what is memory

Computers process information stored in their memory, which consists of data storage units. Storage devices such as CD and DVD drives are called the external or auxiliary storage units, whereas the principal memory devices directly accessible from computers are called the internal or main storage units, which rely on semiconductor memory chips.
There are mainly two types of semiconductor memory: random-access memory (RAM) and read-only memory (ROM). RAM is a temporary data storage domain, whereas ROM serves as a semi-permanent storage domain. If RAM is likened to notebooks or memo pads, then ROM is comparable to dictionaries and textbooks.

Since random-access memory (RAM) is principally used as temporary storage for the operating system and the applications, it does not much matter that some types of RAM lose data when they are powered off. What matters more is the cost and the read/write speed. There are mainly two types of RAM: one is DRAM (dynamic RAM), and the other is SRAM (static RAM). DRAM stores information in capacitors, and since the capacitors slowly discharge, the information fades away unless the capacitor charge is refreshed periodically. In practice, the data on DRAM need to be read and rewritten (i.e., refreshed) dozens of times per second. In contrast, SRAM needs no refreshing because it uses flip-flop circuits* to preserve the data. SRAM is more expensive than DRAM because of the complex circuitry involved, but is also faster.
Flip-flop circuit: An electronic circuit that stores a single bit of data that represents either 0 or 1.

Read-only memory (ROM) is used for retrieving stored data that are permanently fixed and cannot be rewritten. Many home appliances such as washing machines and rice cookers use ROM devices to store pre-set programs.
ROM is non-volatile memory, meaning that the data stored on ROM are not lost even when the power is shut off. ROM is designed specifically for reading data. It may be possible to erase or write data on ROM, but it takes an inordinately long time to do so. To correct this shortcoming, new kinds of devices have emerged in recent years that serve as a cross between ROM and RAM, including flash memory and EPROM.
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