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What are the different addressing modes in 8085?
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Answer Posted / surabhi deshmukh
Subject: Microprocessor Design
Chapter: Microprocessor Architecture and Its Applications
Type Of Question: Long
Marks For Question: 8
Question: What are the addressing modes of 8085 microprocessor?
Answer: Following is the answer to the above question:
Types of Addressing Modes
The way by which the microprocessor identifies the
operands for a particular instruction is known as Addressing
mode.
The process of identifying the operands for a particular
instruction can be carried out in several ways. The various
types of addressing modes are as shown below.
Immediate addressing mode
Direct addressing mode
Register addressing mode
Register indirect addressing mode
Implicit addressing mode
Immediate Addressing Mode:
In this type of addressing mode the operand is specified
within the instruction itself. Let us discuss with an example.
Consider this instruction:
ADI 34H - This instruction adds the immediate data, 34H
to the accumulator. 34H is the data here. H represents
Hexadecimal value and the immediate value is added to the
accumulator. In this case 34H is added to the accumulator.
Suppose if accumulator has a value 8H and when this
instruction is executed, 34H is added to the 8H and the
result is stored in accumulator.
In the above instruction the operand is specified within
instruction itself.
Direct Addressing Mode:
In this mode of addressing, the address of the data
(operand) is specified within the instruction.
There is a subtle difference between the direct
addressing modes and immediate addressing modes. In
immediate addressing mode the data itself is specified
within instruction, but in direct addressing mode the
address of the data is specified in the instruction.
Example:
OUT 10H
LDA 4100H
STA 2000H
Consider the instruction STA 2000H
When this instruction is executed, the contents of the
accumulator are stored in the memory location specified. In
the above example the contents of accumulator are stored in
memory location 2000H.
Register Addressing Mode:
In this type of addressing mode the instruction
specifies the name of the register in which the data is
available and Opcode specifies the name (or) address of the
register on which the operation would be performed.
Example:
MOV A, B
Here the Opcode is MOV. If the above instruction is
executed, the contents of Register B are moved to the
Register A, which is nothing but the accumulator.
Other examples:
ANA B
On executing the above instruction the contents of
Register B or logically ANDed with contents of register A
(accumulator).
Register Indirect Addressing Mode:
This is indirect way of addressing. In this mode the
instruction specifies the name of the register in which the
address of the data is available.
Example:
MOV A, M
SUB M
DCR M
Consider MOV A, M. This instruction will move the
contents of memory location, whose address is in H-L
register pair to the accumulator.
M represents the address present in the H-L register
pair. So when MOV A, M is executed, the contents of the
address specified in H-L register pair are moved to accumulator.
Implicit Addressing Mode:
There are certain instructions in 8085 which does not
require the address of the operand to perform the operation.
They operate only upon the contents of accumulator.
Example:
CMA
RAL
RAR
CMA complements the contents of accumulator.
If RAL is executed the contents of accumulator is
rotated left one bit through carry.
If RAR is executed the contents of accumulator is
rotated right one bit through carry.
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