1. 8051 Addressing Modes
The way, using which the data source or destination addresses are specified in the instruction mnemonic for moving the data, is called “Addressing Mode”.
8051 has the following addressing modes:
1) Register Addressing Mode
2) Direct Byte Addressing Mode
3) Register Indirect Addressing Mode
4) Immediate Addressing Mode
5) Register Specific Addressing Mode
6) Index Addressing Mode
7) Stack Addressing Mode

2. Register Addressing
The 8051 can access eight “working registers” (R0-R7). Three bit
code within the instruction selects one of the eight registers from
the selected register bank. The programmer can select a register
bank by modifying bits 4 and 3 in the PSW.
Destination
Register
Source
Example:
Step 1: Select register bank
MOV PSW, # B
Step 2: Add the contents of R3 and R4
MOV A,R3
MOV B,R4
ADD R3,R4

3. Direct Byte Addressing
Direct addressing can access any on – chip variable or hardware
register. i.e. on-chip RAM and special function register. The most
significant bit of the address decides whether it is a location within
on-chip RAM (MSB = 0) or in special function register (MSB=1).
Memory
Destination Register
Data From
Selected
Location
Address of Memory
Within the Instruction
Example:
MOV A, 50H
ADD A, 51H

4. Register Indirect Addressing
In this addressing mode R0 and R1 of each register bank can be used
as an index or pointer register. R0 and R1 point to the contents in the
RAM.
The Instruction with indirect addressing uses the Sign.
Indirect addressing accesses data in dynamic manner rather than static
Manner. Looping is not possible in direct addressing mode
In indirect addressing we can increment the index or pointer register
to access successive locations.

5. Example: MOV A, @R0 ADD A, @R1 Memory Destination Register Data From
Selected
Location
Register
Contents of registers
are used to point memory
Example:
MOV
ADD

6. Immediate Addressing
In this addressing mode source operand is a constant rather than a
variable. So the constant can be incorporated in the instruction.
Sign “#” indicates it is a immediate addressing mode.
Destination
Register
Data Specified
In the instruction
Example:
MOV A, #52

7. Register Specific
Inherent in the instruction, these refers to a specific register such as
accumulator or DPTR.
Example:
SWAP A

8. Index Only program memory can be accessed in the index addressing.
Either the DPTR or PC can be used as an index register.
Example:
MOVC

9. + DPTR Register Program Memory Contents of DPTR Register A Register
Data From
Selected
Memory
Address of
Memory
Contents of Register A

10. Stack Addressing Mode
It is subtype of direct addressing mode in which stack instructions
(PUSH and POP) are used. Instruction such as ‘PUSH A’ is invalid.
Here, we have to specify the address of register A. thus, PUSH 0E0H
is a valid instruction; it pushes/stores the contents of accumulator on
the stack.
Example:
PUSH 04
PUSH 06
POP 02
POP OFOH