Lecture 13 – Oct 2nd, 2019

Setup

Log in to clyde.
Create a directory and cd into it.

Task

Implement a function in C that returns the value of the most significant bit of an unsigned int using CHAR_BIT (defined in limits.h) and sizeof(unsigned int) to figure out how many bits are in an unsigned int.
```
unsigned int msb(unsigned int x) {
  /* ... */
}
```
I recommend creating a constant NUM_BITS defined in terms of CHAR_BIT and sizeof(unsigned int) that is the number of bits in an unsigned int.
```
#define NUM_BITS (/* ... */)
```
Test your implementation by printing out the result of msb(0), msb(1), msb(-1). [Fun fact: Because clyde, like all modern machines, uses two’s complement integers, converting a signed integer like -1 to an unsigned integer does not change the bit pattern, instead negative integers become large positive integers.]

Implement a function that extracts the least significant n bits of an unsigned int x.

unsigned int extract_lsbs(unsigned int x, unsigned int n) {
  assert(n < NUM_BITS);
  /* ... */
}

assert(3) is defined in assert.h.

Test extract_lsbs by printing out the result of extracting a few different numbers of bits (printing hex using printf’s %x specifier is by far the easiest way to see this). E.g., extract_lsbs(0x378, 5) should return 0x18.

Implement a function that returns a bit field—a contiguous sequence of bits—from x that starts at start and is n bits long.

unsigned int extract_field(unsigned int x,
                           unsigned int start,
                           unsigned int n) {
  assert(start < NUM_BITS);
  assert(n < NUM_BITS - start);
  /* ... */
}

Test extract_field. E.g., extract_field(0x11223344, 17, 9) should return 0x91.
That second assert in task 5 is unusual looking, but correct. The more natural looking assertion, assert(n + start < NUM_BITS) is incorrect.
Find unsigned integers start and n such that calling the following function with them causes the program to abort, but only on the marked line.
```
void crash_me(unsigned int start, unsigned int n) {
  assert(start < NUM_BITS);
  assert(n + start < NUM_BITS);
  assert(n < NUM_BITS - start); // Abort on this line.
}
```
When it crashes, it should look like this (although file names and line numbers may differ).
```
a.out: bit_fields.c:27: void crash_me(unsigned int, unsigned int): Assertion `n < NUM_BITS - start' failed.
Aborted (core dumped)
```