Quick Intro to Various Types of Memory

The word memory can be used to refer generally to anywhere we store data for later use, but it is also often used to refer to one of those types of memory in particular. Here is a quick introduction to the types of memory that come up in this class:

• Registers: a small number of memory locations incorporated in the central processing unit (CPU). This is the closest, fastest memory to the arithmetic logic unit (ALU).
• Memory, often also known as "main memory" or RAM for Random Access Memory: the memory being used by currently running processes. It is close to the CPU, but not as close as the registers. As a result, accessing data from main memory is much slower than accessing registers.
• Cache: a small-ish block of memory that sits between the registers and main memory. A cache is used to speed up data access from main memory.
• Secondary memory: where programs and files are stored when not currently being used. Secondary memory remains stable even when the computer is turned off. Examples include internal drives, USB drives, etc.
• Virtual memory: blocks of memory spanning the actual main memory and additional space in secondary memory. Virtual memory is useful when the currently running processes need more memory than what the real memory can actually hold -- in that case some of it is in the actual, physical main memory and some of it is temporarily stored in a special area of a hard drive or other type of secondary memory.