Memory - Hierarchy

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A memory/storage hierarchy in computer storage distinguishes each level in the hierarchy by:

Hierachy Storage

The transfer of memory from primary storage to secondary storage is done through virtual memory.

Storage levels

There are three major storage levels


Primary Storage:

  • the main memory system RAM and controller cards – may take hundreds of cycles, but can be multiple gigabytes. Access times may not be uniform, in the case of a NUMA machine.


Secondary storage and On-line mass storage principally Disk storage – millions of cycles latency if not cached, but very large


Tertiary and Off-line bulk storage - several seconds latency, can be huge because it happens via a network connection.


Internal / External

The internal or external type of memory is a grouping by location of the memory.

When the memory is internal, it's on the motherboard, otherwise, it's external.

There is only one internal memory and it's the primary (main/internal).

Level Type
Primary Internal
Secondary External
Tertiary External

Main / Auxiliary

Historically, memory and storage were respectively called main memory and auxiliary storage.

Auxiliary storage (or auxiliary memory units) is used to represent memory which are not directly accessible by the CPU (ie secondary or tertiary storage).

Volatile / Non-volatile

Memory can also be categorized by their volatility.

Documentation / Reference

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