1. Monitors

2. Controls Condition variables –Value –Queue –ONLY used in a monitor Signal "promotes" or "de-queues" a waiter Two types –Signal & exit(C.A.R. Hoare) –Signal & continue (Per Brinch Hansen- Mesa)

3. Signal & Continue Waiting thread still waits for actual exit Allows system to 'prepare' the waiter Used in Java

4. Inside the Monitor May want to wait for something Condition variable controls the wait While condwait thread is moved outside the monitor

5. Sample – acts like a semaphore boolean busy = false; Conditionavail; MonitorEntry void getRes() // same as "P" {if (busy) wait(avail); // put me in queue busy=true;// now held } MonitorEntry void freeRes() // same as "V" {busy=false; signal (avail); signal (avail);}

6. Readers & Writers Condition canwrite, canread; boolean writelock=false; int rdrs=0; monitorEntry void beginread() {if (writelock || queue(canwrite)) wait canread; ++rdrs; signal (canread); } MonitorEntry void endread() {--rdrs; if (rdrs==0) signal(canwrite); } monitorEntry void beginWrite() {if (rdrs>0 || writelock) wait (canwrite); writelock=true;} monitorEntry void endwrite() {writelock=false; if (queue(canread)) signal(canread); else signal(canwrite); }