Immediate Addressing

A high percentage of arithmetic operations involve one constant operand.Without special instructions, all of the constants would have to be loaded into memory locations before they could be used.MIPS provides a set of instructions that contain the constant operand inside the operation itself, using the I-type or immediate format.

I-type format

Using this instruction format, any constant that can be represented in 16 bits can be added to a register.Constants larger than 16 bits must be loaded into a register, using the lui instruction to load the high 16-bits and then the addi instruction to add the last 16 bits.

Register Addressing

Register addressing is used in arithmetic functions when the value for the operand is translated into the number of the register to access.

Base Addressing

Base or displacement addressing is used in data transfer functions.A byte-offset is added to a register to determine what value should be loaded or stored.

PC-relative Addressing

The I-type format is used by branch and transfer instructions, with the constant field being used as the low bits of the address where control should be transferred.Since all MIPS instructions are stored on word boundaries, there is no need to access individual bytes within an instruction when transferring control.Thus, the last 16 bits of a branch or transfer instruction are used as an offset, and are added to the program counter.This is called PC-relative addressing.

Since MIPS instructions are stored on word boundaries, there is no need to access individual bytes when transferring control.Thus the operand in branch, transfer, jump, and jump-and-link instructions is a word value.In the case of branch instructions, this allows them to reach new operations within ±2¹⁵ words of the current location, or four times as far as if byte-offsets were used.Branch instructions are usually found in if-then-else constructs and loops, and almost all of these constructs can fit in 2¹⁶ words.

Pseudodirect Addressing

Jump and jump-and-link instructions use a different instruction format, J-type.

The target address is absolute on jumps and jump-and-links, but is still word based.This means that the highest four bits of the PC are concatenated to the low 28 bits determined by the target address word value.This is called pseudodirect addressing.

References:

Hennesey, John L. and David A. Patterson (1998). Computer Organization and Design: the Hardware/Software Interface (2^nd ed.). San Francisco: Morgan Kaufmann.