Processor: Addressing Modes

Addressing modes define how the Central Processing Unit (CPU) accesses data in memory. Understanding different addressing modes is essential to grasp how a processor functions, and they can significantly affect a program’s performance. Here are some common addressing modes:

Immediate Mode

In Immediate Mode, the operand is part of the instruction itself.
The advantage of this mode is that no memory access is required to fetch the operand.
However, as the operand is part of the instruction, modifying it requires changing the program’s code.

Direct Mode

In Direct Mode, the memory address of the operand is specified directly as part of the instruction.
This mode is straightforward and efficient for accessing memory.
A potential issue can arise if a large memory is used as only limited addresses can be specified directly within the instruction.

Indirect Mode

In Indirect Mode, the address of the operand is given indirectly. The instruction contains a memory address that points to another memory location holding the effective address of the operand.
This allows for dynamic address computation and is helpful in implementing data structures like arrays and records.

Indexed Mode

Indexed Mode works by adding a constant value to a register to get the effective memory address.
This mode is typically used to index an array in memory, and it is also useful in executing loops.

Base Register Mode

In Base Register Mode, the effective address of the operand is obtained by adding an offset to the contents of a base register.
This mode is especially useful in dealing with data structures such as stacks and queues.

Relative Mode

In Relative Mode, the effective address of the operand is determined by adding a displacement to the program counter (PC).
This mode can be useful in branch instructions and in implementing loops and jumps.