1. Function of the Program Counter: The PC holds the memory address of the next instruction to be fetched and executed from the program memory. After each instruction fetch, the PC is automatically incremented to point to the next instruction in sequence, unless a jump, call, or interrupt occurs.
2. Address Space and Bit Width: The 8051 architecture is designed to access up to 64 KB ($2^{16}$ bytes) of external program memory. To uniquely address every byte within this 64 KB range, the address must be 16 bits long.
$$2^{16} = 65,536 \text{ bytes} = 64 \text{ KB}$$
3. Hardware Characteristics: Because the program address bus in the 8051 is 16 bits wide, the
Program Counter must also be 16 bits wide. It is one of the few 16-bit registers in the 8051, which is otherwise primarily an 8-bit architecture. Unlike other registers like the Accumulator or B-register, the PC does not have an internal address and cannot be accessed directly by the user as a Special Function Register (SFR).
Therefore, the 16-bit width of the Program Counter is what allows the 8051 to handle its standard 64 KB program memory limit.