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Bus organization of 8085 microprocessor | 8085 Bus Structure

8085 Bus Structure

There are three buses in Microprocessor:

  1. Address Bus

2. Data Bus

3. Control Bus

  1. Address Bus:- Generally, the Microprocessor has a 16-bit address bus. The bus over which the CPU sends out the address of the memory location is known as the Address bus. The address bus carries the address of the memory location to be written or to be read from.

The address bus is unidirectional. It means bits flowing occurs only in one direction, only from microprocessor to peripheral devices.

We can find how much memory location it can use the formula 2^N. where N is the number of bits used for address lines.

here,    2^16 = 65536bytes or 64Kb

So we can say that it can access up to 64 kb memory location.

Q.>If a processor has 4 GB memory then how many address lines are required to access this memory?

Ans: 4GB= 4 * 1GB

         4 = 2^2

         1GB = 2^30

          4GB = 2^2   *   2^30 = 2^32

          So 32 address lines are required to access the 4 GB memory.

2. Data Bus:- 8085 Microprocessor has an 8-bit data bus. So it can be used to carry the 8-bit data starting from 00000000H(00H) to 11111111H(FFH). Here ‘H’ tells the Hexadecimal Number. It is bidirectional. These lines are used for data flowing in both directions means data can be transferred or received through these lines. The data bus also connects the I/O ports and CPU. The largest number that can appear on the data bus is 11111111.

It has 8 parallel lines of data bus. So it can access up to 2^8 = 256 data bus lines.

3. Control Bus:- The control bus is used for sending control signals to the memory and I/O devices. The CPU sends a control signal on the control bus to enable the outputs of addressed memory devices or I/O port devices.

Some of the control bus signals are as follows:

  1. Memory read

2. Memory write

3.I/O read

4.I/O write.

Uses of Bus organization in 8085 microprocessor

  1. Data Transfer: The bus organization facilitates the transfer of data between the microprocessor and external memory devices, input/output devices, and other peripheral components. It defines the data bus, which is used to transmit data between these entities.
  2. Address Bus: The 8085 microprocessor has a 16-bit address bus, allowing it to address up to 64KB of memory locations. This address bus is responsible for specifying the memory location from which data should be read or to which data should be written.
  3. Control Bus: The control bus carries various control signals that coordinate the operations of the microprocessor and connected devices. These control signals include read, write, memory enable, input/output control, and others.
  4. Instruction Fetch: During the execution of a program, the microprocessor uses the bus organization to fetch instructions from memory. The address bus specifies the memory location, and the data bus carries the instruction to the microprocessor.
  5. Data Transfer Between Registers: The data bus is used for transferring data between different registers within the microprocessor, enabling arithmetic and logic operations.
  6. Input/Output Operations: The bus organization facilitates communication between the microprocessor and input/output devices. Data is transferred between the microprocessor and these devices using the data bus.
  7. Interrupt Handling: When an interrupt occurs, the bus organization allows the microprocessor to temporarily suspend its current task and jump to an interrupt service routine. The interrupt control signals on the control bus are crucial for this operation.
  8. Memory Mapping: The address bus plays a crucial role in memory mapping, allowing the microprocessor to access different memory locations within its addressable memory space.
  9. Control of Timing Signals: The control bus carries various timing signals that synchronize the activities of the microprocessor and ensure that data is transferred and processed at the correct time.
  10. DMA Operations: In some cases, the bus organization is used to support Direct Memory Access (DMA) operations, where external devices can transfer data directly to and from memory without CPU intervention.


  • Simple Architecture: The 8085 microprocessor has a straightforward and well-defined architecture, making it relatively easy to understand and program. This simplicity is advantageous for educational purposes and for embedded systems with limited complexity requirements.
  • Low Power Consumption: The 8085 microprocessor was designed to be power-efficient, which is beneficial for battery-powered devices and applications where power consumption is a critical concern.
  • Low Cost: Due to its simple design and lower component count, the 8085 microprocessor is cost-effective, making it suitable for applications with budget constraints.
  • Wide Range of Support: Despite its age, the 8085 has a wealth of documentation, textbooks, and online resources available for learning and troubleshooting. This extensive support can be beneficial for engineers and hobbyists.
  • Compatibility: The 8085 microprocessor is compatible with a wide range of peripheral devices and memory chips, making it versatile and adaptable for various applications.
  • Reliability: The 8085 is known for its reliability and robustness, making it suitable for applications where stability and consistency are critical.
  • Interrupt Handling: The 8085 microprocessor has robust interrupt-handling capabilities, allowing it to respond to external events or requests promptly.
  • Parallel Processing: While not as advanced as modern processors, the 8085 supports basic parallel processing operations, enabling concurrent execution of certain instructions.
  • Large Addressable Memory: With a 16-bit address bus, the 8085 can address up to 64KB of memory, which was a significant amount at the time of its introduction.
  • Legacy Support: The 8085 microprocessor is still used in various legacy systems and industrial applications, and there is a need for engineers with expertise in this architecture.
  • Educational Value: The simplicity and historical significance of the 8085 microprocessor make it an excellent tool for teaching the fundamentals of microprocessor architecture and programming.


Limited Bandwidth: The bus organization used in the 8085 microprocessor has a limited bandwidth, which can limit the performance of the processor in high-performance applications.

Latency: The bus organization can introduce latency, which is the delay between the time a command is issued and the time the response is received. This latency can be a problem in real-time applications that require immediate responses.

Data Integrity: The bus organization used in the 8085 microprocessor is vulnerable to data corruption due to electromagnetic interference and other sources of noise. This can lead to errors in data transmission and processing.

Complexity: The bus organization used in the 8085 microprocessor can be complex to implement and troubleshoot, which can increase the cost and time required to develop and maintain computer systems.

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