Memory refresh

Memory refresh is the process of periodically reading information from an area of computer memory and immediately rewriting the read information to the same area without modification, for the purpose of preserving the information. Memory refresh is a background maintenance process required during the operation of semiconductor dynamic random-access memory (DRAM), the most widely used type of computer memory, and in fact is the defining characteristic of this class of memory.

In a DRAM chip, each bit of memory data is stored as the presence or absence of an electric charge on a small capacitor on the chip. As time passes, the charges in the memory cells leak away, so without being refreshed the stored data would eventually be lost. To prevent this, external circuitry periodically reads each cell and rewrites it, restoring the charge on the capacitor to its original level. Each memory refresh cycle refreshes a succeeding area of memory cells, thus repeatedly refreshing all the cells in a consecutive cycle. This process is conducted automatically, in the background, by the memory circuitry, while the computer is on, and is transparent to the user. While a refresh cycle is occurring the memory is not available for normal read and write operations, but in modern memory this "overhead" time is not large enough to significantly slow down memory operation.

Electronic memory that does not require refreshing is available, called static random-access memory (SRAM). SRAM circuits require more area, because an SRAM memory cell requires four to six transistors, compared to a single transistor and a capacitor for DRAM. As a result, data density is much lower in SRAM than in DRAM, and SRAM has higher price per bit. Therefore, DRAM is used for the main memory in computers, video game consoles, graphics cards and applications requiring large capacities and low cost. The need for memory refresh makes DRAM timing significantly more complicated than SRAM circuits, but the density and cost advantages of DRAM justify this complexity.

While the memory is operating, each memory cell must be refreshed repetitively, within the maximum interval between refreshes specified by the manufacturer, which is usually in the millisecond region. Refreshing does not employ the normal memory operations (read and write cycles) used to access data, but specialized cycles called refresh cycles which are generated by separate counter circuits in the memory circuitry and interspersed between normal memory accesses.

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