Register addressing mode : In this mode of addressing, data is in the register and instruction specifies the particular register as shown in the figure below.
This addressing mode is normally preferred because the instructions are compact and fastest executing of all instruction forms. The reason why it is fastest executing is just because, all the register reside on chip, therefore data transfer is within the chip and external bus is not at all required.
Register may be used as source operands, destination operands or both.
The register may be 8/16 bit.
e.g. MOV BX, AX
The instruction copies the contents of BX register to AX register.