Computer memory retains data that is to be processed by the CPU. Computer memory falls into two categories: nonvolatile memory, which is retained even when the computer is powered off, and volatile memory, data that is lost when the computer loses power.

Read-only memory (ROM): this is a type of nonvolatile memory that is preserved when the computer is powered off. When ROMs are manufactured a chip containing its contents is “burned in” and can’t be modified. These chips contain what’s known as “bootstrap” data and the power-on self test (POST) diagnostics the computer uses to start up prior to loading the operating system.

Random access memory (RAM): a type of volatile memory that is vulnerable to power being lost on the computer. When this occurs, all data held in RAM is lost. The CPU relies on RAM for readable and writable memory for processing tasks. RAM maintains its data when power is supplied to it. “Random” refers to the CPU being able to access or move data to and from any addressable RAM on the system.

Cache memory: This component contains a specialized, high-speed memory chip that keeps frequently accessed data located on or near the CPU. With faster speed than RAM, Cache memory is more expensive. The CPU operates in a perpetual mode of requesting and processing data, and executing code and relies on fast access to the data. This is why the memory chip is located close to the CPU for fast retrieval and execution of data. Caches are arranged in layers. The highest layer is known as the level 1 (L1) cache and is closest to the CPU. Modern computers can also have level 2 (L2) and level 3 (L3) cache memory.

Virtual memory: The operating system uses virtual memory for its management functions. Located on the hard disk drive, virtual memory can be utilized if the system’s running low on RAM. A swap file is an example of virtual memory. Accessing virtual memory requires a good amount of system overhead and can slow down processing.

Active memory: Data that is directly accessible to the CPU and held in memory is called active memory. Data that is held externally to the computer system’s active memory is held in storage or in secondary memory.

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