Computer Memory

A. VOLATILE MEMORY Volatile memory, also known as volatile storage, is computer memory that requires power to maintain the stored information, unlike non-volatile memory which does not require a maintained power supply. It has been less popularly known as term Dynamic random access memory (DRAM) is a type of random access memory that stores each bit of data in a separate capacitor within an integrated circuit. Since real capacitors leak charge, the information eventually fades unless the capacitor charge is refreshed periodically.

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Because of this refresh requirement, it is a dynamic memory as opposed to SRAM and other static memory. The main memory (the “RAM”) in personal computers is Dynamic RAM (DRAM), as is the “RAM” of home game consoles (PlayStation, Xbox 360 and Wii), laptop, notebook and workstation computers. Here are the types of volatile memory: •DRAM The advantage of DRAM is its structural simplicity: only one transistor and a capacitor are required per bit, compared to six transistors in SRAM. This allows DRAM to reach very high density. Unlike flash memory, it is volatile memory (cf. on-volatile memory), since it loses its data when power is removed. The transistors and capacitors used are extremely small—millions can fit on a single memory chip. •DDR SDRAM Double data rate synchronous dynamic random access memory (DDR SDRAM) is a class of memory integrated circuits used in computers. Compared to single data rate (SDR) SDRAM, the DDR SDRAM interface makes higher transfer rates possible by more strict control of the timing of the electrical data and clock signals. Implementations often have to use schemes such as phase-locked loops and self-calibration to reach the required timing accuracy.

The interface uses double pumping (transferring data on both the rising and falling edges of the clock signal) to lower the clock frequency. One advantage of keeping the clock frequency down is that it reduces the signal integrity requirements on the circuit board connecting the memory to the controller. The name “double data rate” refers to the fact that a DDR SDRAM with a certain clock frequency achieves nearly twice the bandwidth of a single data rate (SDR) SDRAM running at the same clock frequency, due to this double pumping •SRAM

Static random access memory (SRAM) is a type of semiconductor memory where the word static indicates that, unlike dynamic RAM (DRAM), it does not need to be periodically refreshed, as SRAM uses bistable latching circuitry to store each bit. SRAM exhibits data remanence, but is still volatile in the conventional sense that data is eventually lost when the memory is not powered. Each bit in an SRAM is stored on four transistors that form two cross-coupled inverters. This storage cell has two stable states which are used to denote 0 and 1.

Two additional access transistors serve to control the access to a storage cell during read and write operations. A typical SRAM uses six MOSFETs to store each memory bit. In addition to such 6T SRAM, other kinds of SRAM chips use 8T, 10T, or more transistors per bit. Upcoming Volatile Memory: •T-RAM T-RAM, short for “Thyristor RAM” or “thyristor random-access memory”, is a new type of DRAM computer memory invented and developed by T-RAM Semiconductor, which departs from the usual designs of memory cells, combining the strengths of the DRAM and SRAM: high speed and high volume.

This technology, which exploits the electrical property known as negative differential resistance and is called Thin-Capacitively-Coupled-Thyristor, is used to create memory cells capable of very high packing densities. Due to this, this memory is highly scalable, and already has a storage density that is several times higher than found in conventional six-transistor SRAM memory. It is expected that the next generation of T-RAM memory will have the same density as DRAM.

It is assumed that this type of memory will be used in the next-generation processors by AMD, produced in 32nm and 22nm , replacing the previously licensed but unused Z-RAM technology. •Z-RAM Z-RAM, short for “zero capacitor RAM”, is a registered trademark for a novel DRAM computer memory technology developed by Innovative Silicon based on the floating body effect of silicon on insulator (SOI) process technology. Z-RAM has been licensed by Advanced Micro Devices for possible use in future microprocessors.

Innovative Silicon claims the technology offers memory access speeds similar to the standard six-transistor SRAM cell used in cache memory but uses only a single transistor, therefore affording much higher packing densities. Z-RAM relies on the floating body effect, an artifact of the SOI process technology which places transistors in isolated tubs (the transistor body voltages “float” with respect to the wafer substrate underneath the tubs). The floating body effect causes a variable capacitance to appear between the bottom of the tub and the underlying substrate, and was a problem that originally bedeviled circuit designs.

The same effect, however, allows a DRAM-like cell to be built without adding a separate capacitor, the floating body effect taking the place of the conventional capacitor. Because the capacitor is located under the transistor (instead of adjacent to, or above the transistor as in conventional DRAMs), another connotation of the name “Z-RAM” is that it extends in the negative z-direction. •TTRAM Twin Transistor RAM (TTRAM) is a new type of computer memory in development by Renesas.

TTRAM is similar to conventional one-transistor, one-capacitor DRAM in concept, but eliminates the capacitor by relying on the floating body effect inherent in a silicon on insulator (SOI) manufacturing process. This effect causes capacitance to build up between the transistors and the underlying substrate, originally considered a nuisance, but here used to replace a part outright. Since a transistor created using the SOI process is somewhat smaller than a capacitor, TTRAM offers somewhat higher densities than conventional DRAM. Since prices are strongly related to density, TTRAM is theoretically less expensive.

However the requirement to be built on SOI fab lines, which are currently the “leading edge”, makes the cost somewhat unpredictable at this point. In the TTRAM memory cell, two transistors are serially connected on an SOI substrate. One is an access transistor, while the other is used as a storage transistor and fulfils the same function as the capacitor in a conventional DRAM cell. Data reads and writes are performed according to the conduction state of the access transistor and the floating-body potential state of the storage transistor.

