What is instruction set? Explain various kind of instructions of 8085 microprocessor.

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An instruction of a computer is a command given to the computer to perform a specified operation on given data. In microprocessor, the instruction set is the collection of the instructions that the microprocessor is designed to execute.

An instruction is a binary pattern designed inside a microprocessor to perform a specific function. The entire group of instructions, called the instruction set, determines what functions the microprocessor can perform. These instructions can be classified into the following five functional categories: data transfer (copy) operations, arithmetic operations, logical operations, branching operations, and machine-control operations.

1 Data Transfer Croup

The data transfer instructions move data between registers or between memory and registers.

These instructions are used by the microprocessor in order to transfer the data from one location to another. More specifically, it helps in transferring the data from source operand to destination operand, without making any changes in the source data.

As the data present in the source is not modified. Thus it is more convenient to call it a data copy instruction. As while transferring the data from source to destination, the data present in the source is just copied to the destination also. Due to this no change in source data is noticed.

Basically, the instructions that come under this instruction set, are as follows:

instruction set of 8085 microprocessor - 01

This instruction helps to transfer the contents of register r2 into register r1. While the data in register r2 will remain the same.

2.Logical Instruction

These instructions are used to perform logical or Boolean operations over the data present in either register or memory. The logical instructions modify the flag bits according to the operation performed.

The logical instruction set is as follows:instruction set of 8085 microprocessor - 35

This instruction is used to compare the data at the accumulator with the data present at the register or memory which is given as operand. According to the result obtained by the comparison, the flags are set. While the data being compared remains unchanged.

Suppose,

  • If the data present at the accumulator is smaller than that of register or memory, then carry flag is set.
  • If the data in the accumulator is equal to the data in the register or memory, then it sets the zero flag.
  • However, if the data in the accumulator is greater than the data present in the memory or register, then both carry and zero flags are reset.

instruction set of 8085 microprocessor - 36

This instruction is used to compare the 8-bit data given as operand in the instruction with the data present in the accumulator. The comparison result is shown by the flags.

3.Branching Instruction

These instructions are used in order to switch or transfer the control of the microprocessor from one position to another. More specifically we can say, it changes the general sequential flow.

The branching instruction set is given below: example

instruction set of 8085 microprocessor - 50

This instruction is used to transfer the present program sequence to that memory location whose 16-bit address is specified in operand of the instruction.

4.Control Instruction

As the name itself is suggesting that these instructions are used to control the operations of the microprocessor. The control instructions are as follows: example

instruction set of 8085 microprocessor - 55

It stands for no operation. Whenever the microprocessor fetches this instruction, then it is decoded but not executed.

5.Arithmetic Instruction

This instruction set allows arithmetic operations to be performed over the data in memory and register inside the 8085 microprocessor.

The arithmetic instruction set is given as follows:

instruction set of 8085 microprocessor - 19

This instruction specifies that the data present in the register is to be added with the data present in the accumulator and the result is to be stored in the accumulator itself.

Example: ADD B

instruction set of 8085 microprocessor - 20

This instruction is used to add the data of the memory address specified at the operand with the data present at the accumulator. And the result of the addition is stored in the accumulator.

 

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