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Microprocessor 8085 – Configuration, Architecture, Functional Units

A microprocessor is derived from words micro-small and processor -to process the program. 8085 Processor is a 8-bit microprocessor which operates on 8 bits at a time. It is created with N-MOS technology. Due to its unique features, it holds popularity among the other microprocessors.


More About Microprocessor

The Microprocessor 8085 was introduced by Intel in the year 1977. The performance is calculated in terms of processor clock speed that is expressed in megahertz (MHz) or Gigahertz (GHz). Also, 8085 processor was the first commercial microprocessor by Intel. There were some of the architectural drawbacks in 8080 which was cleared by 8085.

Configuration of 8085 Microprocessor

Following are the main configuration pins of the 8085 microprocessor. They play a crucial role in controlling the memory and I/O operations and managing interrupts and resets. Here are the configuration pins of the 8085 microprocessor:


1. A15-A8 (Address Bus)

These pins are used to transmit the higher-order address bits during memory and I/O operations. A15 represents the most significant bit, while A8 represents the least significant bit.

2. AD7-AD0 (Data Bus)

These pins are bidirectional and used to transmit data between the microprocessor and external devices. AD7 represents the most significant bit, and AD0 represents the least significant bit.

3. ALE (Address Latch Enable)

ALE is a control output signal that is used to latch the address from the microprocessor onto the address bus. It is active high and indicates the availability of a valid address on the bus.

4. RD (Read)

RD is an active low input signal used to initiate a read operation. When RD is asserted, the microprocessor reads data from the addressed memory location or I/O port.

5. WR (Write)

WR is an active low input signal used to initiate a write operation. When WR is asserted, the microprocessor writes data to the addressed memory location or I/O port.

6. IO/M (Input/Output and Memory Select)

It is a control output signal that indicates whether the microprocessor is performing an I/O operation or a memory operation. IO/M = 1 indicates an I/O operation, and IO/M = 0 indicates a memory operation.

7. S0, S1 (Status Flags)

These pins indicate the status of various conditions in the microprocessor. Different combinations of S0 and S1 represent specific conditions such as interrupt enable, hold acknowledge, DMA acknowledge, etc.

8. INTA (Interrupt Acknowledge)

It is an active low output signal which is used to acknowledge an interrupt request from an external device. It indicates that the microprocessor has recognized and accepted the interrupt.

9. RESET IN (Reset Input)

RESET IN is an active low input signal used to reset the microprocessor. When asserted, it initializes the program counter and other internal registers, forcing the microprocessor to start execution from the address 0000H.

10. CLK (Clock)

CLK is an input signal that provides the timing for the microprocessor’s operations. It synchronizes the internal operations of the microprocessor with external devices.

Architecture of 8085 Microprocessor

Here is a architecture of 8085 microprocessor that shows what are the operations to be executed and how are they performed. In order to perform all the below operations, this processor needs a control unit, arithmetic logic unit, registers and buses etc.


  • It operates and stores on 8-bit data.
  • It executes arithmetic and logic operations.
  • It stores the data temporarily
  • 8085 keeps the instructions to be executed in order.

Functional Units of 8085 Microprocessor

Below are some of the functional units of 8085 Microprocessor-




It is an 8-bit register which is used to store the result of the operation executed by the ALU.

Arithmetic and Logic Unit

As it suggests, this microprocessor performs arithmetic and logical operations like Addition, Subtraction etc. on 8-bit.

General Purpose Register

This microprocessor contains 6 general purpose registers inside them and stores the 8-bit data for executing the program. The register are B,C,D,E,H and L and each of them can work in pair to hold 16-bit data. Their pairing combinations are – B-C, D-E, and H-L.

Program Counter

For the next instruction to be performed, this program counter which is a 16-bit register stores the memory address location. Hence, it is clear from the above information that in order to fetch an instruction from the memory, the microprocessor should be aware about its addresses.

Stack Pointer

It is also a part of a memory and contains 16-bit register. Stack is based on LIFO as the data stored in the stack will be in the serial format and this pointer generally stores the address of the last element stored in the stack.

When a new data is added in the stack, the pointer begins to point towards the next memory location and at the same time, when the data is deleted from the stack, the pointer starts pointing to the previous occupied memory location.

Temporary Register

It is an 8-bit register that are used by ALU in order to store the data on the temporary basis and these are not accessed by the programmer.

