Addressing modes are the method used to determine which part of memory is being referred to by a machine instruction. There are various types of addressing modes. Which addressing mode is used is dependent on what type of computer architecture is being used.

Random access memory (RAM) is the primary area of memory for a computer. This is where any application must be loaded to if it is to be run. The central processing unit (CPU) reads machine instructions from the RAM and acts on those instructions. This is what happens whenever any application is run on a computer.

The machine instructions given to the CPU often must refer to specific portions of the RAM. In order to do this, the CPU must have a way of knowing which portion of RAM the machine instruction is referring to. This is where addressing modes come into play.

Addressing modes are used to divide up the sections of RAM into individual portions that may be referenced individually. This is similar to how each house has an address. This address can then be used by a machine instruction to refer to a specific portion of memory. The CPU will then access that portion of memory and perform the action specified by the machine instruction.

There are many different types of addressing modes. Different types of computer architecture feature different types of addressing modes. This results in an incompatibility of software. If an application is designed for one type of addressing mode, then it will not be able to run when used on a system that uses a different type of addressing mode. It will be much like speaking to someone in a language he does not understand.

The specifics of each type of addressing mode are important for computer programmers using assembly language. This type of computer language is a direct representation of the machine instructions sent to the CPU. This is what makes assembly language able to produce programs that can run several times faster than other programming languages.

Assembly language is used in the development of operating systems. A computer programmer must know the type of addressing modes used on the specific computer architecture before he can write a functioning operating system or application in assembly. The differences between addressing modes are part of the reason that applications are unable to run on different computer architectures.