TEXT A. MEMORY OR STORAGE UNIT

1. The part of a digital computer which stores information is called storage or memory. The computer's memory stories the numbers to be operated on; it stores intermediate results that are generated during the course of a computation; and it stores the final results. The instructions themselves are also stored in the computer's memory.

2. There are two important factors about the memory unit: an access time and a capacity. The time required to transmit one computer word out of the memory to where it will be used is called the memory access time; it usually amounts to a few millionths of a second or less in modern fast computers. The speed of modern computers is the speed of access to their memories. The capacity of a computer is the quantity of data that its memory unit can hold.

3. There are many ways of memorizing information in memory cells of a digital computer. External memory or storage units may use magnetic tapes, magnetic drums," magnetic disks and floppy disks. The magnetic drum and magnetic disk are called a Direct Access, or Random Access, Storage Device (DASD).

4. The magnetic disk is very similar to the magnetic drum but is based upon the use of a flat disk with a series of concentric circles of magnetizable material, one read/write head, being for each concentric circle, i.e., for each track. Memory units on magnetic disks may store more than 100,000,000 bytes. The magnetic disk is illustrated in Figure 1.

5. Internal or main memory units were constructed of magnetic cores about 8 hundredths of an inch in diameter, each core storing one 'yes ' or 'no', that is, each core representing one bit of information.

6. Information that is stored inside a computer is stored in registers, electronic units of hardware in which the positioning of physical objects stores information. Each register holds one machine word consisting usually of 32 bits or 4 bytes. Registers hold information temporarily during processing. The slower models of registers use magnetic cores; the faster models use special electronic circuits or film memory devices.

7. Usually the registers are of three types:

1. General-Purpose Registers are sixteen registers, each being able to contain one word. These registers are used for storing the integer operands taking part in binary arithmetic operations.

2. Floating-Point Registers are four registers, each being able to contain a doubleword. These registers hold the operands taking part in arithmetic operations on floating-point numbers.

3. Control Registers form a group of registers differing from one model to another.

8. The development of semiconductor integration technology has led to creation of memories on LSI circuits. For constructing memory units on LSI circuits either bipolar or MOS memory are used. The access time of bipolar memory is about 100 nanoseconds, while the access time of MOS memory is 500 nanoseconds. But on the other hand the density of memorizing elements allocation in the latter is very high and amounts to 4 thousand memorizing elements for one chip. The latest achievements of modern electronics are creation of memories on electronic circuits made by spraying layers of different memorizing materials.

9. Nowadays the main memory RAM which is regularly used in microcomputers can accept new instructions or information from a peripheral device. Terms synonymous with the computer's working memory RAM are: core, core storage, main memory, main storage, primary storage, read/write memory. Other memories, such as ROM or PROM, which are used in microcomputers as well, store instructions or information permanently. ROM, PROM, EPROM, and EEPROM are all together called firmware which is 'hard' software.