Memory device which supports such access is called a Direct Access Memory. Direct Access Memoryĭirect access memory or Random Access Memory, refers to conditions in which a system can go directly to the information that the user wants.
#ACCESS INTERNAL MEMORY NOTE 3 DEVICE OFF SERIAL#
Magnetic tape is an example of serial access memory. Memory device which supports such access is called a Sequential Access Memory or Serial Access Memory. Sequential access means the system must search the storage device from the beginning of the memory address until it finds the required piece of data. Hence, the process of re-programming is flexible but slow. EEPROMs can be erased one byte at a time, rather than erasing the entire chip. In EEPROM, any location can be selectively erased and programmed. Both erasing and programming take about 4 to 10 ms (millisecond). It can be erased and reprogrammed about ten thousand times. The EEPROM is programmed and erased electrically. EEPROM (Electrically Erasable and Programmable Read Only Memory) During normal use the quartz lid is sealed with a sticker. This exposure to ultra-violet light dissipates the charge. For erasing this charge, ultra-violet light is passed through a quartz crystal window (lid). The charge is retained for more than ten years because the charge has no leakage path. During programming an electrical charge is trapped in an insulated gate region. Usually, an EPROM eraser achieves this function. The EPROM can be erased by exposing it to ultra-violet light for a duration of upto 40 minutes. EPROM (Erasable and Programmable Read Only Memory) It can be programmed only once and is not erasable. Inside the PROM chip there are small fuses which are burnt open during programming. The user buys a blank PROM and enters the desired contents using a PROM programmer. PROM is read-only memory that can be modified only once by a user. These kind of ROMs are known as masked ROMs. The very first ROMs were hard-wired devices that contained a pre-programmed set of data or instructions. ROM chip are not only used in the computer but also in other electronic items like washing machine and microwave oven.įollowing are the various types of ROM − MROM (Masked ROM) The information is stored permanently in such memories during manufacture.Ī ROM, stores such instruction as are required to start computer when electricity is first turned on, this operation is referred to as bootstrap. The memory from which we can only read but cannot write on it. These cells are composed of one capacitor and one transistor. DRAM is used for most system memory because it is cheap and small. This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second. Dynamic RAM (DRAM)ĭRAM, unlike SRAM, must be continually refreshed in order for it to maintain the data. Static RAM is used as cache memory needs to be very fast and small. Transistors do not require power to prevent leakage, so SRAM need not have to be refreshed on a regular basis.īecause of the extra space in the matrix, SRAM uses more chips than DRAM for the same amount of storage space, thus making the manufacturing costs higher. SRAM chips use a matrix of 6-transistors and no capacitors. However, data is lost when the power gets down due to volatile nature. The word static indicates that the memory retains its contents as long as power remains applied. RAM is small, both in terms of its physical size and in the amount of data it can hold. Hence, a backup uninterruptible power system (UPS) is often used with computers. data stored in it is lost when we switch off the computer or if there is a power failure. We can reach into the memory at random & extremely fast but can also be quite expensive. Since access time in RAM is independent of the address to the word that is, each storage location inside the memory is as easy to reach as other location & takes the same amount of time. Frequency of access of the memory by the CPU decreases.Ī RAM constitutes the internal memory of the CPU for storing data, program and program result.Capacity in terms of storage increases.Internal Memory − cache memory and primary/main memoryĮxternal Memory − magnetic disk / optical disk etc.Ĭharacteristics of Memory Hierarchy are following when we go from top to bottom. The address of these locations varies from 0 to 65535. Each location or cell has a unique address which varies from zero to memory size minus one.įor example if computer has 64k words, then this memory unit has 64 * 1024 = 65536 memory location. The memory is divided into large number of small parts. Computer memory is the storage space in computer where data is to be processed and instructions required for processing are stored. It is used to store data and instruction.
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