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Primary storage, or internal memory, is computer memory that is accessible to the central processing unit of a computer without the use of computer's input/output channels. Primary storage is used to store data that is likely to be in active use. Primary storage is typically very fast, as in the case of RAM. RAM is also volatile, losing the stored information in an event of power loss, and quite expensive. ROM is not volatile, but is not suited to storage of large quantities of data because it is expensive to produce. Typically, ROM must also be completely erased before it can be rewritten, making large scale use impractical, if not impossible. Therefore, separate secondary storage, or external memory, is usually required for long-term persistent storage.

Confusingly, the term primary storage has recently been used in a few contexts to refer to online storage (hard disks), which is usually classified as secondary storage.

Primary storage may include several types of storage, such as main storage, cache memory, and special registers, all of which can be directly accessed by the processor. Primary storage can be accessed randomly, that is, accessing any location in storage at any moment takes the same amount of time. 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.

Today, primary storage is typically random access memory, a type of semiconductor memory. Over the history of computing hardware, a variety of technologies have been used for primary storage. 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 primary memory on rapidly rotating 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:

• a drum holding capacitors (used in Atanasoff-Berry Computer)
• Williams tube
• Delay line memory
• Drum memory
• Core memory
• Twistor memory
• Bubble memory

• Federal Standard 1037C