Assume that a program and data are already entered in the R/W memory. The program includes binary instructions to add given data and to display the answer at the seven-segment LEDs. When the microprocessor is given a command to execute the program, it reads and executes one instruction at a time and finally sends the result to the seven-segment LEDs for display.
This process of program execution can best be described by comparing it to the process of assembling a radio kit. The instructions for assembling the radio are printed in a sequence on a sheet of paper. One reads the first instruction, then picks up the necessary components of the radio and performs the task. The sequence of the process is read, interpret, and perform. The microprocessor works the same way.
The instructions are stored sequentially in the memory. The microprocessor fetches the first instruction from its memory sheet, decodes it, and executes that instruction. The sequence of fetching, decoding and executing continues until the microprocessor comes across an instruction to stop.
During the entire process, the microprocessor uses the system bus to fetch the binary instructions and data from the memory. It uses registers from the register section to store data temporarily, and it performs the computing function in the ALU section. Finally, it sends out the result in binary, using the same bus lines, to the seven-segment LEDs.