Välismälu

Välismälu, often translated as external memory or secondary storage, refers to non-volatile computer storage that is not directly accessible by the CPU. Unlike primary memory (RAM) which is volatile and used for active processing, välismälu retains data even when the power is turned off. This makes it crucial for storing the operating system, applications, and user data.

Common examples of välismälu include hard disk drives (HDDs), solid-state drives (SSDs), USB flash drives, memory cards, and optical discs like CDs and DVDs. These devices store information magnetically, electronically, or optically.

The primary advantage of välismälu is its capacity and persistence. It can store significantly more data than RAM and holds that data permanently. However, accessing data from välismälu is generally slower than accessing it from RAM due to the physical or electronic mechanisms involved. This speed difference is why computers use a hierarchy of memory, with faster, smaller primary memory working in conjunction with slower, larger välismälu.

When a program or file is needed, it is loaded from välismälu into RAM for the CPU to process. Once the task is complete, or when changes are made, the data is often saved back to välismälu to ensure its preservation. The choice of välismälu technology depends on factors such as cost, speed, capacity, and durability requirements.