Main memory

Generally speaking, memory is a facility for temporarily storing program instructions or data during the course of processing. Within the main memory (sometimes called primary storage), instructions and data are stored in distinct locations so that they can be distinguished easily. The main memory is often referred to as RAM (random access memory). It acts as a staging post between the hard disk and processor. The more data it is possible to have available in the RAM, the faster the PC will run.

Main memory is attached to the processor via its address and data buses. Each transaction between the CPU and memory is called a bus cycle. The number of data bits a CPU is able to transfer during a single bus cycle affects a computer’s performance.

One distinguishing characteristic of RAM is that it is possible both to read data from the memory and to write new data into the memory easily and rapidly. Both the reading and writing are accomplished through the use of electrical signals.

The other distinguishing characteristic of RAM is that it is volatile. A RAM must be provided with a constant power supply. If the power is interrupted, then the data is lost.

The two traditional forms of RAM used in computers are DRAM (dynamic RAM) and SRAM (static RAM). A dynamic memory cell is simpler and smaller than a static memory cell. Thus, a DRAM is more dense (smaller cells = more cells per unit area) and less expensive than a corresponding SRAM. DRAMs tend to be favored for large memory requirements. SRAMs are generally somewhat faster than DRAMs. Because of these relative characteristics, a static RAM is used for cache memory (an intermediate layer of memory that is smaller but faster, and can hold portions of programs that are likely to be used again shortly), and a dynamic RAM is used for main memory.

Input/Output Units

All of the computing power in the world wouldn’t be useful if we couldn’t input values into the calculations from outside or report to the outside the results of the calculations. Input and output units are the channels through which the computer communicates with the outside world.

An input unit is a device through which data and programs from the outside world are entered into the computer. The first input units interpreted holes punched on paper tape or cards. Modern input devices include: the mouse, keyboard, touch screen monitor, scanner, track pad, microphone, joystick, and web camera.

An output unit is a device through which results stored in the computer memory are made available to the outside world. The most common output devices are printers, video display terminals, speakers, and plotters.