The memory is mainly used to store programs and memory components of various data information. Memory can be divided into two categories: main memory (referred to as main memory or memory) and auxiliary memory (referred to as auxiliary memory or external memory). It is main memory that exchanges information directly with the CPU.

A memory is a collection of many storage cells, arranged in cell number order. Each unit is composed of several binary bits to represent the value stored in the storage unit. This structure is very similar to the structure of an array, so in VHDL language, an array is usually used to describe the memory.

The memory of a computer can be divided into internal memory and external memory. Internal memory is frequently used by the computer during program execution and is directly accessible during an instruction cycle. External storage requires the computer to read information from an external storage device such as tape or disk. This is similar to students taking notes in class. If the student knows the content without looking at the notes, the information is stored in "internal memory". If the student has to consult the notes, the information is in "external memory".