Question

State any three addressing modes of 8085 Microprocessor instructions with one example of each.

Answer 1

1. Direct addressing mode: Direct addressing involves storing the address after the opcode of the instruction in program memory. The address of the operand is provided within the instruction. The instructions in direct addressing mode are three bytes long. Byte 1 represents the instruction opcode, whereas Byte 2 is the lower order address and Byte 3 is the high order address.
   E.g.: LDA 9FFFH 
   i.e., This instruction loads the data of memory location 9FFF H into the accumulator. 
2. Register addressing mode: In register addressing mode, the source operands are general-purpose registers with names given in the instruction. These are single-byte instructions. All actions happen within the CPU.
   E.g., MOV A, B.
   i.e., This instruction transfers the data of register B to the accumulator without altering them.
3. Immediate addressing: Immediate addressing means that data is stored in program memory immediately following the instruction's opcode. These instructions specify the actual data within the instruction. These operations are stated using either two or three byte instructions.
   E.g., ADD 05H
   i.e., this instruction adds immediate- data 05 H to the content of accumulator. The result is stored in accumulator.
4. Register indirect addressing mode: In register indirect addressing the content of register pair points to the address of the operand. A register pair (H-L pair) is specified for addressing 16-bit address of memory. location. These are generally 1-byte inshuctjon. 
   E.g. ADO M 
   i.e. this instruction will add the content of memory location whose address is stored in H-L pair to the content of accumulator.
5. Implicit addressing mode: In this type of instruction, generally operand is not specified within the instruction and is predetermined. Generally, the operand is an accumulator. Most of the logical group instructions belong to this addressing mode. These are single byte instructions. All actions occur within the CPU.
   E.g. CMA
   i.e., this instruction will complement the content of accumulator. Here, the actual operand is not specified in the instruction but is predetermined (accumulator). The result is stored in accumulator.