RAM, ROM and memory types

The memory inside a computer falls into broad categories defined by whether it can be written freely and whether it keeps its contents when power is switched off. RAM (random access memory) is the read-write working memory in which a running program and its data reside; it is volatile, meaning its contents vanish when the machine is turned off. ROM (read-only memory) holds fixed information that survives without power, such as the firmware that starts the computer. The name “random access” reflects that any location can be reached directly, without reading through everything before it.

RAM itself comes in two main forms. Static RAM (SRAM) stores each bit in a small circuit of transistors that holds its state as long as power is applied, making it fast but expensive and less dense, so it is used for small, speed-critical memories such as caches. Dynamic RAM (DRAM) stores each bit as a charge on a capacitor; this is cheaper and far denser, allowing large capacities, but the charge leaks and must be refreshed periodically, which is why DRAM serves as the bulk of a system's main memory.

Between fully fixed and freely writable memory lie reprogrammable types. An EPROM (erasable programmable read-only memory) can be erased, traditionally by exposure to ultraviolet light, and then reprogrammed, while later electrically erasable and flash memories made rewriting more convenient. Supporting chips on the motherboard, including the chipset and a small battery-backed CMOS memory that stores settings, work alongside the main memory, and faster variants such as the DDR family of DRAM have steadily raised the speed of system memory.

Frequently asked questions

What is the difference between SRAM and DRAM?

SRAM stores bits in transistor circuits that hold their state while powered, making it fast but costly and used for caches, while DRAM stores bits as capacitor charge that must be refreshed, making it cheaper and denser for main memory.

What is an EPROM?

An EPROM is an erasable programmable read-only memory that can be erased, traditionally with ultraviolet light, and reprogrammed, sitting between fixed ROM and freely writable RAM.

Why is RAM called volatile?

RAM is volatile because it keeps its contents only while powered; when the computer is turned off, the data it holds is lost, unlike ROM which retains its contents.