Addressing Modes

Addressing Modes

AU : Dec.-04, 12, 14, 15, 16, 17, 19, May-17, 18]

Part of the programming flexibility for each microprocessor is the number and different kind of ways the programmer can refer to data stored in the memory. The different ways that a microprocessor can access data are referred to as addressing modes. The 8085 has 5 addressing modes. These are :

1. Immediate  2. Register

3. Direct      4. Indirect   5. Implied

1. Immediate addressing mode :

In an immediate addressing mode, 8 or 16 bit data can be specified as a part of instruction. In 8085, the instructions having T letter fall under this category. T indicates immediate addressing mode.

Example :

MVI A, 20 H ; Moves 8 bit immediate data (20H) into accumulator

MVI M, 30 H ; Moves 8 bit immediate data (30H) into the

 ; memory location pointed by HL register pair.

LXI SP, 2700H ; Moves 16 bit immediate data (2700H) into SP.

LXI D, 10FFH ; Moves 16 bit immediate data (10FFH) into DE

; register pair ( D = 10H and E = FFH).

2. Register addressing mode :

The register addressing mode specifies the source operand, destination operand, or both to be contained in an 8085 registers. This results in faster execution, since it is not necessary to access memory locations for operands.

Example :

MOV A, B ; Moves the contents of register B into the accumulator.

SPHL ; Moves the contents of HL register pair into stack pointer.

ADD C ; Adds the contents of register C into the contents of accumulator ; and stores result in the accumulator.

3. Direct addressing mode :

The direct addressing mode specifies the 16 bit address of the operand within the instruction itself. The second and third bytes of instruction contain this 16 bit address.

Example :

LDA 200H ; Loads the 8 bit contents of memory location

; 2000H into the accumulator.

SHLD 3000H ; Stores the HL register pair into two consecutive memory

; locations. Lower byte i.e. the contents of L register into memory

; location 3000H and higher byte i.e. the contents of H register

; into memory location 3001H.

4. Indirect addressing mode :

In indirect addressing mode, the memory address where the operand located is specified by the contents of a register pair.

Example :

LDAX B ; Loads the accumulator with the contents of ; memory location pointed by BC register pair.

MOV M, A ; Stores the contents of accumulator into the ; memory location pointed by HL register pair.

5. Implied Addressing Mode :

In implied addressing mode, opcode specifies the address of the operands.

Example :

CMA ; Complements contents of accumulator.

RAL  ; Rotates the contents of accumulator left through cany.

Note : Many of the advanced processors support addressing mode called index addressing mode. In this mode, the address of the operand within the memory is generated by adding the offset/displacement to the register specified in the instruction. The offset/displacement is also a part of the instruction. In 8085 such addressing mode is not available. However, we can implement such kind of program structure, by using memory pointer (HL register), any other register pair and a instruction sequence given below :

LXI H, Baseaddr  ; Loads the base address

LXI B, Offset/Displacement ;  Loads the offset or displacement

DAD B ; Gives the addition of HL and BC in HL register pair.

MOV A, M ; Load the data from memory in the  accumulator

By incrementing or decrementing contents of BC register or loading another contents, we can change the index/offset/displacement

Review Questions

1. With example explain the different addressing modes of 8085 and the different types of instruction.

AU : Dec.-04, 19, Marks 16

2. What are the different addressing modes in 8085 microprocessor ? Explain it with an example.

AU : Dec.-12, 14, Marks 8

3. Explain the addressing modes of 8085 microprocessor with example for each.

AU : Dec-15, 17, May-17, 18, Marks 8