Register addressing is similar to direct addressing. The only difference is that the address field referes to a register rather than a main memory address:
The advantages of register addressing are that only a small address field is needed in the instruction and no memory reference is required. The disadvantage of register addressing is that the address space is very limited.
Figure 4.4: Register Addressing Mode.
The exact register location of the operand in case of Register Addressing Mode is shown in the Figure 34.4. Here, 'R' indicates a register where the operand is present.