Phase 3 Checkpoint (15 questions)

Phase 3

Phase 3 Checkpoint: 15 questions on synchronization.

15 questions · 20 marks · 45 minutes. Negative marking on MCQ only: −1/3 mark for a wrong 1-mark MCQ, −2/3 for a wrong 2-mark MCQ. No negative marking for MSQ or NAT; MSQ has no partial credit.

Q1 · NAT · 1 mark(s) — Race

x = 10. Thread 1 runs x = x + 5 and thread 2 runs x = x * 2 (each a load, a compute and a store). How many different final values are possible?

Numerical answer: ________

Q2 · MCQ · 1 mark(s) — Definition

A race condition is:

1. (A) a situation where the result depends on the order in which threads run
2. (B) a deadlock between two threads
3. (C) a hardware fault
4. (D) a scheduling algorithm

Q3 · NAT · 2 mark(s) — Loops

x = 0; two threads each execute x++ 5 times. Largest possible final value?

Numerical answer: ________

Q4 · MCQ · 1 mark(s) — Strict alternation

Strict alternation guarantees mutual exclusion but violates:

1. (A) nothing
2. (B) progress
3. (C) mutual exclusion
4. (D) atomicity of turn

Q5 · MCQ · 2 mark(s) — Peterson

Which line order is correct for Peterson's entry section of process i?

1. (A) turn = j; flag[i] = true; while (flag[j] && turn == j);
2. (B) flag[i] = true; turn = j; while (flag[j] && turn == j);
3. (C) turn = i; while (turn == j); flag[i] = true;
4. (D) flag[i] = true; turn = i; while (flag[j] && turn == j);

Q6 · MCQ · 1 mark(s) — TSL

The test-and-set instruction:

1. (A) only works on one CPU
2. (B) returns the new value
3. (C) needs interrupts to be enabled
4. (D) sets the lock and returns the old value, atomically

Q7 · NAT · 1 mark(s) — CAS

v = 9. CAS(&v, 9, 2) is called, then CAS(&v, 9, 5). Final v?

Numerical answer: ________

Q8 · NAT · 1 mark(s) — Arithmetic

A semaphore starts at 4; then 9 wait and 3 signal operations are done (blocking version). How many processes are waiting?

Numerical answer: ________

Q9 · NAT · 2 mark(s) — Outputs

S = 1, T = 0. P1 runs twice: wait(S); print "0"; signal(T); P2 runs twice: wait(T); print "1"; signal(S); How many different outputs are possible?

Numerical answer: ________

Q10 · NAT · 2 mark(s) — Outputs

Same code with S = 1 and T = 1. How many different outputs are possible?

Numerical answer: ________

Q11 · MCQ · 1 mark(s) — Use

A semaphore initialised to 0 is typically used for:

1. (A) ordering one event after another
2. (B) counting free buffer slots
3. (C) mutual exclusion
4. (D) nothing

Q12 · NAT · 1 mark(s) — Bounded buffer

Producer–consumer with a 10-slot buffer: after the producer adds 4 items and the consumer removes 1 (no one waiting), what is the value of full?

Numerical answer: ________

Q13 · MCQ · 1 mark(s) — Philosophers

All five philosophers pick up the left chopstick at the same moment. The result is:

1. (A) two philosophers eating
2. (B) mutual exclusion violated
3. (C) deadlock
4. (D) starvation of one philosopher only

Q14 · MCQ · 1 mark(s) — Monitors

The main difference between signal on a condition variable and signal on a semaphore is that the condition variable's signal:

1. (A) needs busy waiting
2. (B) can wake all processes
3. (C) is lost if no process is waiting
4. (D) increments a counter

Q15 · NAT · 2 mark(s) — Race with semaphore

x = 0. W and X each do x = x + 1; Y and Z each do x = x − 2; each brackets its read-modify-write with wait(S) … signal(S), and S starts at 1. Final value of x?

Numerical answer: ________