Monitors and Priority Waits

Introduction

This is an example of a monitor using priority waits.  It implements a simple alarm clock; that is, a process calls 
alarmclock.wakeme(n), and suspends for n seconds.  Note that we are assuming the hardware invokes the pro-
cedure tick to update the clock every second.

Algorithm

	1	alarmclock: monitor;

	2	var	now:	integer;

	3			wakeup: condition;

	4	procedure entry wakeme(n: integer);

	5	begin

	6		alarmsetting := now + n;

	7		while now < alarmsetting do

	8			wakeup.wait(alarmsetting);

	9		wakeup.signal;

	10	end;

	11	procedure entry tick;

	12	begin

	13		now := now + 1;

	14		wakeup.signal;

	15	end.

Comments

lines 2-3	Here, now is the current time (in seconds) and is updated once a second by the procedure tick.  
When a process suspends, it will do a wait on the condition wakeup.

line 6	This computes the time at which the process is to be awakened.

lines 7-8	The process now checks that it is to be awakened later, and then suspends itself.

line 9	Once a process has been woken up, it signals the process that is to resume next.  That process 
checks to see if it is time to wake up; if not, it suspends again (hence the while loop above, rather 
than an if statement).  If it is to wake up, it signals the next process-

line 14	This is done once a second (hence the addition of 1 to now).  The processes to be woken up are 
queued in order of remaining time to wait with the next one to wake up first.  So, when tick sig-
nals, the next one to wake up determines if it is in fact time to wake up.  If not, it suspends itself; if 
so, it proceeds.