(computer science) A ranking of computer memory devices, with devices having the fastest access time at the top of the hierarchy, and devices with slower access times but larger capacity and lower cost at lower levels.

Computer Desktop Encyclopedia: memory hierarchy

The levels of memory in a computer. From fastest to slowest speed, they are:

1. CPU registers

2. L1 cache

3. L2 cache

4. Main memory

5. Virtual memory

6. Disk

A memory hierarchy in computer storage distinguishes each level in the 'hierarchy' by response time. Since response time, complexity, and capacity are related[1], the levels may also be distinguished by the controlling technology. There are four major storage levels.[1]
Internal – Processor registers and cache.
Main – the system RAM and controller cards.
On-line mass storage – Secondary storage.
Off-line bulk storage – Tertiary and Off-line storage.

This is a most general memory hierarchy structuring. Many other structures are useful. For example, a paging algorithm may be considered as a level for virtual memory when designing a computer architecture.

The memory hierarchy in most computers is:
Processor registers – fastest possible access (usually 1 CPU cycle), only hundreds of bytes in size
Level 1 (L1) cache – often accessed in just a few cycles, usually tens of kilobytes
Level 2 (L2) cache – higher latency than L1 by 2× to 10×, often 512 KiB or more
Level 3 (L3) cache – higher latency than L2, often 2048 KiB or more
Main memory – may take hundreds of cycles, but can be multiple gigabytes. Access times may not be uniform, in the case of a NUMA machine.
Disk storage – millions of cycles latency if not cached, but very large
Tertiary storage – several seconds latency, can be huge

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Answered by naziakhan85 , an ibibo Citizen, at 11:17 AM on November 02, 2009