Addressing modes of 8051 Microcontroller

8051 Addressing Modes

8051 has four addressing modes.

1.  Immediate Addressing:

Data is immediately available in the instruction. For example -

ADD A, #77; Adds 77 (decimal) to A and stores in A

ADD A, #4DH; Adds 4D (hexadecimal) to A and stores in A MOV DPTR, #1000H;

Moves 1000 (hexadecimal) to data pointer

2.  Bank Addressing or Register Addressing:

This way of addressing accesses the bytes in the current register bank. Data is available in the register specified in the instruction. The register bank is decided by 2 bits of Processor Status

Word (PSW). For example-

ADD A, R0; Adds content of R0 to A and stores in A 

3. Direct Addressing:

The address of the data is available in the instruction. For example - MOV A, 088H; Moves content of SFR TCON (address 088H)to A 

4. Register Indirect Addressing:

The address of data is available in the R0 or R1 registers as specified in the instruction. For example - MOV A, @R0 moves content of address pointed by R0 to A .

5. External Data Addressing:

Pointer used for external data addressing can be either R0/R1 (256 byte access) or DPTR (64kbyte access).

For example -

MOVX A, @R0; Moves content of 8-bit address pointed by R0 to A

MOVX A, @DPTR; Moves content of 16-bit address pointed by DPTR to A

6. External Code Addressing:

Sometimes we may want to store non-volatile data into the ROM e.g. look-up tables. Such data may require reading the code memory. This may be done as follows -

MOVC A, @A+DPTR; Moves content of address pointed by A+DPTR to A

MOVC A, @A+PC; Moves content of address pointed by A+PC to A