Computer systems typically employ a central processing unit (CPU), a display device, input devices, and memory for data storage. Memory devices are typically provided as internal storage areas in the computer. Memory for data storage generally comes in the form of integrated circuit chips. Memory for a computer system is technically any form of electronic, magnetic or optical storage. It is generally divided up into different categories based in part upon speed and functionality. In general, memory types can be categorized according to the storage state into volatile memory and non-volatile memory according to storage type. The primary difference between volatile memory and non-volatile memory is that a volatile memory needs to be supplied with external power in order to hold and refresh data while a non-volatile memory can maintain data for extended periods of time without any power being supplied to the device. Data intended for high-speed short-term access is typically stored in volatile memory. Data intended for long term future access is typically stored in non-volatile memory. Volatile memory currently has faster access times and higher data transfer rates than non-volatile memory. This makes it an appealing alternative for systems requiring very high-speed access to data typically stored in long term, non-volatile storage devices. Most computer systems utilize both volatile and non-volatile memory in the same system or device. In a typical computer system, data intended for high-speed short-term access, such as on-chip memory for the CPU, and often first and second level off-chip memory, are typically stored in volatile memory devices such as a cache or random access memory (RAM, DRAM, SRAM) which typically have nanosecond to microsecond access times. Data intended for long-term storage or mass storage are typically stored in non-volatile storage devices such as magnetic disks, hard disk drives, zip drives, floppy disk drives, tape drives and optical storage media which typically have access times on the order of milliseconds or seconds.

Random access memory (RAM) is the main memory of a computer system used for storing programs and data. RAM provides temporary read/write storage while hard disks offer semi-permanent storage. All programs must be run through RAM before they can be used. The term random derives from the fact that the CPU can retrieve data from any individual location, or address, within RAM. Most RAM is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM is lost. The volatile memory typically comprises random access memory (RAM) and is considered the main memory for the computer system. To facilitate quick access for processing, a typical modern computer has a main memory connected by a memory bus directly to the processor. Random access memory is much faster to read from and write to than the other kinds of storage devices in a computer such as the hard disk, floppy disk, and CD-ROM. In contrast to the relatively slow storage memory, the main memory is generally comprised of fast, expensive volatile random access memory (RAM) with access times generally less than 100 nanoseconds. Volatile random access memory (RAM) devices may be further divided into two categories, including static random access memory (SRAM) and dynamic random access memory (DRAM). Static random access memory (SRAM) consists of flip-flop latches, which each retain one bit of data for as long as power is maintained. In dynamic random access memory (DRAM), each memory cell is made up from one transistor and a capacitor. Random access memory is usually used to designate a data memory having a multiplicity of memory cells, each of which can store a datum and which can be accessed selectively and directly to selectively write in or read out data. Random access memories, such as static random access memories or dynamic random access memories, generally comprise a multiplicity of addresses for writing therein data. Data in the addresses may be accessed, for example, through data latches for performing operations, e.g., programming, on a memory cell array, e.g., a non-volatile memory cell array. As a computer system may be used in a variety of ways the amount of RAM deemed appropriate in one instance may be insufficient or superfluous in another. For example, an image processing and manipulation application may not only be time consuming to initialize for use, but also may require the majority of available main memory RAM resources, while a simple text editor may hardly be noticeable to the system.

Static random access memory (SRAM) devices have been employed for decades to store electronic data. An SRAM device includes an array of memory cells organized into rows and columns of memory cells. An SRAM cell includes a pair of inverters with the outputs of the inverters cross-coupled to form a flip-flop. The typical SRAM cell includes four transistors for storing data and two transistors for selection of a particular cell. An addressable word line is coupled to the memory cells in a distinct row of memory cells. The memory cells of an SRAM typically have first and second inverters whose inputs and outputs are connected to each other, and first and second transfer transistors that connect the output ends of the first and second inverters to a bit line pair. The first and second inverters include a load transistor and a driver transistor. The memory cells in a column of memory cells are coupled to an addressable pair of bit lines. Data is written to and read from a memory cell in the memory cell array by selecting a row of memory cells and accessing memory cells therein that are coupled to selected bit line pairs. Static random access memory cells typically provide memory storage for bits that can be rapidly read from and written to. Unlike dynamic random access memory (DRAM) cells, because of the flip-flop feedback effect, SRAM cells typically enable storage of static data even without refresh operations. The static random access memory is the main stream of the on-chip memories to be mounted on an LSI together with other parts. In spite of this, the SRAM, since it is composed of six transistors, requires a large space for disposing memory cells, encountering a problem of mounting space when it employed to be mounted on an LSI together with other parts. Due to its larger memory cell size, an SRAM is typically more expensive to manufacture than a DRAM. An SRAM typically has a smaller read access time and lower power consumption than a DRAM. Therefore, where fast access to data or low power is needed, SRAMs are often used to store the data.

