Random-Access Memory
Random-access memory (that; engl. Random [ -] access memory to German: " random / direct access " = Random Access Memory) acronym, RAM is an information store that is used as a memory type of computer data, usually in the form of memory modules. The most common forms are among the semiconductor memories. RAM is implemented as an integrated circuit silicon technology, and mainly used in all types of electronic devices.
Characteristics
The name of the storage type as " optional " in this context means that each memory cell can be addressed directly via its memory address. The memory does not have to be read sequentially or in blocks. In large memory devices, however, the address is not on the individual cells, but over a word, the width of which is dependent on the memory architecture. This differs from the RAM blocks to be written memories, the so-called flash memory.
The term Random Access Memory is now always in the sense of " write - read - RAM " (read -write random-access memory - RWRAM ) used. There are other types of memory with random access, in particular read-only memory devices (Read Only Memory, ROM). Since the term RAM is misleading, was temporarily tried the name "read-write memory" ( RWM, read-write memory ) to establish, but who could not prevail.
History
The origin of the term goes back to the early days of modern computing in which all data were available to be read sequentially storage forms such as punched cards or magnetic tapes that have been loaded for processing in fast arithmetic register. To facilitate the provision interim results faster, delay lines were temporarily ( engl. delay line ) used for intermediate values until then the Ferritkernspeicher were introduced. This writable memory already had the same shape of the matrix Zugriffes as today's RAM. At that time the fast memory types were all written and the main innovation consisted in the random access magnetic core memory and the subsequently touching down on semiconductor memories RAM modules.
Control of RAM chips
Depending on the type of RAM device, the control is effected in synchronism with a clock signal without clock or asynchronously. The main difference is that in the asynchronous version the data is available only after a certain block- dependent travel time available or are written. These, among other things, material-dependent, temporal parameters have manufacturing tolerances and are dependent, so for asynchronous storing the maximum throughput is more limited than with synchronous memory activations of different influences. For synchronous Save the temporal alignment of the control signals is determined by a clock signal, resulting in significantly higher throughput rates.
Synchronous RAM can both static and dynamic RAMs (see below). Examples of synchronous SRAMs are burst SRAMs or ZBTRAMs. Asynchronous SRAMs are usually slower low-power SRAMs, which are used for example in smaller microcontrollers as an external data storage application. In the dynamic RAMs are the usual since the late 1990s synchronous SDR SDRAMs and their successors, the DDR SDRAMs, to name an example, while earlier conventional DRAMs represent as EDO DRAM asynchronous DRAM devices.
Supply voltage
The energy requirements of volatile RAM types is highly dependent on the operating voltage thereof: In general, the energy consumption increases quadratically increasing voltage. It can be depending on memory size several watts, which is particularly noticeable effect on the battery life in mobile devices. Therefore, the manufacturers continuously try to reduce the energy demand and to allow a lower supply voltage.
The supply voltage of ( JEDEC -compliant ) DDR2 SDRAM is 1.8 V. The predecessor DDR SDRAM requires 2.5 V, even older memory as SDR SDRAM need 3.3 V. In the currently used DDR3 SDRAM was the voltage 1.5 V reduced.
Types of RAM
There are different technical implementations of RAM. The most commonly used today are mainly used in computers and are "volatile" (also: volatile ), that is, the stored data will be lost after switching off the power supply. However, there are types of RAM that get their information even without a power supply (not volatile ). These are called NVRAM. The following list is arranged according to the basic principle:
- Random Access Memory (RAM) volatile ( volatile ) RAM Static RAM (SRAM)
- Dynamic RAM (DRAM) Synchronous Dynamic Random -Access Memory (SDRAM, DDR SDRAM, etc.)
- Pseudo Static RAM ( Psiram )
- Ferroelectric RAM (FRAM, FeRAM )
- Magnetic RAM ( MRAM), Racetrack memory, magnetic bubble memory
- Phase-change RAM (PRAM, PCRAM )
- RRAM
Static RAM (SRAM)
Static RAM (SRAM), usually referred to smaller electronic memory devices in the range up to several Mibit. As a special feature they retain their memory content, which is stored in bistable multivibrators, without continuous refresh cycles - it is sufficient that a supply voltage. From this circumstance the name derives; it is true historically also for core memory, which does not change its state even without voltage over the years.
SRAM requires significantly more components ( and chip area ) than DRAM ( see below) - specifically four to six transistors per bit of memory versus a (plus a storage capacitor ) in a DRAM cell - and is therefore too expensive for large amounts of memory. However, it provides very fast access times and does not require refresh cycles as DRAM.
Applications are for example in computers as cache and main memory as microcontrollers. Its content is volatile ( volatile; engl: volatile. ), That is, the stored information is lost when the power supply is removed. In combination with a buffer battery can be realized from the static RAM a special form of non-volatile memory NVRAM, since SRAM cell without access cycles have only a very low power requirement and the buffer battery can keep for several years the data content in the SRAM.
Dynamic RAM (DRAM)
Dynamic RAM (DRAM) means an electronic memory module which is used mainly as a work memory in computers. Its content is volatile ( volatile ), that is, the stored information is quick even with maintained operating voltage (!) Lost.
The information is stored in the form of the state of charge of a capacitor. This very simple structure makes the memory cell, although very small ( 6 to 10 F ²), however, the capacitor discharges in the small potential capacity by the leakage currents occurring quickly, and the information content is lost. Therefore, the memory cells must be refreshed periodically.
Compared to SRAM DRAM is much cheaper per bit, which is why it is used mainly for RAM. DRAM modules with built-in control circuit for refreshing can behave outwardly as SRAM. This is referred to as a pseudo- static RAM.
Phase -change RAM ( PCRAM, PRAM)
Phase -change RAM ( PRAM) is, inter alia, Samsung still in development. It should serve as a replacement of S and DRAM, and have advantages over NOR flash memory, for example, should be much faster write accesses and the number of Schreib-/Lese-Zyklen should be higher by a multiple than NOR flash memory. In this case, it occupies less space and is easier to manufacture.
Resistive RAM ( RRAM, ReRAM, memristor )
Resistive RAM ( RRAM, English) is a non- volatile type of memory consisting of an "active " layer of thin-film metal oxide ( materials? ) Whose electrical resistance can be greatly changed by means of electrical impulses. The change in static and is reversible, this results in the possibility of using the material as a non-volatile memory. The data rewriting is "very good " and the energy consumption ( the thus occurs only in writing and reading ) referred to as " very low".
ReRAM is a (as of 2013) recent development, which was still, for example, in 2010 the prototype stage in the development of some companies in the semiconductor industry. Applies to state or damaligem was: 1 RRAM has some similarities and commonalities with the PCRAM (which?) 2 Various forms of RRAM based on most dielectric materials, ranging from perovskites on transition metal oxides to chalcogenides. Already in 1967 it was shown resistance switching itself on silica ( SiO2).