Describe auto increment and auto decrement addressing modes with proper example ?

Auto-increment and auto-decrement addressing modes are used in computer architectures to simplify the process of accessing elements in memory sequentially.

Auto-increment mode:

In auto-increment mode, the effective address (EA) of the operand is determined by the content of a register specified in the instruction. After accessing the operand, the content of this register is automatically incremented to point to the next item in memory.

Example:

Add (R2)+, R0

Here’s what happens:

1. The content of register R2 is used as the effective address (EA) for the operand.
2. After accessing the operand, the content of register R2 is automatically incremented to point to the next item in memory.

Auto-decrement mode:

In auto-decrement mode, the content of a register specified in the instruction is decremented, and then this decremented value is used as the effective address of the operand.

Example:

Add -(R2), R0

Here’s what happens:

1. The content of register R2 is decremented.
2. The decremented value of R2 is then used as the effective address (EA) for the operand.
3. The operand is accessed using this effective address, and the result is added to the content of register R0.

Simplified Explanation:

• Auto-increment mode: Think of it like stepping forward on a staircase. You start at a certain step, use it, and automatically move to the next step.
• Auto-decrement mode: Picture stepping backward on a staircase. You start at a step, move back one step, and use that step as your current position.

These modes are particularly useful when dealing with arrays or lists in memory where you need to access consecutive elements efficiently. They automate the process of updating the memory address after each access.