A dynamic random access memory (DRAM) device is a typical volatile memory device constructed from an array of memory cells. Each memory cell comprises an active device and a capacitor. Furthermore, each memory cell is electrically connected to a word line (WL) and a bit line (BL). A dynamic random access memory includes a large number of memory cells, each of which can store at least one bit of data. In typical DRAM, the coupling of memory cells results in a differential voltage appearing on a bit line (or bit line pair). The differential voltage is amplified by a sense amplifier, resulting in amplified data signals on the bit lines. The applied memory address also activates column decoder circuits, which connect a given group of bit lines to input/output circuits. The memory cells are arranged in an array having a number of rows and columns. Memory cells within the same row are commonly coupled to a word line and memory cells within the same column are commonly coupled to a bit line. The memory cells within the array are accessed according to various memory device operations. Such operations include read operations, write operations and refresh operations. DRAM memory cell stores data by placing charge on, or removing charge from, a storage capacitor. According to the type of capacitor used in each memory cell, dynamic random access memory can be further sub-divided into a stack capacitor DRAM and a deep trench capacitor DRAM. Computer systems and other electronic devices containing a microprocessor or similar device typically include system memory, which is generally implemented using dynamic random access memory. The primary advantage of DRAM is that it uses relatively few components to store each bit of data, and is thus relatively inexpensive to provide relatively high capacity system memory. A disadvantage of DRAM is that their memory cells must be periodically refreshed. Pseudo static random access memory (PSRAM) is another type of DRAM. PSRAM is a low power DRAM having a static random access memory interface for wireless applications.

DRAM uses a main clock signal and a data strobe signal (DQS) for addressing the array of memory cells and for executing commands within the memory. The clock signal is used as a reference for the timing of commands such as read and writes operations, including address and control signals. The DQS signal is used as a reference to latch input data into the memory and output data into an external device. Dynamic random access memory uses an interface with address lines that are typically multiplexed in time. DRAM memory devices generally employ a row decoder circuit comprising a decoding unit and a wordline driver to drive a voltage level of a wordline high or low in order to "open" or "close" access to an associated row of memory cells. Such circuits operate to drive the wordline voltage level between a range of a positive voltage, which is greater than a maximum available power supply voltage and a negative voltage, which is less than a reference voltage, such as a ground reference. A synchronous DRAM (SDRAM) is a type of DRAM that can run at much higher clock speeds than conventional DRAM memory. A synchronous DRAM can perform various functions in synchronism with a clock signal that is supplied from an external source. The synchronous DRAM can refresh itself independently of the computer system which incorporates the synchronous DRAM, by generating refresh addresses inside of the memory using internal addresses. While the synchronous DRAM is being refreshed, word lines are selected to satisfy a refresh period required by the refresh characteristics of cells and a refresh rate required by the computer system. When the synchronous DRAM is in normal operation, a common output word line is provided for a plurality of banks in an interleaved configuration, and only one of the banks is selected at a time. The SDRAM has internal logic used to advance the data address. In addition to the timing signals, certain control registers of the internal logic of the SDRAM must be loaded with timing control parameters before the sequential access mode may be used.

Recent advances in memory technology have included the development of magnetic RAM (MRAM). The MRAM is a memory device for reading and writing information wherein multi-layer ferromagnetic thin films is used by sensing current variations according to a magnetization direction of the respective thin films. MRAM uses a ferromagnetic material as one of the next generation memory devices. The MRAM embodies a memory device by using a giant magneto resistive (GMR) or spin-polarized magneto-transmission (SPMT) phenomenon generated when the spin influences electron transmission. MRAM stores information magnetically, so it does not require a constant power supply. This quality is known as non-volatility. A magnetic random access memory (MRAM) element typically has a structure that includes first and second magnetic layers which are separated by a non-magnetic layer. A magnetic vector in one of the two magnetic layers is magnetically fixed or pinned, while the magnetic vector of the other of the two magnetic layers is not fixed and thus its magnetization direction is free to be controlled and switched. Information is written to and read from the element as a logic "1" or a logic "0" by changing the direction of the non-fixed magnetization vector in the other of the two magnetic layers. The differences in magnetization vector direction cause resistance variations within the element which can be measured. MRAM can offer all the advantages in speed and size that volatile memory offers and brings the added advantage of being non-volatile and, in some architectural configurations, cheaper to manufacture. MRAM can operate at speeds similar to either SRAM or DRAM, thus allowing it to be utilized within main memory. The MRAM has a high speed and low power consumption, and allows high integration density due to its unique properties of the magnetic thin film, and also performs a nonvolatile memory operation such as a flash memory.

