Single-Processor Systems:
Most systems use a single processor. There are different types of single-processor systems from PDAs to mainframe computers. A single-processor system has one main CPU to execute a general-purpose instruction set including the instructions from user processes.
Almost all systems also have other special-purpose processors. They may come in the form of device-specific processors such as disk, keyboard and graphics controllers. They may come in the form of more general-purpose processors in mainframes such as I/O processors to move data quickly among the system components.
The special-purpose processors run a limited instructions set and do not execute user processes. Sometimes they are managed by the operating system. The operating system sends them information about the next task and monitors their status. For example, a disk controller microprocessor receives requests from the main CPU and implements its own disk queue and scheduling algorithm. The main CPU has to perform the disk scheduling. The personal computers have a microprocessor in keyboard to convert keystrokes into codes to send to CPU. The special-purpose processor in some systems are low-level components built into the hardware. The operating system cannot communicate with these processors. They perform their jobs autonomously. The use of special-purpose microprocessors is very common. However, it is still considered a single-processor system not multiprocessor.
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Multiprocessor Systems
Multiprocessing is a type of processing in which two or more processors work together to process more than one program simultaneously. It allows the system to do work in a shorter period of time. UNIX is one of the most widely used multiprocessing systems. more
Multiprocessor system is also known as parallel system or tightly-coupled system. It means that multiple processors are tied together in some manner. Generally, the processors are in close communication with each other. They share common data structures and a common system clock.
Advantages of Multiprocessor Systems
1. Reduced Cost
Some advantages of multiprocessor system are as follows: Multiple processors share same resources. Separate power supply or mother board for each chip is not required. This
2. Increased Reliability
The reliability of system is also increased. The failure of one processor does not affect other processors though it will slow down the machine. Several mechanisms are required to achieve increased reliability. If a processor fails, a job running on that processor also fails. The system must be able to reschedule the failed job or to alert the user that the job was not successfully completed.
Types of Multiprocessor Systems There are two main types of parallel processing:
1. Symmetric Multiprocessing
A method of processing in which multiple processors work together on the same task is called symmetric multiprocessing. Each processor runs an identical copy of the same operating system. Symmetric multiprocessing treats all processors equally. I/O eded. It processed on any processor. The processors communicate with each other as needed. It enables many processes to be run at once without degrading performance. Each CPU is separate and it is possible for one CPU to be idle while another CPU is overloaded.
Symmetric multiprocessing is easier to implement in operating systems. It is the method mostly used by the operating systems that support multiple processors. The most common multiprocessor systems today use SMP architecture.
2. Asymmetric Multiprocessing
A multiprocessing technique in which each processor is dedicated to a particular task is called asymmetric multiprocessing. It has one master processor and remainder processors are called slave. The master distributes tasks among the slaves. I/O is usually done by the master only. An example is disk management where the responsibility of managing disk activities is not handled by main processor. It is performed by a microprocessor that has its own memory. It only performs disk management tasks such as how to move the head of disk to read data from disk into memory etc.