CIS 314 Assignment 2

CIS 314 Assignment 2 – 100/100 points 
Please submit individual source files for coding exercises (see naming conventions below) and a
single solution document for non-coding exercises (.txt or .pdf only). Your code and answers
need to be documented to the point that the graders can understand your thought process.
Full credit will not be awarded if sufficient work is not shown.
1. [20] B&O’H 2.71.
You just started working for a company that is implementing a set of procedures to
operate on a data structure where 4 signed bytes are packed into a 32-bit unsigned. Bytes
within the word are numbered from 0 (least significant) to 3 (most significant). You have
been assigned the task of implementing a function for a machine using two’s-complement
arithmetic and arithmetic right shifts with the following prototype:
/* Declaration of data type where 4 bytes are packed
 into an unsigned */
typedef unsigned packed_t;
/* Extract byte from word. Return as signed integer */
int xbyte(packed_t word, int bytenum);
That is, the function will extract the designated byte and sign extend it to be a 32-bit int.
Your predecessor (who was fired for incompetence) wrote the following code:
/* Failed attempt at xbyte */
int xbyte(packed_t word, int bytenum)
{
 return (word (bytenum << 3)) & 0xFF;
}
A. What is wrong with this code?
B. Give a correct implementation of the function that uses only left and right shifts, along
with one subtraction.
Example calls with a correct implementation:
xbyte(0x00007700, 1): 0x77
xbyte(0x00EE0000, 2): 0xFFFFFFEE
Also write a main() function to test your function. Name your source file 2-71.c and write your
answer for part A in your comments.
2. [20] B&O’H 2.72.
You are given the task of writing a function that will copy an integer val into a buffer buf,
but it should do so only if enough space is available in the buffer.
Here is the code you write:
/* Copy integer into buffer if space is available */
/* WARNING: The following code is buggy */
void copy_int(int val, void *buf, int maxbytes) {
 if (maxbytes-sizeof(val) = 0)
 memcpy(buf, (void *) &val, sizeof(val));
}
This code makes use of the library function memcpy. Although its use is a bit artificial
here, where we simply want to copy an int, it illustrates an approach commonly used to
copy larger data structures.
You carefully test the code and discover that it always copies the value to the buffer, even
when maxbytes is too small.
A. Explain why the conditional test in the code always succeeds. Hint: The sizeof
operator returns a value of type size_t.
B. Show how you can rewrite the conditional test to make it work properly.
The above code doesn’t no print anything; you should verify that memcpy is only being called
when appropriate. For example:
copy_int(42, buffer, 4): memcpy should be called
copy_int(42, buffer, 2): memcpy should NOT be called
Also write a main() function to test your function. Name your source file 2-72.c and write your
answer for part A in your comments.
3. [15] B&O’H 2.83.
Fill in the return value for the following procedure, which tests whether its first argument
is less than or equal to its second. Assume the function f2u returns an unsigned 32-bit
number having the same bit representation as its floating-point argument. You can
assume that neither argument is NaN. The two flavors of zero, +0 and −0, are considered
equal.
int float_le(float x, float y) {
 unsigned ux = f2u(x);
 unsigned uy = f2u(y);
 /* Get the sign bits */
 unsigned sx = ux 31;
 unsigned sy = uy 31;
 /* Give an expression using only ux, uy, sx, and sy */
 return ; }
Use the following code for f2u:
unsigned f2u(float f) {
 return *((unsigned*)&f);
}

Example calls:
float_le(0.0f, 0.0f): 1
float_le(-0.0f, 0.0f): 1
float_le(-1.0f, 0.0f): 1
float_le(0.0f, 1.0f): 1
float_le(1.0f, 0.0f): 0
float_le(0.0f, -1.0f): 0
Also write a main() function to test your function. Name your source file 2-83.c.
4. [15] Convert the following hex values to decimal assuming that they are stored as 2s
complement integers.
a. (5) 0x0000000F
b. (5) 0xFFFFFCE6
c. (5) 0xFFFFFFFF
Write your answers in your solutions document.
5. [15] Convert the following hex values to decimal assuming that they are encoded as IEEE 754
single-precision floating-point numbers:
a. (5) 0x00000000
b. (5) 0x41120000
c. (5) 0xC39D0000
Write your answers in your solutions document.
6. [15] Convert the following decimal numbers to hex encoded as IEEE 754 single-precision
floating-point numbers. Write your answers in your solutions document.
a. (5) -1.0
b. (5) 10.5
c. (5) -85.125
Write your answers in your solutions document.
Zip the source files and solution document (if applicable), name the .zip file <Your Full
NameAssignment2.zip (e.g., EricWillsAssignment2.zip), and upload the .zip file to Canvas (see
Assignments section for submission link).