Random access memory

See also: Random access memory (homonymy)

The random access memory , memory system or volatile memory ( RAM ( Random Access Memory random access memory ) or RWM ( Read Write Memory memory in reading writing ) in English) is the memory in which a computer places the data during their treatment. The characteristics of this memory are:

  • its speed of access (this speed is essential to quickly provide the data to the Processeur);
  • its volatility (this volatility implies that the data are lost as soon as the computer ceases being fed in electricity).

Designation

The random access memory ( RAM ) is generally opposed to the ''' read-only memory ''' (''' ROMANIAN '''): the data contained in the random access memory are lost when the power supply is cut whereas the read-only memory preserves its data in absence of power supply. The read-only memory is thus not volatile what makes it very useful at the time of the starting of a Ordinateur. Indeed, the random access memory is empty and the Hard drive is inaccessible during the starting of a computer. The computer can thus count only on the read-only memory to provide him information of configuration necessary to starting. This information is contained in a chip EEPROM support of BIOS.

Seldom, one uses the initials RWM (for Read Write Memory , that is to say memory in reading writing ) to indicate RAM rather by stressing the possibility of writing than the arbitrary access.

Attention the " terms; memory RAM " and " random access memory " can carry to confusion because they are not used in the literal sense:

Literally, the term RAM ( Random Access Memory ) implies the possibility of an arbitrary access to the data, i.e. an access to any data any time, in opposition to a Sequential access, like the access to a Magnetic band, where the data are necessarily read in an order defined in advance. In this direction, the ROMANIAN memories and the memories flash enjoy the characteristic RAM ( Random Access Memory ), but this literal interpretation carries to confusion with the everyday usage which opposes RAM and ROMANIAN . Consequently, one does not classify these memories in the memories RAM nor in the random access memories but only in the nonvolatile memories.

There exist memories with arbitrary access, accessible in reading and writing, like RAM , but not birds, like the ROMANIAN : for example, the memory flash. As these memories are not birds, one thus does not classify them in the memories RAM or random access memories because the terms memories RAM and random access memories are regarded as synonyms of volatile memories .

Technology

The Computing memory is component which was initially Magnétique (Tore S of Ferrite), then became electronic in the Années 1970, which makes it possible to store and to read again information binary S. Its role is in particular to store the Donnée S which will be treated by the Central processing unit (or the Microprocesseur). The random access memory has an access time of a few tens or hundreds of Nanoseconde S while that of the Hard drive is of some Milliseconde S (ten thousand to a hundred and thousand times more).

RAM presents the characteristic which one can reach it at the same time in reading and writing. A suitable electronic activation if need be makes it possible to lock temporarily in writing of the physical blocks given. The addressing of a memory (translation of electric tensions on wire in Address memory) is done by a mechanism named the Chip select . It is very easy to provide a microprocessor with a noncontiguous memory (for example from 0 to 4095, then a hole, then memory between 16384 and 32767), which facilitates much the detection of errors of possible addressing.

Information can be organized in words of 8,16 or 32 bits even more. Certain old machines had words of more exotic size, as for example 60 bits for the Control Dated 6600,36 bits for the IBM 7030 “Stretch” or DEC PDP-10 and 12 bits for the majority from the first minicomputers of DEC, the apparatuses of instrumentation working as well as possible on 12 bits at the time. But:

  • In the memories with parity, a ninth bit (known as of parity check ) exists in an invisible way,
  • In the memories with autocorrection of error on 1 bit and detection on more than one bit (ECC), these invisible bits are sometimes six or more,
  • Each word of the memories of modern the waiters known as not-stop or 24x365 lays out in addition to the bits of correction of bits of replacement which take the changing of the faulty bits with measurement of the ageing of the memory: a failure of 10-11 each year indeed results in more than one bit weakening per annum on a memory of 128 Gio.

The manufacturers often recommend to use ECC from 1 Gio of RAM.

There exist also associative memories.

