HOW PROGRAM IS EXECUTED IN 8085

8085 Microprocessor works on the principle of stored program concept which means that instructions are stored in the memory.

For executing the program, microprocessor fetches the instruction one by one and executes them. Steps in execution are Fetch, Decode and Execute.

STEPS INVOLVED IN THE EXECUTION

Step - 1  Program Counter loads the 16 bit memory address of the instruction on the address bus.

Step - 2 Control Unit sends the MEMR⁻ signal to tell the memory that microprocessor wants to read from memory. 

Step - 3 8-bit Instruction Code in the addressed memory location is then placed on the data bus. This code is then transferred to the Instruction Register.

Step - 4 Till Step 3, fetching of the instruction occurred. Now the opcode of the instruction is decoded by instruction decoder. It decodes the meaning of the opcode and number of machine cycles required to execute the complete instruction. 

 Meanwhile, Program Counter is incremented by 1 and next instruction is fetched from the memory in the same way as before..

Step - 5 The decoded opcode information is sent to the control unit so that control unit can generate appropriate control signal to execute that instruction.

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