Addressing Modes of 8051 Micro Controller

Addressing Modes of 8051

The 8051 provides a total of five distinct addressing modes.

ü immediate

ü register

ü direct

ü register indirect

ü indexed

Immediate Addressing Mode

ü The operand comes immediately after the op-code.

ü The immediate data must be preceded by the pound sign, "#".

Register Addressing Mode

Register addressing mode involves the use of registers to hold the data to be manipulated

Direct Addressing Mode

ü It is most often used to access RAM locations 30 - 7FH.

ü This is due to the fact that register bank locations are accessed by the register names of R0 - R7.

ü There is no such name for other RAM locations so must use direct addressing

ü In the direct addressing mode, the data is in a RAM memory location whose address is known, and this address is given as a part of the instruction

Register Indirect Addressing Mode

ü A register is used as a pointer to the data.

ü If the data is inside the CPU, only registers R0 and R 1 are used for this purpose.

ü R2 - R7 cannot be used to hold the address of an operand located in RAM when using indirect addressing mode.

ü When RO and R 1 are used as pointers they must be preceded by the@ sign.

Indexed Addressing Mode

ü Indexed addressing mode is widely used in accessing data elements of look-up table entries located in the program ROM space of the 8051.

ü The instruction used for this purpose is : MOVC A, @ A+DPTR

ü The 16-bit register DPTR and register A are used to form the address of the data element stored in on-chip ROM.

ü Because the data elements are stored in the program (code) space ROM of the 8051, the instruction MOVC is used instead of MOV. The "C" means code.

ü In this instruction the contents of A are added to the 16-bit register DPTR to form the 16- bit address of the needed data.