Memory Hierarchy — HIGH PRIORITY

Memory hierarchy

Organization of storage by speed, cost, capacity, and proximity to the CPU.

Memory hierarchy order

Registers → Cache → Main memory/RAM → Secondary storage.

Registers

Fastest storage in the memory hierarchy and located inside the CPU.

Cache

Small, fast memory located close to the CPU that stores frequently needed data and instructions.

Main memory

RAM used to hold programs and data currently being used by the computer.

Secondary storage

Long-term nonvolatile storage such as SSDs and hard drives.

Volatile memory

Memory that loses its contents when power is removed.

Nonvolatile memory

Memory that retains its contents when power is removed.

RAM

Volatile main memory used for actively running programs and data.

Cache hit

Requested data is found in cache.

Cache miss

Requested data is not found in cache and must be retrieved from a slower memory level.

Hit rate

Percentage of memory accesses that are successfully found in cache.

Miss rate

Percentage of memory accesses that are not found in cache.

Hit time

Time required to access data when it is found in cache.

Miss penalty

Extra time required to retrieve data after a cache miss.

Temporal locality

Recently accessed data is likely to be accessed again soon.

Spatial locality

Data located near recently accessed data is likely to be accessed soon.

Locality

Principle that programs tend to reuse recently accessed data or access nearby memory location