Primary storage

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, in the
case of RAM which is also volatile, losing the stored information in an event of power loss, and
quite expensive. ROM is not volatile, but 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.