3.2.1 Some definitions

Remark : Sequential programming is only the expression of our inability to transfer the natural parallelism of the world to a machine.
Problems occur with the administration of parallel processes :

• totally independent processes (only a common power supply)
  $\Longrightarrow$ absolutely parallel ( $\Longrightarrow$ not in the scope of interest)
• shared use of (limited) resources
  $\Longrightarrow$ multitasking/time-sharing
  $\Longrightarrow$ distribution strategy necessarily
• multiprocessor systems which contribute subtasks
  $\Longrightarrow$ information exchange via shared memory or message passing (distributed memory).

Synchronization :
Prevention of an undefined status of a variable by means of a mechanism which allows processes to access on that variable in a defined way.

Barrier :
Certain point in a program which has to be passed by all processes (or a group of processes) before the execution continues. This guarantees that each single process has to wait until also all remaining processes have reached that program point.

Ex.: Undefined status

An undefined status may occur in the access on shared memory.

Figure 3.3: Undefined status.

The value of $N$ depends on the execution speed of processes A, B and is therefore not predictable, i.e., the status of $N$ is undefined.

Figure 3.4: Results in an undefined status

For omitting the above undefined status, one has to regard operations A and B as atomic operations, i.e., they cannot be split any further.
$\Longrightarrow$ Exclusive access on $N$ for one process during that time is needed to finish the operation.
One realization of exclusive access will be presented in the next section.