Implementation region x when B do S

• Associate with the shared variable x, the following variables:

  semaphore mutex, first-delay, second-delay;
     int first-count, second-count;

• Mutually exclusive access to the critical section is provided by mutex.
• If a process cannot enter the critical section because the Boolean expression B is false, it initially waits on the first-delay semaphore; moved to the second-delay semaphore before it is allowed to reevaluate B.