Q.How many bits of address number can register PC can hold?

To write assembly-language programs a student must know three key registers and, crucially, the number of bits each one holds, because the bit-width sets exactly how much data or how large an address the register can carry.

Register A (the accumulator) is 8 bits, or 1 byte, wide. It is the main working register through which arithmetic and many data-transfer operations pass: a value is loaded into A, operated on, and the result left in A. The largest value an 8-bit register can hold is $2^8 - 1 = 255$, i.e. FFH in hexadecimal.

Register R is a general-purpose 8-bit register used for temporary storage of a single byte while a program runs — a scratch-pad for intermediate values. …