Different types of addressing modes in 8051.

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Different types of addressing modes in 8051.




Addressing:-The CPU can access data in a register or in memory or be provided as an immediate value. These various ways of accessing data are called addressing modes.
      The 8051 provides a total of five distinct addressing modes.
      They are as follows:-
1) Immediate addressing modes.
     
2) Register addressing modes.
      
3) Direct addressing modes.
    
  4) Register indirect - addressing mode.
  5) Indexed addressing mode.


Let us study different modes with selective examples:-
1) Immediate Addressing mode:-
                                    In This addressing mode, the source operated is a constant i.e. immediate data. Note that the immediate data must be preceded by the pound sign"#".
       Example:-
                MOV A, # 82H                        ; load 82H into A
                MOV R4, # 100                       ; load the decimal value 100 in to R4
                MOV B, # 40H                        ; load 40H in to B
               MOV P1, # 55H                        ; load 55H directly to port 1
               MOV DPTR , # 4521 H            ; DPTR =4521 H

          2) Register - Addressing Mode:-                                              
                                 Register addressing mode involves the use of registers to hold data to be manipulated.
      Example:-
              MOV  A,RO                              ;copy the contents of R0 in to A
              MOV R2,A                                ;copy the contents of A in to R2
              ADD A,R5                                 ;Add contents of R5 to A
              MOV  R7,DPL                           ;load R7 with contents of DPL

            3) Direct Addressing mode:-
                               In direct addressing mode the data is in a RAM Memory location whose addressing is known and this address is given as a part of the instruction.
                               Although the entire 128 bytes of RAM com be addressed Using direct addressing mode
                               It is most often used to access RAM locations 30-7FH
     Example:-

              MOV R0, 40H                        ; save content of RAM location 40H in Ro
              MOV 7FH, A                          ; save content of A in to RAM location 7FH
              MOV A,07H                          ;same as move A1 R7 is copies data in R7 to A
              MOV 03H, 04H                        ; it copies data in R4 to R3

        4) Indirect addressing mode:-
                               In the register indirect addressing mode a register is used as a pointer to the data .only register R0 and R1 are used for internal RAM indirect Data transfer.
                          When they hold the address of Rom locations they must be preceded by  the “@” symbol.
            Example :-

         MOV ,A, @R0              ;move contents of Ram location whose address is help
                                                By R0 in to A
        MOV @ R1 ,B               ;move contents of B in to RAM location whose Address is    
                                                help by R1.

      5) Indexed addressing mode :-
                        Indexed addressing mode is widely used in accessing data elements of look-up table entries located  in the program Rom space of the 8051.

            Example :-

MOV A, @ A + DPTR
MOV A, @ A+ PC



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