Working of 8085-based Single board microcomputer

Prerequisites – Pin diagram of 8085 microprocessor, Bus organization of 8085 microprocessor
Single board microcomputers are the system which have a program called Key Monitor or Key Executive permanently stored in memory. This program is stored either in EPROM or in ROM, beginning at the memory location 0000H.

Hardware is the skeleton of the computer and software is its life. The software (programs) makes the computer live; without it the hardware is a dead piece of semiconductor material.

When the power is turned on, the monitor program comes alive. Initially, the program counter has a random address. When the system is reset, the program counter in the 8085 is cleared and it holds the address 0000H. The trainer system includes a “power on” reset circuit, which reset the system and clears the program counter when system is turned on. The MPU places the address 0000H on the address bus. The instruction code stored in location 0000H is fetched and executed, and the execution continues according to the instructions in the monitor program.

The primary functions of monitor program are as follows:

  1. Reading the Hex keyboard and checking for a key closure. Continuing to check the keyboard until a key is pressed.
  2. Displaying the Hex equivalent of the key pressed at the output port, such as the seven segment LEDs.
  3. Identifying the key pressed and storing its binary equivalent in memory, if necessary.
  4. Transferring the program execution sequence to the user program when the Execute key is pressed.

The programmer enters a program in R/W memory in sequential memory locations by using the data keys (0 to F) and the function key called Enter. When the system is reset, the program counter is cleared, and the monitor program begins to check a key closure again. By using the keyboard, the programmer enters the first memory address where the user program is stored in R/W memory and directs the MPU to execute the program by pressing the Run key. The MPU fetches, decodes and executes one instruction code at a time and continues to do so until it fetches the Halt instruction.

The Key monitor program is a critical element in entering, storing and executing a program. Until the Execute key is pushed, the monitor program in the EPROM (or ROM) directs all the operations of the MPU. After the Execute key is pushed, the user program directs the MPU to perform the functions written in the program.

My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using or mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.