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Computer-System Architecture
Single-Processor Systems
Most computer systems used a single processor. On a single processor system, there is one main CPU capable of executing a general-purpose instruction set, including instructions from user processes.
A single processor system contains only one processor. So only one process can be executed at a time and then the process is selected from the ready queue. Most general purpose computers contain the single processor systems as they are commonly in use.
Multiprocessor Systems
Multiprocessor or parallel systems are increasing in importance nowadays. These systems have
multiple processors working in parallel that share the computer clock, memory, bus, peripheral
devices etc. An image demonstrating the multiprocessor architecture
Types of Multiprocessors
There are mainly two types of multiprocessors i.e. symmetric and asymmetric multiprocessors.
Details about them are as follows:
Symmetric Multiprocessors
In these types of systems, each processor contains a similar copy of the operating system and
they all communicate with each other. All the processors are in a peer to peer relationship i.e. no
master - slave relationship exists between them.
An example of the symmetric multiprocessing system is the Encore version of Unix for the
Multimax Computer.
Asymmetric Multiprocessors
In asymmetric systems, each processor is given a predefined task. There is a master processor
that gives instruction to all the other processors. Asymmetric multiprocessor system contains a
master slave relationship.
Asymmetric multiprocessor was the only type of multiprocessor available before symmetric
multiprocessors were created. Now also, this is the cheaper option.
Advantages of Multiprocessor Systems
There are multiple advantages to multiprocessor systems. Some of these are:
More reliable Systems
In a multiprocessor system, even if one processor fails, the system will not halt. This ability to continue working despite hardware failure is known as graceful degradation. For example: If there are 5 processors in a multiprocessor system and one of them fails, then also 4 processors are still working. So the system only becomes slower and does not ground to a halt.
Enhanced Throughput
If multiple processors are working in tandem, then the throughput of the system increases i.e. number of processes getting executed per unit of time increase. If there are N processors then the throughput increases by an amount just under N.
More Economic Systems
Multiprocessor systems are cheaper than single processor systems in the long run because they share the data storage, peripheral devices, power supplies etc. If there are multiple
Clustered systems are quite fault tolerant and the loss of one node does not result in the loss of the system. They may even contain one or more nodes in hot standby mode which allows them to take the place of failed nodes.
Clustered systems are quite scalable as it is easy to add a new node to the system. There is no need to take the entire cluster down to add a new node.
