Chapter 2, Problem 2.71HW

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;

}

1. A.    What is wrong with this code?
2. B. Give a correct implementation of the function that uses only left and right shifts, along with one subtraction.