Primary storage is a category of computer storage, often called main memory. Confusingly, the term primary storage has recently been used in a few contexts to refer to online storage (hard disk), which is usually classified as secondary storage.

Primary storage is used to store data that is likely to be in active use, so it is usually faster than long-term secondary storage. Today, many computers have cache memory located in between the central processing unit and primary storage in order to further increase speed.

A particular location in storage is selected by its physical memory address. That address remains the same, no matter how the particular value stored there changes.

Over the history of computing, a variety of technologies have been used for primary storage. Today, we are most familiar with random access memory (RAM) made out of many small integrated circuits. Some early computers used mercury delay lines, in which a series of acoustic pulses were sent along a tube filled with mercury. When the pulse reached the end of the tube, the circuitry detected whether the pulse represented a binary 1 or 0 and caused the oscillator at the beginning of the line to repeat the pulse. Other early computers stored RAM on high-speed magnetic drums.

Modern primary storage devices include:

• Random access memory (RAM) - includes VRAM, WRAM, NVRAM
• Read-only memory (ROM)

Before the use of integrated circuits for memory became widespread, primary storage was implemented in many different forms:

• Williams tube
• Delay line memory
• Drum memory
• Core memory
• Twistor memory
• Bubble memory

See also:

• Secondary storage