1. under http://www.edsim51.com/8051Notes/index.html
Address Modes
Source:
under

2. What does an assembly instruction do?
Instructions tell the processor which operation to carry out.
For example, the instruction
MOV A, #5EH
tells the processor to move the data 5EH to the accumulator.

3. Instructions result in the processor performing some operation on some data (the data being 5EH in the example above).
The instruction above (i.e., MOV A, #5EH) is an example of what's known as immediate addressing. The reason for this is because the data 5EH immediately follows the instruction in code memory.

4. Address Modes
The available ways of how a processor accesses data from registers or memory are called the “address modes”.
The following are the address modes provided for 8051 processor:
Immediate
Register
Direct
Indirect
Relative
Absolute
Long
Indexed

5. “Immediate” address mode
MOV A, #5EH
The machine code of “MOV A, #5EH” is “74H, 5EH” stored at addresses 0000H and 0001H.
The datum “5EH” is stored immediately after the opcode “74H”.

6. “Register” address mode
MOV A,R5

7. “Direct” address mode

8. Quiz: what are the address modes in “MOV 65H,#43H”
Q: What are the address modes?
Q: How does the datum flow?

9. “Indirect” address mode

10. “Relative” address mode

11. “Absolute” address mode

12. “Long” address mode

13. “Indexed” address mode