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Main memory.

The memory unit of a digital computer typically has a main (or primary) memory, cache, and secondary (or auxiliary) memory. The main memory holds data and instructions for immediate use by the computer's ALU. It receives this information from an input device or an auxiliary storage unit. In most cases, the main memory is a high-speed random-access memory (RAM)--i.e., a memory in which specific contents can be accessed (read or written) directly in a very short time regardless of the sequence (and hence location) in which they were recorded. Two types of main memory are possible with random-access circuits--static random-access memory (SRAM) and dynamic random-access memory (DRAM). A single memory chip is made up of several million memory cells. In a SRAM chip, each memory cell consists of a single flip-flop (for storing the binary digits 1 or 0) and a few more transistors (for reading or writing operation). In a DRAM chip, each memory cell consists of a capacitor (rather than a flip-flop) and a single transistor. When a capacitor is electrically charged, it is said to store the binary digit 1, and when discharged, it represents 0; these changes are controlled by the transistor. Because it has fewer components, DRAM requires a smaller area on a chip than does SRAM, and hence a DRAM chip can have a greater memory capacity, though its access time is slower than that of SRAM.

The cache is a SRAM-based memory of small capacity that has faster access time than the main memory and that temporarily stores data and part of a program for quicker processing by the ALU. 

Auxiliary, or secondary, memory. Auxiliary storage units are an integral part of a computer's peripheral equipment. They can store substantially more information than can a main memory but operate at slow speeds. The most common forms of secondary storage are magnetic disk or tape. 

Magnetic disks are flat, circular plates coated with a magnetic material. There are two types: hard disks, which are made of aluminum or glass and are physically rigid; and floppy disks, which are made of plastic and are flexible. Both types of disks come in diameters of 3.5 and 5.25 inches (9 and 13 cm). Hard disks that can store anywhere from 20 megabytes to 2 gigabytes (20 million to 2 billion bytes, or small groups of adjacent binary digits constituting a subunit of information) are readily available for desktop computers, and still larger ones can be had. Floppy disks have a much smaller capacity of only two to three megabytes. In both types of disk, data on their surfaces is arranged in concentric tracks. A tiny magnet, called a magnetic head, writes a binary digit (1 or 0) by magnetizing a tiny spot on a disk in different directions and reads digits by detecting the magnetization direction of the spots. A magnetic-disk drive is an assembly of one or more disks, magnetic heads, and a mechanical device for rotating the disks for reading or writing purposes.

Magnetic tapes are also sometimes used in auxiliary storage units. They have an even greater memory capacity than disks, but their access time is far slower because they are sequential-access memories--i.e., ones in which data in consecutive addresses are sequentially read or written as a tape is unwound. Magnetic disks are partly random-accessed (because a magnetic head for reading or writing goes to a desired circular track) and partly sequential-accessed (because data is read or written sequentially from that track as the disk rotates).

Hard disks are routinely used for storing current records and applications software on personal and other small computers. Larger computers may use RAID (redundant array of inexpensive drives), which consists of a group of hard-disk drives that work together as one disk drive. A typical RAID consists of five or more drives with 3.5-inch or 5.25-inch hard disks; this array yields reasonably high access speeds and is more reliable yet less expensive than a traditional single drive with large hard disks. RAIDs are widely used with mainframe computers that require auxiliary memory of very large capacity.

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