The fact that TTRAM memory cell operations don’t require a step-up voltage or negative voltage, as DRAM cells do, makes the new cell design suitable for use with future finer processes and lower operating voltage B. NON-VOLATILE Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, in the most basic sense, is computer memory that can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory, flash memory, most types of magnetic computer storage devices (e. g. ard disks, floppy disks, and magnetic tape), optical discs, and early computer storage methods such as paper tape and punched cards. Non-volatile memory is typically used for the task of secondary storage, or long-term persistent storage. The most widely used form of primary storage today is a volatile form of random access memory (RAM), meaning that when the computer is shut down, anything contained in RAM is lost. Unfortunately, most forms of non-volatile memory have limitations that make them unsuitable for use as primary storage. Typically, non-volatile memory either costs more or performs worse than volatile random access memory. ROM Read-only memory (usually known by its acronym, ROM) is a class of storage media used in computers and other electronic devices. Because data stored in ROM cannot be modified (at least not very quickly or easily), it is mainly used to distribute firmware (software that is very closely tied to specific hardware, and unlikely to require frequent updates) In its strictest sense, ROM refers only to mask ROM (the oldest type of solid state ROM), which is fabricated with the desired data permanently stored in it, and thus can never be modified.

However, more modern types such as EPROM and flash EEPROM can be erased and re-programmed multiple times; they are still described as “read-only memory”(ROM) because the reprogramming process is generally infrequent, comparatively slow, and often does not permit random access writes to individual memory locations. Despite the simplicity of mask ROM, economies of scale and field-programmability often make reprogrammable technologies more flexible and inexpensive, so mask ROM is rarely used in new products as of 2007. PPROM A programmable read-only memory (PROM) or field programmable read-only memory (FPROM) or one-time programmable non-volatile memory (OTP NVM) is a form of digital memory where the setting of each bit is locked by a fuse or antifuse. Such PROMs are used to store programs permanently. The key difference from a strict ROM is that the programming is applied after the device is constructed. PROMs are manufactured blank and, depending on the technology, can be programmed at wafer, final test, or in system.

The availability of this technology allows companies to keep a supply of blank PROMs in stock, and program them at the last minute to avoid large volume commitment. These types of memories are frequently seen in video game consoles, mobile phones, radio-frequency identification (RFID) tags, implantable medical devices, high-definition multimedia interfaces (HDMI) and in many other consumer and automotive electronics products. •EPROM An EPROM, or erasable programmable read only memory, is a type of memory chip that retains its data when its power supply is switched off.

In other words, it is non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in digital circuits. Once programmed, an EPROM can be erased by exposing it to strong ultraviolet light from a mercury-vapor light source. EPROMs are easily recognizable by the transparent fused quartz window in the top of the package, through which the silicon chip is visible, and which permits exposure to UV light during erasing. •EEPROM

EEPROM (also written E2PROM and pronounced “e-e-prom,” “double-e prom” or simply “e-squared”) stands for Electrically Erasable Programmable Read-Only Memory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e. g. , calibration tables or device configuration. When larger amounts of static data are to be stored (such as in USB flash drives) a specific type of EEPROM such as flash memory is more economical than traditional EEPROM devices. EEPROMs are realized as arrays of floating-gate transistors. FLASH MEMORY Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It should not be confused with flash drives or pen drives which are USB storage device based on flash memory. It is primarily used in memory cards, USB flash drives, MP3 players and solid-state drives for general storage and transfer of data between computers and other digital products. It is a specific type of EEPROM (electrically erasable programmable read-only memory) that is erased and programmed in large blocks; in early flash the entire chip had to be erased at once.

Flash memory costs far less than byte-programmable EEPROM and therefore has become the dominant technology wherever a significant amount of non-volatile, solid state storage is needed. •FERAM Ferroelectric RAM (FeRAM or FRAM) is a random access memory similar in construction to DRAM but uses a ferroelectric layer instead of a dielectric layer to achieve non-volatility. FeRAM is one of a growing number of alternative non-volatile memory technologies that offer the same functionality as Flash memory. FeRAM advantages over Flash include: lower power usage, faster write performance and a much greater maximum number (exceeding 1016 for 3. V devices) of write-erase cycles. FeRAM disadvantages are: much lower storage densities than Flash devices, storage capacity limitations and higher cost. •MRAM Magnetoresistive Random Access Memory is a non-volatile computer memory (NVRAM) technology that has been under development since the 1990s. Continued increases in density of existing memory technologies – notably flash RAM and DRAM – kept it in a niche role in the market, but its proponents believe that the advantages are so overwhelming that magnetoresistive RAM will eventually become dominant for all types of memory, becoming a true universal memory. •PRAM

Phase-change memory (also known as PCM, PRAM, PCRAM, Ovonic Unified Memory, Chalcogenide RAM and C-RAM) is a type of non-volatile computer memory. PRAM uses the unique behavior of chalcogenide glass, which can be “switched” between two states, crystalline and amorphous, with the application of heat. Recent versions can achieve two additional distinct states, effectively doubling its storage capacity. PRAM is one of a number of new memory technologies that are attempting to compete in the non-volatile role with the almost universal Flash memory. The latter technology has a number of practical problems which these replacements hope to address.

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