Various types of random access memory

Static random access memory

  • the static random access memory SRAM (Static Random Memory Access), Static RAM , uses the principle of the Bascule S electronic, it is very fast, does not require cooling, on the other hand, it is expensive, bulky and, large consumer of electricity. It is used for the hiding places memory, example the plugs L1 report, L2 and L3 of the microprocessors.

  • MRAM ( Magnetic RAM ). Technology using the magnetic load of the electron. The possible performances are rather eloquent, flow about the gigabit a second, access time comparable with memory DRAM (~10 NS) and especially not-volatility of the data. Studied by all the large actors of electronics, it starts just (July 2006) with being marketed and should take its rise by 2010.

  • DPRAM ( Dual Ported RAM ). Technology using a double port which allows quasi simultaneous multiple accesses, in entry and exit.

Dynamic random access memory

The dynamic storage (DRAM, Dynamic RAM ) uses the technique of the Nano - Condensateur. It stores the information only if it “is refreshed” regularly: the controller report is obliged to read the cells all X Milliseconde S) to maintain on the good level the electric charges representing information in the cells, information which if not would become unreliable, impossibility of distinguishing a 1 from a 0 .

In spite of these constraints of cooling, this type of memory is very much used because it is quite cheaper than the static storage. Indeed, the elementary cell memory of the DRAM is very simple and requires only few Silicium.

The chips memories are gathered on supports SIMM (use of the bars per identical pair) or DIMM (possible use in simple bar).

The following types of dynamic random access memory are distinguished:

  • SDRAM ( Synchronous Dynamic RAM ). It is used as main and video memory. It tends to being replaced by GDR SDRAM. For the machines of the generation Pentium II, Pentium III. One distinguishes the SDRAM 66,100 and 133 (frequency of access in MHz). It comprises 168 pins normally.

  • VRAM ( Video RAM ). Present in the graphics cards. It is used to build the video image which will be sent to the screen of computer via the converter RamDac.

  • RDRAM ( Rambus Dynamic RAM ). Developed by the company Rambus, she suffers in particular from a price much higher than the other types of memories and too restrictive patents on behalf of the creative company. She is used for the machines of generation Pentium III and Pentium 4.

  • GDR SDRAM ( Double Dated Spleen Synchronous Dynamic RAM ). Used as main memory and video memory, it is synchronous with the clock system but it also doubles the busy bandwidth while transferring from the data twice by cycles instead of only one for the simple SDRAM. It is also more expensive. One distinguishes GDR PC1600, PC2100, PC2700, PC3200, etc the number represents the maximum theoretical quantity of transfer of information in Megabytes a second (it is necessary to multiply by 8 to obtain this speed in Megabits a second, a byte being composed of 8 bits). For the machines of generation Pentium III and Pentium 4. It comprises 184 pins normally.

  • DDR2 SDRAM ( Double Dated Spleen two SDRAM ). One distinguishes the DDR2-400, DDR2-533, DDR2-667 and DDR2-800. The number (400, 533,…) represent the frequency of operation. Certain manufacturers privilege the technique of name based on the quantity of theoretically transportable data (PC2-4200, PC2-5300, etc), but some seem to turn over at the real speed of operation in order to more clearly distinguish the DDR2 from the former generation. For the machines of generation Pentium 4 and more. It comprises 240 pins normally.

  • DDR3 SDRAM (Double Spleen three SDRAM Dated). It is about the 3e generation of technology GDR. The specifications of this new version are not finalized yet by JEDEC (September 2006). The first Microcomputer S which can use the DDR3 are awaited on the market for the end of 2007.

  • XDR DRAM (XDimm Rambus RAM). Technology based on technology Flexio developed by Rambus. It makes it possible to consider theoretical outputs of 6,4 Go/s with 12,8 Go/s in gust.

Manufacturers of memory

Manufacturers of chips memory:
  • Cypress
  • Elpida
  • Hynix
  • IDT
  • Qimonda (old division memory of Infineon Technologies)
  • Micron, 2nd larger manufacturer (March 2002), it provides in particular the memory for the game consoles Xbox
  • Nanya
  • Powerchip Semiconductor Corporation (PSC)
  • Samsung
  • Winbond

Manufacturers of bars of memory:

See too

Simple: Random access memory

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