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Computer Memory Computer Memory Computer memory is an internal storage area in the computer, in which data and programs are stored either temporarily or permanently. The two types of memories are 1. Primary memory 2. Secondary memory. 1. Primary memory: Primary memory is also known as main memory or internal memory. Primary memory directly holds instructions and data, when a program is executed. Primary memory is the only type of memory, that is directly accessed by CPU. It is volatile in nature. Volatile means, the data and instructions will erase when you power off the computer. Primary memory is expensive and limited storage capacity. Primary memory is non-portable and its data access is high. Primary memory are again two types. They are i). RAM (Random access memory) ii). ROM (Read only memory). i). Random access memory: Random access memory is a volatile storage area within the computer that is typically used to store data temporarily, so that it can be promptly accessed by the processor. The information stored in the RAM is basically loaded from the computer hard disk and includes data related to the operating system and application that are currently being executed by the processor. RAM is considered random access because any memory cell can be directly accessed if its address is known. Memory usually refers to random access memory (RAM) typically DRAM. a). Dynamic Ram (DRAM): DRAM is a common type of memory used in PC, work stations, servers. DRAM must be refreshed in a second to hold its data contents. DRAM chip contains millions of memory cells. Each cell is made up of transistors and capacitors and contains a bit of information. It is cheap and can hold 4 times more data than SRAM. b). Static RAM (SRAM): SRAM holds data without an external refresh as long as it is powered. SRAM is made up of D flip-flops, in which memory cells flip-flip between 0 and 1 without use of capacitors. SRAM occupies more space and expensive. SRAM is faster and more reliable. SRAM chips are used in cars, house hold appliances and hand-held electronic devices. ii). Read only memory: read only memory refers computer memory chips containing permanent or semi-permanent data which is nonvolatile. Most computers contain a small amount of ROM that contains critical programs such as the Basic Input Output System (BIOS) which is used to boot up the computer when it is turned on. ROMs are read only. To update the programs stored in them, the ROM chip had to removed and physically replaced by another that had new versions of the program. But in today ROM chips updates are possible. Rewritable ROM chips are available. Rewritable ROM chips include PROM (programmable read only memory) EPROM (erasable programable read only memory) EEPROM (electrically erasable programable read only memory) PROM: PROM is a memory chip on which data can be written only once. It is also called one-time programmable PROM and can be written or programmed using a special device called a PROM programmer. EPROM: EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light. Once it is erased, it can be reprogrammed. EEPROM: EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge. It is based on a semiconductor structure similar to the EPROM, but allows the entire or selected contents to be electrically erased and rewritten electrically, so that they need not be removed from the computer. The process of writing an EEPROM is also known as Flashing. 2. Secondary Memory: Secondary memory is also known as Auxillary memory or external memory. Secondary memory holds data and programs, that are not currently in use and provides long term storage. Secondary memory is non-volatile in nature. The data and instructions do not erase, when the power is switch off, it is cheap and can store voluminous data. Secondary memory is highly portable. In secondary memory data access is low when compared to primary memory. 8888 .
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