A cache memory and a main memory are used for a large scale integration circuit having a central processing unit. Memory caching is a widespread technique used to improve data access speed in computers and other digital systems. The speed at which processors can execute instructions has typically outpaced the speed at which memory systems can supply the instructions and data to the processors. Due to this discrepancy in the operating speeds of the processors and system memory, the system memory architecture plays a major role in determining the actual performance of the system. Most current memory hierarchies utilize cache memory in an attempt to minimize memory access latencies. A cache is a small, fast memory that acts as a buffer between a device that uses a large amount of memory and a large, slower main memory. The cache's purpose is to reduce average memory-access time. Caches are effective because of two properties of software programs: spatial and temporal locality. Cache memory is used to provide faster access to frequently used instructions and data, which helps improve the overall performance of the system. Caching relies on a property of memory access known as temporal locality. Temporal locality states that information recently accessed from memory is likely to be accessed again soon. Information in cache RAM may be stored based upon two principles, namely spatial locality and temporal locality. The principle of spatial locality is based upon the fact that when data is accessed at an address, there is an above average likelihood that the data which is next required will have an address close to that of the data which has just been accessed. By contrast, temporal locality is based upon the fact that there is an above average probability that data which has just been accessed will be accessed again shortly. Cache memory is typically implemented using static random access memory (SRAM) because such memory need not be refreshed and is thus always accessible for a write or a read memory access.

Computers almost always contain a small amount of read-only memory (ROM) that holds instructions for starting up the computer. Read only memory (ROM) is a non-volatile memory commonly used in electronic equipment such as microprocessor-based digital electronic equipment and portable electronic devices. Read only memory is a type of non-volatile data storage device that can retain stored data even when the power is cut off. Among the memory products, non-volatile memory is one type of memory device having the capacity for writing data into, reading data from and erasing stored data multiples of times. Moreover, data will be retained even if the power to the device is cut off. With these advantages, it has become one of the most widely adopted memory devices in personal computer and electronic equipment. Most standard electrical products are equipped with some read only memory for holding a normal operation. ROM devices typically include multiple memory cell arrays. Each memory cell array may be visualized as including intersecting word lines and bit lines. Each word and bit line intersection can correspond to one bit of memory. In mask programmable metal oxide semiconductor (MOS) ROM devices, the presence or absence of a MOS transistor at word and bit line intersections distinguishes between a stored logic `0` and logic `1`. A ROM array of memory cells is defined by a number of transistors generally arranged in a grid pattern having a plurality of rows and columns. Each individual transistor of each memory cell of the ROM array is placed between a column of the series of columns and a voltage bus. The column is supplied with power at a first predetermined voltage level, and the voltage bus is supplied with power at a second, different predetermined voltage level. A gate of each transistor of a ROM array is connected to a particular row of the series of rows. ROM memories may be included in any type of integrated circuit (IC). In general, ROM memory is used to hold and make available data or code that will not be altered after IC manufacture. Data or code is programmed into ROM memory during fabrication.

According to data storage format, read only memory (ROM) can be further sub-divided into mask ROM, one-time programmable ROM (OTPROM), erasable programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM) and so on. The one-time electrically programmable read only memory (OTEPROM) permits the writing of data into the memory after leaving the factory. The data can be written by the user to fit a particular memory environment, which is more convenient to a user. Since data can be programmed into a one-time programmable ROM outside the factory according to the particular environment the memory is supposed to be working in, one-time programmable ROM is more convenient to work with than the mask ROM. A mask read only memory (Mask ROM) is a semiconductor memory device in which data required is coded during a manufacturing process. There are two types of Mask ROMs: an embedded diffusion-programmable ROM and an embedded metal programmable ROM. Mask ROM is able to write quaternary data into each memory cell transistor. A large amount of information can be stored in small circuits using this mask ROM. A programmable read only memory (PROM) is similar to the mask programmable ROM except that a user may store data values using a PROM programmer. A PROM device is typically manufactured with fusible links at all word and bit line intersections. An erasable programmable read only memory (EPROM) is programmable like a PROM, but can also be erased by exposing it to ultraviolet light. An EPROM integrated circuit is normally housed in a package having a quartz lid, and the EPROM is erased by exposing the EPROM integrated circuit to ultraviolet light passed through the quartz lid. Most PROMs can only be programmed once, typically by blowing open an appropriate word-to-bit connection path. Conversely, EPROMs can be programmed and reprogrammed multiple times. EPROMs are programmed by injecting hot electrons into, for example, a floating gate dielectrically spaced above the transistor channel. The injected electrons can thereafter be removed by irradiating the floating gate with ultraviolet light.

