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The physical machine, consisting of electronic circuits, is called the hardware. It consists of several major units: the Central Processing Unit (CPU), Main Memory, Secondary Memory and Peripherals.
The CPU is the major component of a computer; the ``electronic brain'' of the machine. It consists of the electronic circuits needed to perform operations on the data. Main Memory is where programs that are currently being executed as well as their data are stored. The CPU fetches program instructions in sequence, together with the required data, from Main Memory and then performs the operation specified by the instruction. Information may be both read from and written to any location in Main Memory so the devices used to implement this block are called random access memory chips (RAM). The contents of Main Memory (often simply called memory) are both temporary (the programs and data reside there only when they are needed) and volatile (the contents are lost when power to the machine is turned off).
The Secondary Memory provides more long term and stable storage for both programs and data. In modern computing systems this Secondary Memory is most often implemented using rotating magnetic storage devices, more commmonly called disks (though magnetic tape may also be used); therefore, Secondary Memory is often referred to as the disk. The physical devices making up Secondary Memory, the disk drives, are also known as mass storage devices because relatively large amounts of data and many programs may be stored on them.
The disk drives making up Secondary Memory are one form of Input/Output (I/O) device since they provide a means for information to be brought into (input) and taken out of (output) the CPU and its memory. Other forms of I/O devices which transfer information between humans and the computer are represented by the Peripherals box in Figure 1.1. These Peripherals include of devices such as terminals -- a keyboard (and optional mouse) for input and a video screen for output, high-speed printers, and possibly floppy disk drives and tape drives for permanent, removable storage of data and programs. Other I/O devices may include high-speed optical scanners, plotters, multiuser and graphics terminals, networking hardware, etc. In general, these devices provide the physical interface between the computer and its environment by allowing humans or even other machines to communicate with the computer.