Difference between PC relative and Base register Addressing Modes

Prerequisite – Addressing Modes

1. PC relative addressing mode:
PC relative addressing mode is used to implement intra segment transfer of control, In this mode effective address is obtained by adding displacement to PC.

EA = PC + Address field value
PC = PC + Relative value 

2. Base register addressing mode:
Base register addressing mode is used to implement inter segment transfer of control. In this mode effective address is obtained by adding base register value to address field value.

EA = Base register + Address field value
PC = Base register + Relative value 



Difference between PC Relative And Base Register Addressing modes:

PC Relative Addressing Mode Base Register Addressing Mode
The content of program counter is added to the addressing field of the instruction i to obtain the effective address. The base register content is added to the addressing field of the instruction to obtain the effective address.
The addressing field of the instruction is mostly a signed number which can be either positive or negative. A base register holds a base address and the addressing field of the instruction gives displacement according to the base address.
A program counter always keeps track of the instructions of the program stored in its memory. A particular register has to be selected from the register set, according to the instruction.
Uses more bits as it has to specify a memory address directly. Uses less bits as it has to select a register from a register set.
A program counter always contains the address of the immediately next instruction to be executed. After fetching the address mentioned in the instruction, the program counter value immediately increases. In Base Register addressing mode the displacement value can be the same as the value required to reference the desired address as it does not immediately go to the next instruction.
Effective address of the operand is obtained by adding the program counter content to the addressing field of instruction. Effective address of the operand is obtained by adding the base register content to the addressing field of instruction.
EA = PC + Address field value
PC = PC + Relative value
EA = Base register + Address field value
PC = Base register + Relative value

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