Page 105 - DCAP403_Operating System

Operating System

Notes device driver. At this point, the operating system must decide whether to grant this request,
depending upon whether the printer is already being used by another process. If it is not, the
operating system should grant the request and allow the process to continue; otherwise, the
operating system should deny the request and perhaps classify the process as a waiting process
until the printer becomes available. Indeed, if two processes were given simultaneous access to
the machine’s printer, the results would be worthless to both.

Now that the problem of synchronization is properly stated, consider the following related
defi nitions:
1. Critical Resource: A resource shared with constraints on its use (e.g., memory, fi les,
printers, etc.)
2. Critical Section: Code that accesses a critical resource
3. Mutual Exclusion: At most one process may be executing a Critical Section with respect to
a particular critical resource simultaneously
Figure 6.2: Railway-road Intersection with Signal

In the example given above, the printer is the critical resource. Let’s suppose that the processes
which are sharing this resource are called process A and process B. The critical sections of process
A and process B are the sections of the code which issue the print command. In order to insure
that both processes do not attempt to use the printer at the same, they must be granted mutually
exclusive access to the printer driver. The idea of mutual exclusion with our railroad intersection
by adding a semaphore to the picture.
Semaphores are used in software systems in much the same way as they are in railway systems.
Corresponding to the section of track that can contain only one train at a time is a sequence of
instructions that can be executed by only one process at a time. Such a sequence of instructions
is called a critical section.

6.2 Critical Selection Problem

The key to preventing trouble involving shared storage is find some way to prohibit more than
one process from reading and writing the shared data simultaneously. That part of the program
where the shared memory is accessed is called the Critical Section. To avoid race conditions and

flawed results, one must identify codes in Critical Sections in each thread. The characteristic
properties of the code that form a Critical Section are:
1. Codes that reference one or more variables in a “read-update-write” fashion while any of
those variables is possibly being altered by another thread.

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