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Text of the page (random words):
ltra hd blu ray holographic versatile disc worm in development cbram racetrack memory nram millipede memory ecram patterned media holographic data storage electronic quantum holography 5d optical data storage dna digital data storage universal memory time crystal quantum memory ultraram historical paper data storage 1725 punched card 1725 punched tape 1725 plugboard delay line memory drum memory 1932 magnetic core memory 1949 plated wire memory 1957 core rope memory 1960s thin film memory 1962 disk pack 1962 twistor memory 1968 bubble memory 1970 floppy disk 1971 v t e example of writable volatile random access memory synchronous dynamic ram modules primarily used as main memory in personal computers workstations and servers 8gb ddr3 ram stick with a white heatsink random access memory ram r æ m is a form of computer memory that can be read and changed in any order typically used to store working data and machine code 1 2 a random access memory device allows data items to be read or written in almost the same amount of time irrespective of the physical location of data inside the memory in contrast with other direct access data storage media such as hard disks cd rws dvd rws and the older magnetic tapes and drum memory where the time required to read and write data items varies significantly depending on their physical locations on the recording medium due to mechanical limitations such as media rotation speeds and arm movement ram contains multiplexing and demultiplexing circuitry to connect the data lines to the addressed storage for reading or writing the entry usually more than one bit of storage is accessed by the same address and ram devices often have multiple data lines and are said to be 8 bit or 16 bit etc devices clarification needed in today s technology random access memory takes the form of integrated circuit ic chips with mos metal oxide semiconductor memory cells ram is normally associated with volatile types of memory such as dynamic random access memory dram modules where stored information is lost if power is removed although non volatile ram has also been developed 3 other types of non volatile memories exist that allow random access for read operations but either do not allow write operations or have other kinds of limitations on them these include most types of rom and a type of flash memory called nor flash the two main types of volatile random access semiconductor memory are static random access memory sram and dynamic random access memory dram commercial uses of semiconductor ram date back to 1965 when ibm introduced the sp95 sram chip for their system 360 model 95 computer and toshiba used dram memory cells for its toscal bc 1411 electronic calculator both based on bipolar transistors commercial mos memory based on mos transistors was developed in the late 1960s and has since been the basis for all commercial semiconductor memory the first commercial dram ic chip the intel 1103 was introduced in october 1970 synchronous dynamic random access memory sdram later debuted with the samsung km48sl2000 chip in 1992 contents 1 history 1 1 mos ram 2 types 3 memory cell 4 addressing 5 memory hierarchy 6 other uses of ram 6 1 virtual memory 6 2 ram disk 6 3 shadow ram 7 recent developments 8 memory wall 9 timeline 9 1 sram 9 2 dram 9 3 sdram 9 4 sgram and hbm 10 see also 11 references 12 external links history these ibm tabulating machines from the mid 1930s used mechanical counters to store information 1 megabit mbit chip one of the last models developed by veb carl zeiss jena in 1989 early computers used relays mechanical counters 4 or delay lines for main memory functions ultrasonic delay lines were serial devices which could only reproduce data in the order it was written drum memory could be expanded at relatively low cost but efficient retrieval of memory items required knowledge of the physical layout of the drum to optimize speed latches built out of vacuum tube triodes and later out of discrete transistors were used for smaller and faster memories such as registers such registers were relatively large and too costly to use for large amounts of data generally only a few dozen or few hundred bits of such memory could be provided the first practical form of random access memory was the williams tube starting in 1947 it stored data as electrically charged spots on the face of a cathode ray tube since the electron beam of the crt could read and write the spots on the tube in any order memory was random access the capacity of the williams tube was a few hundred to around a thousand bits but it was much smaller faster and more power efficient than using individual vacuum tube latches developed at the university of manchester in england the williams tube provided the medium on which the first electronically stored program was implemented in the manchester baby computer which first successfully ran a program on 21 june 1948 5 in fact rather than the williams tube memory being designed for the baby the baby was a testbed to demonstrate the reliability of the memory 6 7 magnetic core memory was invented in 1947 and developed up until the mid 1970s it became a widespread form of random access memory relying on an array of magnetized rings by changing the sense of each ring s magnetization data could be stored with one bit stored per ring since every ring had a combination of address wires to select and read or write it access to any memory location in any sequence was possible magnetic core memory was the standard form of computer memory system until displaced by solid state mos metal oxide silicon semiconductor memory in integrated circuits ics during the early 1970s 8 prior to the development of integrated read only memory rom circuits permanent or read only random access memory was often constructed using diode matrices driven by address decoders or specially wound core rope memory planes citation needed semiconductor memory began in the 1960s with bipolar memory which used bipolar transistors while it improved performance it could not compete with the lower price of magnetic core memory 9 mos ram the invention of the mosfet metal oxide semiconductor field effect transistor also known as the mos transistor by mohamed m atalla and dawon kahng at bell labs in 1959 10 led to the development of metal oxide semiconductor mos memory by john schmidt at fairchild semiconductor in 1964 8 11 in addition to higher performance mos semiconductor memory was cheaper and consumed less power than magnetic core memory 8 the development of silicon gate mos integrated circuit mos ic technology by federico faggin at fairchild in 1968 enabled the production of mos memory chips 12 mos memory overtook magnetic core memory as the dominant memory technology in the early 1970s 8 an integrated bipolar static random access memory sram was invented by robert h norman at fairchild semiconductor in 1963 13 it was followed by the development of mos sram by john schmidt at fairchild in 1964 8 sram became an alternative to magnetic core memory but required six mos transistors for each bit of data 14 commercial use of sram began in 1965 when ibm introduced the sp95 memory chip for the system 360 model 95 9 dynamic random access memory dram allowed replacement of a 4 or 6 transistor latch circuit by a single transistor for each memory bit greatly increasing memory density at the cost of volatility data was stored in the tiny capacitance