Electrically erasable programmable read-only memory (EEPROM) is a non-volatile memory device that allows multiple data writing, reading, and erasing operations. The structure of EEPROMs is similar to that of erasable programmable read-only memories (EPROMS) since both of them have a floating gate for storing charges and a control gate for controlling data access. EEPROM comprise a large number of memory cells having electrically isolated gates (floating gates). Data is stored in the memory cells in the form of charge on the floating gates. The electrical charge modifies the electrical characteristics of the EEPROM cell so that the information can be later read back using the modified electrical characteristics. The electrical charge is typically blocked in a trapping layer that gives to the EEPROM cell its memory capability. A typical EPROM device has a floating gate MOS transistor at all word and bit line intersections. Each MOS transistor has two gates: a floating gate and a non-floating gate. The floating gate is not electrically connected to any conductor, and is surrounded by a high impedance insulating material. The floating gates in the EEPROM device are surrounded by a much thinner insulating layer, and accumulated negative charges on the floating gates can be dissipated by applying a voltage having a polarity opposite that of the programming voltage to the non-floating gates. To program the EPROM device, a high voltage is applied to the non-floating gate at each bit location where a logic value is to be stored. This causes a breakdown in the insulating material and allows a negative charge to accumulate on the floating gate. When the high voltage is removed, the negative charge remains on the floating gate. An electrically erasable programmable read-only memory allows multiple data writing, reading and erasing operations. In addition, the stored data will be retained even after power to the device is removed. The EEPROM is very suitable to be used in an embedded function, such as an address book in cell phones, because of its byte program/erase feature. In addition, EEPROM products usually have good high reliability performance, which increases applicability in application fields requiring repetitive programming, reading, and erasing. With these advantages, it has been broadly applied in personal computer and other electronic equipment.

A flash memory is a type of flash EEPROM device that can be erased and reprogrammed in blocks instead of one byte at a time. Flash memory devices are different from EEPROM devices in that electrical erasure involves large sections of, or the entire contents of, a flash memory device. A flash memory cell includes a field effect transistor (FET) having a selection gate, a floating gate, a source and a drain. Data is stored in the flash memory cell by variations in the amount of charge stored in the floating gate, which causes a variation in a threshold voltage (Vt) of the flash memory cell. The data stored in the flash memory cell is read out by applying a selection voltage to a word line connected to the selection gate. The flash memory electrically deletes the data using a same method as that of an electrically erasable and programmable ROM (EEPROM), and the memory may be entirely deleted in one second or several seconds. The data stored in the flash memory is deleted throughout the chip in a block unit, but it is impossible to delete the data in a byte unit. The flash memory stores a correctable control program, which is used instead of an auxiliary memory. The flash memory is divided into a NAND flash memory and a NOR type flash memory. The NOR type flash memory uses an interface method as an SRAM or a ROM to easily construct a circuit with a processor. The NOR flash memory employs memory cell arrays that suppress the parasitic resistance. The NOR flash memory lowers the resistance by providing one through-hole to bit line for two cells connected in parallel. A NAND flash memory device is comprised of memory cells serially connected between a drain selection transistor and a source selection transistor in the unit of 16 or 32 in number. The flash memory cells of the NAND flash memory device include a current path formed between the source and drain on a semiconductor substrate, and a floating gate and a control gate that are connected over the semiconductor substrate with an insulator intervened between them. NAND flash memory devices are typically used as mass data storage devices, and NOR flash memory devices are typically used as information storage devices for high speed data processing.

Flash memory devices have achieved a commercial success in an electronic industry because they are able to store data for a relatively long time even without a power supply. Flash memory devices are applicable for multiple operations of data writing, reading and erasing, and have the advantage that stored data will not been vanished even after power supply is cut off. Thus, flash memory devices are widely used as non-volatile memory devices for personal computers and other electronic products. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include portable computers, personal digital assistants (PDA), digital cameras, and cellular phones. Program code, system data such as a basic input/output system (BIOS), and other firmware can typically be stored in flash memory devices. Most electronic devices are designed with a single flash memory device. Flash memory devices are finding increasing applications in smart cards for recording, storing and transporting digital information. Flash memory cards are currently used in digital cameras for recording and storing pictures that can be later displayed on personal computers, TVs or printed. Flash memories in smart cards are being used not only for storing data but also for storing application programs such as fingerprint identification, identification cards, health records, transportation programs and many more applications which include encryption for personal security, and also applications such as e-passport, credit card, JAVA card subscriber identity module (SIM).

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