What is the bit size of each temporary Internal Registers in 8051? 

8051 microcontroller

8051 is an 8-bit microcontroller built with 40 pins DIP (dual inline package).

8051 has internal RAM (128 Bytes) and ROM (4K Bytes).

Of the 128-byte internal RAM, only 16 bytes are bit-addressable.

Storage Registers in 8051

The most widely used registers of the 8051 are Accumulator, R registers, B registers, DPTR (data pointer), and PC (program counter).

All these registers are 8 bits, except DPTR and PC.

1.) Accumulator

• The accumulator, register A, is used for all arithmetic and logic operations. If the accumulator is not present, then every result of each calculation (addition, multiplication, shift, etc.) is to be stored in the main memory.
• Access to main memory is slower than access to a register like an accumulator because the technology used for the large main memory is slower (but cheaper) than that used for a register.

2.) The "R" Register

• The "R" registers are a set of eight registers, namely, R0, R1 to R7.
• These registers function as auxiliary or temporary storage registers in many operations.

3.) The "B" Register

• The "B" register is very similar to the Accumulator in the sense that it may hold an 8-bit (1-byte) value.
• The "B" register is used only by two 8051 instructions: MUL and DIV.

4.) The Data Pointer

• The Data Pointer (DPTR) is the 8051’s only user-accessible 16-bit (2-byte) register.
•  It is used by the 8051 to access external memory using the address indicated by DPTR.
• DPTR is the only 16-bit register available and is often used to store 2-byte values.

5.) The Program Counter

• The Program Counter (PC) is a 2-byte address that tells the 8051 where the next instruction to execute can be found in the memory.
• PC starts at 0000 H when the 8051 initializes and is incremented every time an instruction is executed.
• PC is not always incremented by 1. Some instructions may require 2 or 3 bytes; in such cases, the PC will be incremented by 2 or 3.