Addressing Modes of 8051

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  ADDRESSING MODES OF 8051

 

Ans:

Various methods of accessing the data are called addressing modes. 8051 addressing modes are classified as follows.

                    i.            Immediate addressing.
-          In this addressing mode the data is provided as a part of instruction
itself.
-          In other words data immediately follows the instruction.
Eg.
MOV A,#30H
ADD A, #83

                 ii.            Register addressing.
-          In this addressing mode the register will hold the data.
-          One of the eight general registers (R0 to R7) can be used and specified as the operand.
Eg.
MOV A, R0
ADD A, R6
-          R0 – R7 will be selected from the current selection of register bank.
-          The default register bank will be bank 0.

               iii.            Direct addressing
-          In direct addressing, an 8 bit internal data memory address is specified as part of the instruction and hence, it can specify the address only in the range of 00H to FFH.
-          In this addressing mode, data is obtained directly from the memory.
Eg.
MOV A, 60h
ADD A, 30h

               iv.            Indirect addressing
-          The indirect addressing mode uses a register to hold the actual address that will be used in data movement.
-          Registers R0 and R1 and DPTR are the only registers that can be used as data pointers.
Eg.
MOV A,@R0
ADD A,@R1
MOVX A,@DPTR

                  v.            Indexed addressing.
-          In indexed addressing, either the program counter (PC), or the data pointer DTPR) —is used to hold the base address, and the A is used to hold the offset address.
-          Adding the value of the base address to the value of the offset address forms the effective address.
-          Indexed addressing is used with JMP or MOVC instructions.
-          Look up tables are easily implemented with the help of index addressing.
Eg.
MOVC A, @A+DPTR          //copies the contents of memory location pointed by the sum of the accumulator A and the DPTR into accumulator A.

MOVC A, @A+PC       // copies the contents of memory location pointed by the sum of the accumulator A and the program counter into accumulator A

               vi.            Relative Addressing.
-          Relative addressing is used only with conditional jump instructions. The relative address, (offset), is an 8 bit signed number, which is automatically added to the PC to make the address of the next instruction. The 8 bit signed offset value gives an address range of +127 to —128 locations. The jump destination is usually specified using a label and the assembler calculates the jump offset accordingly. The advantage of relative addressing is that the program code is easy to relocate and the address is relative to position in the memory.
Eg.
SJMP LOOP1
JC BACK

             vii.            Absolute addressing
-          Absolute addressing is used only by the AJMP (Absolute Jump) and ACALL (Absolute Call) instructions.
-          These are 2 bytes instructions.
-          The absolute addressing mode specifies the lowest 11 bit of the memory address as part of the instruction.
-          The upper 5 bit of the destination address are the upper 5 bit of the current program counter.
-          Hence, absolute addressing allows branching only within the current 2 Kbyte page of the program memory.
Eg.
AJMP LOOP1
ACALL LOOP2

          viii.            Long Addressing
-          The long addressing mode is used with the instructions LJMP and LCALL.
-          These are 3 byte instructions.
-          The address specifies a full 16 bit destination address so that a jump or a call can be made to a location within a 64 Kbyte code memory space
.
Eg.
LJMP FINISH
LCALL DELAY

                ix.            Bit Direct Addressing
-          In this addressing mode the direct address of the bit is specified in the instruction.
-          The RAM space 20H to 2FH and most of the special function registers are bit addressable.
-          Bit address values are between 00H to 7FH
.
Eg.
CLR 07h         ; Clears the bit 7 of 20h RAM space
SETB 07H       ; Sets the bit 7 of 20H RAM space.

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