This article is started in the middle. You should see the Functions Of An Operating System Part I and Part 2 before you read this
Multiprocessing and Parallel Processing
Both multiprocessing and parallel processing involve using two or more CPUs (or multiple cores in a single CPU) in one computer system to perform work more efficiently. The primary difference between these two techniques is that, with multiprocessing, each CPU typically works on a different job; with parallel processing, the processors usually work together to complete one job more quickly. In either case, the CPUs can perform tasks simultaneously (at the exactly the same time), in contrast with multitasking and multithreading which use a single CPU and process tasks sequentially(by rotating through tasks, as discussed previously).
Multiprocessing is supported by most operating systems and is used with personal computers that have multi-core CPUs as well as with servers and mainframe computers that have multi-core CPUs and/or multiple CPUs. Parallel processing is used most often with supercomputers and supercomputing clusters.
Memory Management
Another key function of the operating system is memory management, which involves optimizing the use of main memory (RAM). The operating system allocates RAM to programs as needed and then reclaims that memory when the program is closed. With today’s memory-intensive programs, good memory management can help speed up processing. Since each additional running program or open window consumes memory, users can also help with memory management by limiting the number of programs running in the background(as discussed in an earlier section) to only the ones that are absolutely necessary, as well as by closing windows when they are no longer needed.
One memory management technique frequently used by operating systems is virtual memory, which uses a portion of the computer’s hard drive as additional RAM. All programs and data located in RAM are divided into fixed-length pages or variable length segments, depending on the operating system being used. When the amount of RAM.
Buffering and Spooling
Some input and output devices are exceedingly slow, compared to today’s CPUs. If the CPU had to wait for these slower devices to finish their work, the computer system would experience a horrendous bottleneck. For example, suppose a user sends a 100-page document to the printer. Assuming the printer can output 20 pages per minute, it would take 5 minutes for the document to finish printing. If the CPU had to wait for the print job to be completed before performing other tasks, the computer would be tied up for 5 minutes.
To avoid this problem, most operating systems use two techniques—buffering and spooling. A buffer is an area in RAM or on the hard drive designated to hold input and output on their way in or out of the system. For instance, a keyboard buffer stores characters as they are entered via the keyboard, and a print buffer stores documents that are waiting to be printed. The process of placing items in a buffer so they can be retrieved by the appropriate device when needed is called spooling.
The most common use of buffering and spooling is print spooling. Print spooling allows multiple documents to be sent to the printer at one time and to print, one after the other, in the background while the computer and user are performing other tasks. The documents waiting to be printed are said to be in a print queue, which designates the order the documents will be printed. While in the print queue, most operating systems allow the user to cancel print jobs and pause the printer. Some also allow the user to prioritize the documents in the print queue.
Although originally used primarily for keyboard input and print jobs, most computers and operating systems today use several other buffers to speed up operations. For instance, it is common today for computers to use buffers to assist in redisplaying images on the screen and to temporarily store data that is in the process of being burned onto a CD or DVD.
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