^ | + << >>
 * Max ops: 16
 * Rating: 3
 */
int conditional(int x, int y, int z) {
 return 2;
}
// Extra Credit: Rating: 4
/*
 * isPower2 - returns 1 if x is a power of 2, and 0 otherwise
 * Examples: isPower2(5) = 0, isPower2(8) = 1, isPower2(0) = 0
 * Note that no negative number is a power of 2.
 * Legal ops: ! ~ & ^ | + << >>
 * Max ops: 20
 * Rating: 4
 */
int isPower2(int x) {
 return 2;
}

/*
 * CSE 351 HW1 (Data Lab - Pointers)
 *
 * <Please put your name and userid here>
 *
 * pointer.c - Source file with your solutions to the Lab.
 *          This is the file you will hand in to your instructor.
 *
 * WARNING: Do not include the <stdio.h> header; it confuses the dlc
 * compiler. You can still use printf for debugging without including
 * <stdio.h>, the following function declaration should prevent a
 * compiler warning. In general, it's not good practice to ignore
 * compiler warnings, but in this case it's OK.
 */
int printf(const char *, ...);

#if 0
/*
 * Instructions to Students:
 *
 * STEP 1: Read the following instructions carefully.
 */

You will provide your solution to this homework by
editing the collection of functions in this source file.

INTEGER CODING RULES:

  Replace the "return" statement in each function with one
  or more lines of C code that implements the function. Your code
  must conform to the following style:

  int Funct(arg1, arg2, ...) {
      /* brief description of how your implementation works */
      int var1 = Expr1;
      ...
      int varM = ExprM;

      varJ = ExprJ;
      ...
      varN = ExprN;
      return ExprR;
  }

  Each "Expr" is an expression using ONLY the following:
  1. Integer constants 0 through 255 (0xFF), inclusive. You are
      not allowed to use big constants such as 0xffffffff.
  2. Function arguments and local variables (no global variables).
  3. For 1-4, only Unary integer operations *, & and Binary integer
      operations - + * are allowed. For the last three, you may also
      use shifts (<<, >>), ~, ==, and ^.

  Some of the problems restrict the set of allowed operators even further.
  Each "Expr" may consist of multiple operators. You are not restricted to
  one operator per line.

  You are expressly forbidden to:
  1. Use any control constructs such as if, do, while, for, switch, etc.
  2. Define or use any macros.
  3. Define any additional functions in this file.
  4. Call any functions.
  5. Use any other operations, such as &&, ||, ?: or sizeof.
  6. Use any data type other than those already in the declarations provided.

  You may assume that your machine:
  1. Uses 2s complement, 32-bit representations of integers.
  2. Performs right shifts arithmetically.
  3. Has unpredictable behavior when shifting an integer by more
     than the word size.

/*
 * STEP 2: Modify the following functions according the coding rules.
 *
 * Test the code below in your own 'main' program.
 *
 */

#endif

/*
 * Return the size of an integer in bytes.
 */
int intSize() {
  int intArray[10];
  int * intPtr1;
  int * intPtr2;
  // TODO: Write code to compute size of an integer.

  return 2;
}

/*
 * Return the size of a double in bytes.
 */
int doubleSize() {
  double doubArray[10];
  double * doubPtr1;
  double * doubPtr2;
  // TODO: Write code to compute size of a double.

  return 2;
}

/*
 * Return the size of a pointer in bytes.
 */
int pointerSize() {
  double * ptrArray[10];
  double ** ptrPtr1;
  double ** ptrPtr2;
  // TODO: Write code to compute size of a pointer.

  return 2;
}

/*
 * Modify intArray[5] to be the value 351 using only &intArray and
 * pointer arithmetic.
 */
int changeValue() {
  int intArray[10];
  int * intPtr1 = intArray;
  int * intPtr2;
  // TODO: Write code to change value of intArray[5] to 351 using only
  //       intPtr1 and the + operator.

  return intArray[5];
}


/*
 * Return 1 if ptr1 and ptr2 are within the *same* 64-byte aligned
 * block (or word) of memory. Return zero otherwise.
 *
 * Operators / and % and loops are NOT allowed.
 */
int withinSameBlock(int * ptr1, int * ptr2) {
  // TODO
  return 2;
}

/*
 * Return 1 if ptr points to an element within the specified intArray,
 * 0 otherwise.
 */
int withinArray(int * intArray, int size, int * ptr) {
  // TODO
  return 2;
}
/*
 * Return x with the n bits that begin at position p inverted (i.e.,
 * turn 0 into 1 and vice versa) and the rest left unchanged. Consider
 * the indices of x to begin with the low-order bit numbered as 0.
 */
int invert(int x, int p, int n) {
  // TODO
  return 2;
}