Non-volatile random-access memory

Non-volatile random-access memory (NVRAM) is random-access memory that retains data without applied power. This is in contrast to dynamic random-access memory (DRAM) and static random-access memory (SRAM), which both maintain data only for as long as power is applied, or forms of sequential-access memory such as magnetic tape, which cannot be randomly accessed but which retains data indefinitely without electric power. Read-only memory devices can be used to store system firmware in embedded systems such as an automotive ignition system control or home appliance. They are also used to hold the initial processor instructions required to bootstrap a computer system. Read-write memory can be used to store calibration constants, passwords, or setup information, and may be integrated into a microcontroller. If the main memory of a computer system were non-volatile, it would greatly reduce the time required to start a system after a power interruption. Current existing types of semiconductor non-volatile memory have limitations in memory size, power consumption, or operating life that make them impractical for main memory. Development is going on for the use of non-volatile memory chips as a system's main memory, as persistent memory. A standard for persistent memory known as NVDIMM-P has been published in 2021. Early computers used core and drum memory systems which were non-volatile as a byproduct of their construction. The most common form of memory through the 1960s was magnetic-core memory, which stored data in the polarity of small magnets. Since the magnets held their state even with the power removed, core memory was also non-volatile. Other memory types required constant power to retain data, such as vacuum tube or solid-state flip-flops, Williams tubes, and semiconductor memory (static or dynamic RAM). Advances in semiconductor fabrication in the 1970s led to a new generation of solid state memories that magnetic-core memory could not match on cost or density. Today dynamic RAM forms the vast majority of a typical computer's main memory.