ECS 150, Operating Systems Fall Quarter 2008
Sample Midterm
1. What are the values of the integer variables xand ywhen the following program completes? (If either variable
could have more than one value, say why.)
y = 2 ;
parbegin
x=y∗2 ;
y = 6 ;
parend ;
Answer: By the Bernstein conditions, the value of xwill be undefined, since it uses a variable that is both read
and written concurrently. The value of ywill be 6, since it is written to only at one place in the concurrent
statements.
2. Is the following true or false? Justify your answer. “When several processes access shared information in
primary storage, mutual exclusion must be enforced to prevent the production of indeterminant results.”
Answer: This statement is false because it is too general. If many processes are reading shared data, and not
writing it, mutual exclusion is unnecessary. If however any process does alter that data, then mutual exclusion
becomes necessary.
3. Process A should finish before process B starts, and process B should finish before either of processes C or D
start. Show how these processes may use two semaphores to provide the necessary synchronization.
Answer: Define two semaphores Adone and Bdone, both of which are initialized to 0. Have B, C, and D execute
as their first instructions down(Adone), down(Bdone), and down(Bdone), respectively. Have A execute as its
last instruction up(Adone); this enables B to proceed past its first instruction. Have B execute two up(Bdone)s as
its last two instructions; this enables C and D to proceed past their first instructions. This arrangement provides
the necessary synchronization.
4. A bounded semaphore s is a counting semaphore that cannot exceed a given value smax >0. The corresponding
up and down operations are:
up(s): wait until s < smax; then increment s by 1
down(s): wait until s > 0; then decrement s by 1
Write a monitor to implement bounded semaphores. (Hint: assume the semaphore is to be initialized to the
constant SINIT and the maximum value is SMAX.)
typedef mo ni to r {
/∗s c ou n t i s t he i n t e g e r v a l ue o f th e se ma p ho re ; i f t h i s i s to o bi g
an d t h e p r o c e s s t r i e s t o u p i t , t h e p r o c e s s w i l l b l o c k on t o o b i g ;
s i m i l a r l y , i f t h e v a l u e i s t o o s m a l l an d th e p r o c e s s t r i e s t o d own
i t , t h e p r o c e s s w i l l b l o c k on t o o s m a l l . ∗/
int scount = 0;
condition t oo b i g , t o o s m a l l ;
/∗s t r a i g h f o r w a r d i m p l e m e n t a t i o n o f up ∗/
v o i d e n t r y u p ( vo i d )
{
/∗i f t oo b i g , w a it u n t i l o k ∗/
while ( scount >= SMAX)
toobig . w a i t ;
/∗i n c r e m e n t an d n o t i f y so m eo n e
w a i t i n g f o r t h e s e ma p ho r e t o be
n o n z e r o t h a t i t i s ∗/
Version of October 28, 2008 at 7:44am Page 1 of 2
