According to DIN 44300, an electrical or electronic memory is a functional unit in a digital computing system that records, stores or outputs data and commands. Electronic memories can be classified according to various criteria, areas of application and characteristic values. For example, according to the storage technology, the storage medium and the associated storage method, the storage organization and the area of application in computer and information technology.
Classification of electronic memories according to storage medium
The range of storage media is characterized by historical development and areas of application. An essential aspect is to be seen in the storage of short-term retrievable and long-term usable data. While in the fifties of the last century, short-term retrievable data for program execution was stored in relays, flip-flops and core memories, long-term usable data was stored as punched tape or punched card on paper or cardboard, which was later replaced by magnetic technologies such as disk storage, magnetic tapes, hard disks or floppy disks.
As far as short-term and read-only storage is concerned, semiconductor memories have dominated the scene since the 1970s and are still being constantly developed today in terms of their storage capacities, access times and supply voltages. In parallel, optical storage media such as compact discs, DVDs and Blu-Ray discs were developed for long-term storage.
Classification of electronic memories according to storage technology
There are various technological differences in semiconductor memories, which are expressed in the fact that certain semiconductor memories can store the stored data only for a short time or for a longer period by regular refreshing. Other technologies can store their charge permanently without regular refreshing. Therefore, a distinction is made between volatile and non-volatile, dynamic and static memories. With volatile memories, the data is lost when the supply voltage is switched off. The memory content can only be saved by the constant supply of a buffer battery. In contrast, the data in a non-volatile memory continues to be stored even after the supply voltage is switched off, without any special measures being required.
In the organizational form of memories, a general distinction is made between word-organized memories and block-organized memories. Both organizational forms can operate with different access methods such as random, quasi-random, direct, implicit or sequential access. Dynamic memories work with random access in which each memory cell is directly accessible. In contrast, static memories operate with fixed memory states that are lost only when signals are applied or when the supply voltage fails.
Classification of electronic memories according to characteristic values
Memories are distinguished by various characteristic values. The most important of these are memory capacity, access time, storage time, readout time and, in the case of semiconductor memories, supply voltage. The memory capacity is specified in bytes (B). When specifying the storage capacity, the prefixeskilobyte( KB), megabyte ( MB), gigabyte( GB), terabyte( TB) and petabyte( PB) are used. However, the prefixes are based on the dual system and therefore differ considerably from the prefixes of the decimal system. Another application-specific characteristic value is the access time. It is the amount of time required to access the storage medium. Depending on the type of memory being accessed, this can be a few nanoseconds (ns), as in the case of semiconductor memories, or a few seconds or minutes, as in the case of magnetic tape devices.
Classification of electronic memories according to their application
From the memory hierarchy, memories can be divided into primary memories, secondary memories, and tertiary memories according to speed and access. Primary memories include registers, caches, main memories and program memories. Secondary storage includes removable disk and hard disk drives, and tertiary storage includes mass storage such as optical storage media, tape drives, tape libraries, disk arrays, Just a Bunch Of Disks( JBOD) and RAID systems.
With hierarchically located storage media, it is important that the speed of the larger and slower storage is matched to the faster and smaller storage. Hierarchically, the central memory, the working memory or main memory, is the fastest storage unit with constantly changing memory contents. Local memories are located in the central processing unit( CPU), as registers with partly special functions, as instruction counters, index or universal registers. In addition, there are the intermediate and buffer memories, known as cache.
Data that does not need to be constantly available is stored in peripheral memory units or mass storage devices, the background memories. These include floppy disk drives, disk storage and tape drives, HVD discs and DVD drives. These storage units, which are characterized by high storage capacities, can be equipped with both fixed storage media and removable storage media.