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Working of 8085-based Single board microcomputer

Last Updated : 12 May, 2023
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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.


Cost-effective: The 8085-based single board microcomputer is a cost-effective solution for many applications, as it provides a complete system on a single board, eliminating the need for additional hardware.

Easy to use: The 8085 microprocessor is a well-established architecture that is easy to use and program, making it suitable for a wide range of applications.

Availability of software tools: A wide range of software tools and compilers are available for the 8085 microprocessor, which makes it easier to develop and debug applications.

Expandability: The single board microcomputer can be expanded with additional peripherals such as I/O devices, memory, and communication interfaces, making it suitable for a variety of applications.

Low power consumption: The 8085 microprocessor is a low power device, which makes it suitable for battery-powered applications.


Limited processing power: The 8085 microprocessor has limited processing power compared to more modern microprocessors, which can limit the performance of the system.

Limited memory: The 8085 microprocessor has a limited address space, which can limit the amount of memory that can be used in the system.

Limited peripherals: The 8085 microprocessor has limited built-in peripherals, which can limit the functionality of the system.

Limited connectivity: The 8085 microprocessor has limited connectivity options, which can limit the ability to interface with external devices.

Limited availability: The 8085 microprocessor is an older architecture, which may limit its availability and support in the future.


8-bit microprocessor: The 8085 microprocessor is an 8-bit processor, which means it can handle 8 bits of data at a time. This limits its processing power compared to newer processors, but it is still capable of running a wide range of software and applications.

Memory: The microcomputer typically includes memory, which is used to store the program instructions and data being processed by the microprocessor. The memory may include both read-only memory (ROM) and random access memory (RAM).

I/O ports: The microcomputer typically includes a variety of I/O ports, such as serial and parallel ports, which are used to interface with other devices.

Clock: The microprocessor requires a clock signal to synchronize its operations, and the microcomputer typically includes a clock generator to generate the necessary clock signal.

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