114 System overhead An operating system is itself a computer program which must

114 system overhead an operating system is itself a

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1.1.4 System overhead An operating system is itself a computer program which must be executed. It therefore requires its own share of a computer's resources. This is especially true on multitasking systems, such as UNIX, where the OS is running all the time along side users' programs. Since user programs have to wait for the OS to perform certain services, such as allocating resources, they are slowed down by the OS 1.3 . The time spent by the OS servicing user requests is called the system overhead . On a multi-user system one would like this overhead to be kept to a minimum, since programs which make many requests of the OS slow not only themselves down, but all other programs which are queuing up for resources. In the UNIX C-shell (csh) environment, it is possible to find out the exact fraction of time spent by the OS working on a program's behalf by using the time function. 1.1.5 Caching Caching is a technique used to speed up communication with slow devices. Usually the CPU can read data much faster from memory than it can from a disk or network connection, so it would like to keep an up-to-date copy of frequently used information in memory. The memory area used to do this is called a cache . You can think of the whole of the primary memory as being a cache for the secondary memory (disk). Sometimes caching is used more generally to mean `keeping a local copy of data for convenience'. 1.2 Hardware Here we list the main hardware concepts. 1.2.1 The CPU The CPU, or central processor unit is the heart and soul of every computer. This is the part which does the work of executing machine instructions. Traditionally, it is just one microprocessor with lots of pins to connect is to memory and devices - usually identifiable by being the largest chip. On modern machines, there may be several CPUs which can work in parallel. Also VLSI or very large scale integration technology has made it possible to put very many separate processors and memory into a single package,
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so the physical distinction between the CPU and its support chips is getting blurred. Nevertheless, the CPU is still logically separate from the memory and devices. The CPU is driven by a `clock' or pulse generator. Each instruction completes in a certain number of `clock cycles'. Traditionally CPUs are based on CISC (Complex Instruction Set Computing) architecture, where a single instruction takes one or more clock cycles to complete. A new trend is to build RISC (Reduced Instruction Set Computing) processors which aim to be more efficient for a subset of instructions by using redundancy. These have simpler instructions but can execute much more quickly, sometimes with several instructions per clock cycle. 1.2.2 Memory The primary memory is the most important resource a computer has. Since CPUs are only made with instructions for reading and writing to memory, no programs would be able to run without it. There are two types of memory: RAM - random access memory, or read/write memory, which loses its contents when the machine is switched off, and ROM - read only memory, which never loses its contents unless destroyed. ROM is normally
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