Addressing modes specify how the data/arguments for an instruction are retrieved/stored.
Addressing modes may include:
- Implicit - Any arguments are implied by the instruction (e.g., a return-from-subroutine instruction, which will pop a value off the stack or retrieve it from a Link Register and place it in the program counter - no additional arguments are required to complete the operation).
- Register - Data is read from (or written to) a register.
- Absolute - An exact memory address is specified.
- Indirect - A memory address is specified, which contains the address of the memory for reading/writing. Effectively, the argument in this addressing mode is the address of a pointer to the actual data.
- Base plus offset - An absolute or indirect memory address is specified as a base address along with a register (or immediate value) that contains an offset value. The base and offset values are added together, and the resulting address is used to read/write data. In some cases, the computed address may be written back to a register, so that as you iterate through a loop, successive data locations are accessed.
- Relative - A signed or unsigned offset is added to the current program counter to obtain a new address. This is most often used for jumps but is also used for some kinds of data access. Code that uses relative addresses can be easily relocated in memory and is therefore sometimes called position-independent code (PIC).
- Stack - Values are pushed to or popped from the stack.
(There are many other modes available on some architectures).
The addressing modes available, and the valid combinations of operations and addressing modes, are significant features of an Instruction Set Architecture.