The 8051 provides a total of five distinct addressing modes.
- register indirect
1. Immediate Addressing Mode
- The operand comes immediately after the op-code.
- The immediate data must be preceded by the pound sign, "#".
2. Register Addressing Mode
- Register addressing mode involves the use of registers to hold the data to be manipulated
3. Direct Addressing Mode
- It is most often used to access RAM locations 30 - 7FH.
- This is due to the fact that register bank locations are accessed by the register names of R0 - R7.
- There is no such name for other RAM locations so must use direct addressing
- 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
4. Register Indirect Addressing Mode
- A register is used as a pointer to the data.
- If the data is inside the CPU, only registers R0 and R 1 are used for this purpose.
- R2 - R7 cannot be used to hold the address of an operand located in RAM when using indirect addressing mode.
- When RO and R 1 are used as pointers they must be preceded by the @ sign.
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.
- The instruction used for this purpose is : MOVC A, @ A+DPTR
- The 16-bit register DPTR and register A are used to form the address of the data element stored in on-chip ROM.
- 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.
- 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