(638) L-2.10: Base Register Addressing Mode || Computer Organisation and Architecture

Base Register Addressing Mode

• Purpose: Used for Program Relocation

  • Allows multiple programs to utilize limited RAM size.

  • Enables programs to be swapped in and out of memory:

    • Swap out running processes (e.g., P1) when RAM is full.

    • Bring back programs later possibly at a different memory location.

    • Relocate programs to different addresses as necessary.

Key Concepts

• Program Relocation:

  • Involves changing the memory address at which a program is loaded.

  • Programs may not always return to their original address after being swapped out.

• Base Register:

  • Holds the base address for the current program in memory.

  • Essential for calculating the effective address of instructions.

Memory Example

• When a program (e.g., P1) occupies memory from 100 to 199, a branch instruction (e.g., Branch 160) indicates to go to address 160.

• Issue: If P1 is relocated (e.g., moved to 400-499) and the instruction still references the absolute address (160), it may cause illegal access errors.

Effective Address Calculation

• To avoid confusion with absolute addressing, the program uses a displacement (offset).

  • Example: Original addresses -> P1 from 100 to 199 with a required jump to 160.

  • Relocation: If P1 is moved to 400-499, instead of Jump 160, use Jump 60:

    • Calculated as Base Address (400) + Displacement (60) = Effective Address (460).

Summary of Base Register Addressing

• Instead of using absolute addresses, which change with relocation, use offsets for addressing.

• The effective address calculation facilitates program relocation without the need for adjusting all instructions with absolute addresses.

• Allows program flexibility and efficient memory management.