Addressing Modes: 8051

keywords: 8051, microcontroller

8051 Addressing Modes:

When instructions operate on data, the question arises: "Where are the data?" The answer to this question lies in the 8051's "addressing modes."   An "addressing mode" refers to how you are addressing a given memory location. The8051 instructions use eight addressing modes. These are:

1. Register 
2. Immediate
3. Direct
4. Indirect

Each of these addressing modes provides important flexibility.

 

Register Addressing:

The register banks, which contains registers R0 through R7, can be accessed by instructions whose opcodes carry a 3-bit register specification. Instructions that access the registers this way make efficient use of code, since this mode eliminates an address byte. When the instruction is executed, one of four banks is selected at execution time by the row bank select bits in PSW.

                 MOV A, Rn                   ; copy contents of Rn into A

     MOV A, #n                     ; copy the immediate data byte n to the register A.

     MOV Rn, A                    ; copy contents of A into Rn.

     MOV Rn, #n                   ; copy the immediate data byte n to the register Rn.

     MOV DPTR, #nn            ; copy the immediate 2 byte data nn to the register DPTR.

*Note: The source and destination registers must match in size.

 

Immediate Addressing:

When a source operand is a constant rather than a variable, then the constant can be incorporated into the instruction as a byte of "immediate" data. An additional instruction byte contains the value.

In assembly language, immediate operands are preceded by a number sign (#). The operand may be a numeric constant, a symbolic variable, or an arithmetic expression using constants, symbols, and operators. The assembler computes the value and substitutes the immediate data into the instruction. For example, the instruction

             MOV A, #25H;                load 25H into A

             MOV R4, #62;                 load 62 into R4

             MOV DPTR, #4521H;        DPTR=4512H

 

Direct Addressing:

It differs from register addressing in that any byte within the first 256 addresses can be accessed by specifying 8-bit address. When using this mode, there is few things watch out for. The first RAM addresses (080h to 0FFh, if the device you are using has RAM at these locations) cannot be accessed by direct memory addressing.

            MOV R0, 40H;                 save content of 40H in R0

            MOV 56H, A;                   save content of A in 56H

 

Indirect Addressing:

As the name indicates it is an indirect mode of addressing. It uses R0 or R1 as an 8-bit index register to access a byte in the first 256 addresses of the data space. The register indirect addressing mode is identified by the symbol '@' before either R0 or R1. Using any other bank (R2 to R7) will result in an error.

                             MOV     A, @R0;       move contents of RAM whose address is held by R0 into A

                             MOV     @R1, B;       move contents of B into RAM whose address is held by R1

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, @R1;               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

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 

keywords: 8051, microcontroller