Flag Register

This register will have the current result that is generated by the ALU, but it is not actually generated result. However, it can be known as testing the data conditions. 8085 microprocessor has five different flags which shows five different conditions such as- carry, sign, zero, parity and auxiliary carry flags. But most often used is sign, carry and zero.

Register of Instruction and Decoder

This is an 8-bit register. The instruction fetched from the memory is stored in the instruction register and the decoder will decodes the information present in the instruction register.

Timing and Control Unit

To execute the operations, it provides the timing and control signals to microprocessor. Below are the timing and control signals that controls external and internal circuits –

    • Control Signals – READY, RD, WR, ALE
    • Status Signals – S0,S1, IO/M’
    • DMA Signals – HOLD, HLDA

Interrupt Control

As the name suggests, it controls the interruptions during the process. Moreover, the microprocessor executes the main program during this period. When there is any interruptions, it will shift the control from main program to process the requests that are incoming. Accordingly, the control moves back to the main program once the request is completed. We can see five interrupt signals in 8085 microprocessor like – INTR, RST 7.5, RST 6.5, RST 5.5 and TRAP.

Input /Output Serials

In this, we can find two instructions, SID (serial input data) and SOD (serial output data) used to control the serial data communication.

Address Buffer and Address-Data Buffer

The data saved in the stack pointer and program counter will be loaded into the address buffer and address-data buffer in order to communicate with the CPU. The memory and I/O chips are connected to these buses and the CPU will exchange the required data with the memory and I/O chips.

Address Bus and Data Bus

Address bus is unidirectional, as it carries the location to where the data is supposed to be stored. Meanwhile, data bus is bidirectional. It carries the data to be stored. They are used to transfer the data and their addresses to I/O devices.

Applications of 8085 Microprocessor

There is a huge change in the applications of devices post introduction of 8085 processor. lIt provides effective cost, minimal weight, use of minimal power, etc. Earlier they were designed for small and dedicated applications. However, now a days, they are used in various applications from small household to large applications. Below are some applications of 8085 microprocessor-

  • These microprocessors are used in switching.
  • They are used in banking and financial systems.
  • Used as programmable calculators.
  • Also used for Numeric control and environment monitoring systems.
  • Use in medical field.
  • They are used for communications.
  • They were used in personal computers.

Features of 8085 Microprocessor

  • These 8085 microprocessors are an 8-bit register that receives, operates, outputs 8-bit information simultaneously.
  • The capacity of this processor is 64KB.
  • This processor has 6200 transistors and built of single NMOS chip device.
  • Overall, there are 246 operational codes and 80 instructions are available.
  • This microprocessor is available in a DIP package of 40pins.
  • Internal clock generator is available.
  • It functions on a clock cycle that has a duty cycle of 50%.
  • The 8085 microprocessor shares its bus with the DMA controller in order to transfer huge information from I/O to memory and again memory to I/O.
  • It supports in handling interruption mechanism.
  • This microprocessor can be operated as a three-chip microcomputer with the use of IC 8355 and IC 8155 circuits.
  • 8085 microprocessor has 8-bit Input/Output address lines and this has the capacity to address 256 Input/Output port

Advantages and Disadvantages of 8085 Microprocessor

Advantages Disadvantages
High speed processing data Limitations on the data size
Intelligence is enabled on the systems Physical address space has its limits for the application.
Industrial process and office administration are automatic. Error occurs while digitizing the analog signals
More reliable, less cost, smaller in size and less number of components. The analog signals cannot be processed directly
Maintenance and operation is much easier. Most of the microprocessors do not support floating-point operations.
Flexibility to alter the system by the changing the software alone as the devices are programmable. Execution speed is slow.


The 8085 microprocessors has developed widely in various fields and it changed people’s lives. When compared to the systems, these 8085 microprocessors reduce the processing cost, consumption of power, increase in reliability, focuses on real time applications and faster in speed. This processor is used in smallest embedded system to largest main frames and super computers.

Also Read:
Microcontroller – Classification, Architecture, Application, Advantage
Integrated Circuit – Various Types, Construction, Application and Advantage
Chakrasthitha is a B.E (Medical Electronics) and has work experience in MatLab and Lab View Software as Design Engineer at BCS innovations and Manipal hospital as Biomedical Engineer. She is an author, editor and partner at Electricalfundablog.


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