I-4 Other operating modes

I-4-1 Personal computer systems

Personal computers appeared in the 1970s. They are microcomputers much smaller and less expensive than mainframes, their operating systems were neither multi-user nor multi-tasking; their goals have changed over time. Instead of maximizing CPU and peripheral usage, they opted for user convenience and responsiveness. Personal computer systems are:

• interactive,
• friendly,
• without sharing the processor.

Example: MS-DOS

I-4-2 Real-time systems

A real-time system is used when there are compelling response time requirements for the operation of a processor or for the flow of data. It is used in the control of dedicated applications (control of a nuclear power plant, robotic assembly line, autopilot, etc.). The sensors bring data to the computer, which must analyze it and possibly adjust the controls to modify the sensor inputs.

There are two types of real-time systems:

• Rigid real-time system ensures that critical tasks finish on time. This goal requires that all system delays be limited, from the retrieval of stored data until the end of a query time.
• Soft real-time system is a less restrictive type where a critical real-time task takes priority over other tasks and maintains that priority until it completes. Example: Unix, MachI-4-3 Systèmes parallèles

I-4-3 Parallel systems

Most systems are monoprocessor systems, i.e. they have a single main CPU. Multiprocessor systems have more than one processor in close communication sharing the computer bus, clock, and sometimes memory and peripherals. These systems are called strongly coupled systems.
The reasons for creating such systems are as follows:

• increase in processing capacity,
• save the acquisition cost compared to several mono-systems,
• increase reliability (harmonious degradation),

Multiprocessor systems use two OS models:

• symmetric multiprocessing: each processor executes an identical copy of the OS and these copies communicate with each other.
• asymmetric multiprocessing: a master processor controls the system, the other systems receive instructions from the master. In a master-slave scheme.

Example: The Encore version of Unix for the Multimax computer is a symmetrical multi-processing system.

The Sun OS Version 4 OS is an asymmetric multiprocessing OS.

I-4-4 Distributed systems

In these systems the processors are loosely coupled, each processor has its own local memory and its clock. They communicate with each other through various communication lines or telephone lines. Processors may vary in size and functions.

The reasons for building these systems can be:

• resource sharing,
• acceleration of calculations (load distribution),
• reliability and
• communication (e-mail)