CSE 430
FALL 2003 MIDTERM 2 NAME
Put name on EVERY page
1. (23+3points extra credit) Process Coordination
As shown below, a counting semaphore can be implemented using a binary semaphore.
(a) (2) What range of values can a binary semaphore take on?
(b) (2) What range of values can a counting semaphore take on?
(c) (4) What is the reason for implementing a counting semaphore using a binary semaphore.
Note: Just saying binary semaphores are easier to implement gets you zero points. You have to
say why.
(d) (3) What is binary semaphore S1 used for?
(e) (3) What is binary semaphore S2 used for?
(f) (3) What does the signal(S2) at step 4 of the signal operation do?
(g) (3) What does the signal(S1) at step 6 of the signal operation do and when is it needed?
(h) (3) If C was > 0when a process performed the wait operation, what does the signal(S1) at step
7 of the wait operation do?
(i) (3points extra credit) If C was ≤ 0 when a process performed the wait operation, what does the
signal(S1) at step 7 of the wait operation do and why is it needed?
Data structures:
binary-semaphore S1, S2;
int C:
Initialization:
S1 = 1
S2 = 0
C = initial value of counting semaphore S
wait operation
1wait(S1);
2C--;
3if (C < 0) {
4signal(S1);
5wait(S2);
6}
7signal(S1);
signal operation
1wait(S1);
2C ++;
3if (C <= 0)
4signal(S2);
5else
6signal(S1);