of each transistor and had to be periodically refreshed every few milliseconds before the charge could leak away toshiba s toscal bc 1411 electronic calculator which was introduced in 1965 15 16 17 used a form of capacitive bipolar dram storing 180 bit data on discrete memory cells consisting of germanium bipolar transistors and capacitors 16 17 while it offered improved performance over magnetic core memory bipolar dram could not compete with the lower price of the then dominant magnetic core memory 18 mos technology is the basis for modern dram in 1966 dr robert h dennard at the ibm thomas j watson research center was working on mos memory while examining the characteristics of mos technology he found it was capable of building capacitors and that storing a charge or no charge on the mos capacitor could represent the 1 and 0 of a bit while the mos transistor could control writing the charge to the capacitor this led to his development of a single transistor dram memory cell 14 in 1967 dennard filed a patent under ibm for a single transistor dram memory cell based on mos technology 19 the first commercial dram ic chip was the intel 1103 which was manufactured on an 8 µm mos process with a capacity of 1 kbit and was released in 1970 8 20 21 synchronous dynamic random access memory sdram was developed by samsung electronics the first commercial sdram chip was the samsung km48sl2000 which had a capacity of 16 mbit 22 it was introduced by samsung in 1992 23 and mass produced in 1993 22 the first commercial ddr sdram double data rate sdram memory chip was samsung s 64 mbit ddr sdram chip released in june 1998 24 gddr graphics ddr is a form of ddr sgram synchronous graphics ram which was first released by samsung as a 16 mbit memory chip in 1998 25 types the two widely used forms of modern ram are static ram sram and dynamic ram dram in sram a bit of data is stored using the state of a six transistor memory cell typically using six mosfets metal oxide semiconductor field effect transistors this form of ram is more expensive to produce but is generally faster and requires less dynamic power than dram in modern computers sram is often used as cache memory for the cpu dram stores a bit of data using a transistor and capacitor pair typically a mosfet and mos capacitor respectively 26 which together comprise a dram cell the capacitor holds a high or low charge 1 or 0 respectively and the transistor acts as a switch that lets the control circuitry on the chip read the capacitor s state of charge or change it as this form of memory is less expensive to produce than static ram it is the predominant form of computer memory used in modern computers both static and dynamic ram are considered volatile as their state is lost or reset when power is removed from the system by contrast read only memory rom stores data by permanently enabling or disabling selected transistors such that the memory cannot be altered writeable variants of rom such as eeprom and nor flash share properties of both rom and ram enabling data to persist without power and to be updated without requiring special equipment ecc memory which can be either sram or dram includes special circuitry to detect and or correct random faults memory errors in the stored data using parity bits or error correction codes in general the term ram refers solely to solid state memory devices either dram or sram and more specifically the main memory in most computers in optical storage the term dvd ram is somewhat of a misnomer since unlike cd rw or dvd rw it does not need to be erased before reuse nevertheless a dvd ram behaves much like a hard disc drive if somewhat slower memory cell main article memory cell computing the memory cell is the fundamental building block of computer memory the memory cell is an electronic circuit that stores one bit of binary information and it must be set to store a logic 1 high voltage level and reset to store a logic 0 low voltage level its value is maintained stored until it is changed by the set reset process the value in the memory cell can be accessed by reading it in sram the memory cell is a type of flip flop circuit usually implemented using fets this means that sram requires very low power when not being accessed but it is expensive and has low storage density a second type dram is based around a capacitor charging and discharging this capacitor can store a 1 or a 0 in the cell however the charge in this capacitor slowly leaks away and must be refreshed periodically because of this refresh process dram uses more power but it can achieve greater storage densities and lower unit costs compared to sram sram cell 6 transistors dram cell 1 transistor and one capacitor addressing to be useful memory cells must be readable and writeable within the ram device multiplexing and demultiplexing circuitry is used to select memory cells typically a ram device has a set of address lines a0 an and for each combination of bits that may be applied to these lines a set of memory cells are activated due to this addressing ram devices virtually always have a memory capacity that is a power of two usually several memory cells share the same address for example a 4 bit wide ram chip has 4 memory cells for each address often the width of the memory and that of the microprocessor are different for a 32 bit microprocessor eight 4 bit ram chips would be needed often more addresses are needed than can be provided by a device in that case external multiplexors to the device are used to activate the correct device that is being accessed memory hierarchy main article memory hierarchy one can read and over write data in ram many computer systems have a memory hierarchy consisting of processor registers on die sram caches external caches dram paging systems and virtual memory or swap space on a hard drive this entire pool of memory may be referred to as ram by many developers even though the various subsystems can have very different access times violating the original concept behind the random access term in ram even within a hierarchy level such as dram the specific row column bank rank channel or interleave organization of the components make the access time variable although not to the extent that access time to rotating storage media or a tape is variable the overall goal of using a memory hierarchy is to obtain the highest possible average access performance while minimizing the total cost of the entire memory system generally the memory hierarchy follows the access time with the fast cpu registers at the top and the slow hard drive at the bottom in many modern personal computers the ram comes in an easily upgraded form of modules called memory modules or dram modules about the size of a few sticks of chewing gum these can quickly be replaced should they become damaged or when changing needs demand more storage capacity as suggested above smaller amounts of ram mostly sram are also integrated in the cpu and other ics on the motherboard as well as in hard drives cd roms and several other parts of the computer system other uses of ram a so dimm stick of laptop ram roughly half the size of desktop ram in addition to serving as temporary storage and working space for the operating system and applications ram is used in numerous other ways virtual memory main article virtual memory most modern operating systems employ a method of extending ram capacity known as virtual memory a portion of the computer s hard drive is set aside for a paging file or a scratch...