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In computing, memory refers to a device that is used to store information for immediate use in a computer or related computer hardware device.[1] It typically refers to semiconductor memory, specifically metal–oxide–semiconductor (MOS) memory,[2][3] where data is stored within MOS memory cells on a silicon integrated circuit chip. The term "memory" is often synonymous with the term "primary storage". Computer memory operates at a high speed, for example random-access memory (RAM), as a distinction from storage that provides slow-to-access information but offers higher capacities. If needed, contents of the computer memory can be transferred to secondary storage;[a] a very common way of doing this is through a memory management technique called virtual memory. An archaic synonym for memory is store.[4]

The term "memory", meaning "primary storage" or "main memory", is often associated with addressable semiconductor memory, i.e. integrated circuits consisting of silicon-based MOS transistors,[5] used for example as primary storage but also other purposes in computers and other digital electronic devices. There are two main kinds of semiconductor memory, volatile and non-volatile. Examples of non-volatile memory are flash memory (used as secondary storage) and ROM, PROM, EPROM and EEPROM memory (used for storing firmware such as BIOS). Examples of volatile memory are primary storage, which is typically dynamic random-access memory (DRAM), and fast CPU cache memory, which is typically static random-access memory (SRAM) that is fast but energy-consuming, offering lower memory areal density than DRAM.

Most semiconductor memory is organized into memory cells or bistable flip-flops, each storing one bit (0 or 1). Flash memory organization includes both one bit per memory cell and multiple bits per cell (called MLC, Multiple Level Cell). The memory cells are grouped into words of fixed word length, for example 1, 2, 4, 8, 16, 32, 64 or 128 bit. Each word can be accessed by a binary address of N bit, making it possible to store 2 raised by N words in the memory. This implies that processor registers normally are not considered as memory, since they only store one word and do not include an addressing mechanism.

Virtual memory is a system where all physical memory is controlled by the operating system. When a program needs memory, it requests it from the operating system. The operating system then decides in what physical location to place the program's code and data.

This offers several advantages. Computer programmers no longer need to worry about where their data is physically stored or whether the user's computer will have enough memory. It also allows multiple types of memory to be used. For example, some data can be stored in physical RAM chips while other data is stored on a hard drive (e.g. in a swapfile), functioning as an extension of the cache hierarchy. This drastically increases t

This approach has its pitfalls. If the location specified is incorrect, this will cause the computer to write the data to some other part of the program. The results of an error like this are unpredictable. In some cases, the incorrect data might overwrite memory used by the operating system. Computer crackers can take advantage of this to create viruses and malware.

Virtual memory is a system where all physical memory is controlled by the operating system. When a program needs memory, it requests it from the operating system. The operating system then decides in what physical location to place the program's code and data.

This offers several advantages. Computer programmers no longer need to worry about where their data is physically stored or whether the user's computer will have enough memory. It also allows multiple types of memory to be used. For example, some data can be stored in physical RAM chips while other data is stored on a hard drive (e.g. in a This offers several advantages. Computer programmers no longer need to worry about where their data is physically stored or whether the user's computer will have enough memory. It also allows multiple types of memory to be used. For example, some data can be stored in physical RAM chips while other data is stored on a hard drive (e.g. in a swapfile), functioning as an extension of the cache hierarchy. This drastically increases the amount of memory available to programs. The operating system will place actively used data in physical RAM, which is much faster than hard disks. When the amount of RAM is not sufficient to run all the current programs, it can result in a situation where the computer spends more time moving data from RAM to disk and back than it does accomplishing tasks; this is known as thrashing.

Protected memory is a system where each program is given an area of memory to use and is not permitted to go outside that range. Use of protected memory greatly enhances both the reliability and security of a computer system.

Without protected memory, it is possible that a bug in one program will alter the memory used by another program. This will cause that other program to run off of corrupted memory with unpredictable results. If the operating system's memory is corrupted, the entire computer system may crash and need to be rebooted. At times programs intentionally alter the memory used by other programs. This is done

Without protected memory, it is possible that a bug in one program will alter the memory used by another program. This will cause that other program to run off of corrupted memory with unpredictable results. If the operating system's memory is corrupted, the entire computer system may crash and need to be rebooted. At times programs intentionally alter the memory used by other programs. This is done by viruses and malware to take over computers. It may also be used benignly by desirable programs which are intended to modify other programs; in the modern age, this is generally considered bad programming practice for application programs, but it may be used by system development tools such as debuggers, for example to insert breakpoints or hooks.

Protected memory assigns programs their own areas of memory. If the operating system detects that a program has tried to alter memory that does not belong to it, the program is terminated (or otherwise restricted or redirected). This way, only the offending program crashes, and other programs are not affected by the misbehavior (whether accidental or intentional).

Protected memory systems almost always include virtual memory as well.