PRIMARY STORAGE

'Primary storage', or 'internal memory', is computer memory that is accessible to the central processing unit of a computer without the use of computer's input/output channels. Primary storage is used to store data that is likely to be in active use and is stored in locations selected by virtual addressing into a physical address. Primary storage is typically very fast, as in the case of RAM. RAM is also volatile, losing the stored information in an event of power loss, and quite expensive. ROM is not volatile, but is not suited to storage of changeable data which is what makes the computer a flexible tool of immense power; it is also somewhat more expensive to produce as ROM chips are machine dependent and so have less effective economies of scale. Typically, Programmable read-only Memory (EEPROM) must also be completely erased before it can be rewritten, making large scale use impractical for frequently changing data, if not impossible. Other types of ROM (PROM and EPROM) are even less flexible but have their niches in imbedded systems.) Therefore the use of ROM is limited to separate 'secondary storage' such as for code to bootstrap a system, basic input output system (hardware driver kernels) or for Memory Management Units, and other applications usually requiring long-term persistent storage.
Confusingly, the term primary storage has recently been used in a few contexts to refer to online storage on a server's or local computer's hard disks, which memory type, is now classified in many instructional contexts as secondary storage (Before the era of large address spaces and cheap RAM chips, it was considered otherwise). Swapping data blocks (virtual memory management) from Hard Disk to Physical RAM and back is so fundamental a part of modern computers as to make the newer academic distinction almost moot, save that one is electronic, and the other electromagnetic and comparatively much slower utilizing dedicated cache memory managers and other tricks to speed up electronic throughput rates. Hence when the data retrieval can be deferred, the data may be offline in the hard disk's portion of virtual memory at any given time, or if it's been needed lately, in physical addresses manged by the virtual memory swapping controlled by the Memory management unit (MMU).
Primary storage, in the technical sense in use today, includes several types of storage, such as main storage in physical address space, cache memory, and internal registers, all of which can be directly accessed by the processor, at least through the Memory management unit (MMU). Primary storage can be accessed ''randomly'', that is, accessing any location in storage at any moment takes (nearly) the same amount of time (Ram locations behind a Memory management unit, such as video display memory, take a few machine instruction cycles longer, as the video memory registers would need programmed first). A particular location in storage is selected by its physical memory address. That address remains the same, no matter how the particular value stored there changes.

Contents

Technology and history

See also

Technology and history

Today, primary storage is typically random access memory, a type of semiconductor memory. Over the history of computing hardware, a variety of technologies have been used for primary storage. Some early computers used mercury delay lines, in which a series of acoustic pulses were sent along a tube filled with mercury. When the pulse reached the end of the tube, the circuitry detected whether the pulse represented a binary 1 or 0 and caused the oscillator at the beginning of the line to repeat the pulse. Other early computers stored primary memory on rapidly rotating magnetic drums.
Modern primary storage devices include:

★ Random access memory (RAM) - includes VRAM, WRAM, NVRAM

★ Read-only memory (ROM)
Before the use of integrated circuits for memory became widespread, primary storage was implemented in many different forms:

★ a drum holding capacitors (used in Atanasoff–Berry Computer)

★ Williams tube

★ Delay line memory

★ Drum memory

★ Magnetic core memory

★ Twistor memory

★ Bubble memory

See also

★ Federal Standard 1037C