The Computer System
Our general description of the computer is valid for all general-purpose computer systems, and also for most devices with computers embedded in them, regardless of brand name or size. In more general terms, every computer system consists of a CPU, or central processing unit, where all the processing takes place; memory to hold the programs and data while they are being processed; and some form of input and output, usually one or more keyboards and flat-screen display devices plus one or more forms of long-term storage, usually disks, CDs or DVDs, and USB or SD plug-in memory. Most modern computer systems provide more than one CPU (or ‘‘core’’) within the computer system. A single CPU can process only one instruction at a time; the use of multiple CPUs can increase processing speed by allowing instructions that do not affect each other to be executed in parallel. The validity of our general description is true regardless of how complex or simple the computer system may seem.
terabytes (TB) of primary storage. They are capable of executing instructions at a rate of tens of billions of instructions per second! The powerful z/OS operating system can keep track of hundreds or thousands of simultaneous users and divides the time among them to satisfy their differing requirements. Even in its smallest configuration, the z10 EC Model S64 system, which is the largest current model at this writing, provides at least 16 GB of memory and processes instructions at the rate of several billion instructions per second. In addition to the CPU, there are many large I/O devices— including tape drives and high speed printers—and disks that store many billions or trillions of characters. The computer alone weighs over 5000 pounds/2200 kilograms! In contrast, the
laptop PC shown in below Figure ,
hard drive is one of many storage options. The entire system, complete with display screen, built-in
webcam, multiple network connections, and battery, weighs about three pounds (1.4 kilograms, if you prefer).
Although these two systems seem very different, the difference is actually one of magnitude, not of concept. The large system operates much faster, can support much more memory, and handles more input and output much faster. It has operating system software that allows many users to share this larger resource. Nonetheless, the fundamental system architecture is remarkably similar in both cases. Even the actual processing performed by the CPU is similar.
distributed computing. The fact that different types of computers can work together, share files, and communicate successfully is known as open computing. Communication technology fulfills the requirements that make open and distributed computing possible. Computers are sometimes divided into categories:
mainframe computers, minicomputers, workstations, and personal computers, but these categories are less significant than they once were. The capability of today’s personal computer far exceeds the capabilities of a mainframe computer of just a few years ago.
minicomputer, or even a small mainframe. Rather than attempting to categorize a particular computer, it is usually more productive to describe its capabilities in comparison to other systems being discussed or considered.
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