·        Microprocessor 8085

  •        A microprocessor is a controlling unit of a micro-computer, fabricated on a small chip capable of performing Arithmetic Logical Unit (ALU) operations and communicating with the other devices connected to it.
  • ·     Microprocessor consists of an ALU, register array, and a control unit. ALU performs arithmetical and logical operations on the data received from the memory or an input device.
  • ·     Register array consists of registers identified by letters like B, C, D, E, H, L and accumulator. The control unit controls the flow of data and instructions within the computer.
  • ·      The Intel 8085 ("eighty-eighty-five") is an 8-bit microprocessor produced by Intel and Introduced in 1976 N-type metal-oxide-semiconductor chips to implement logic gates and other digital circuits.

Two types of buses used in Microprocessor

  •      The data bus carries the data to be stored, Data bus is bidirectional

  •    While address bus carries the location to where it should be stored, and address bus is unidirectional

     How does a Microprocessor Work?

    • The microprocessor follows a sequence: Fetch, Decode, and then Execute.
    • Initially, the instructions are stored in the memory in a sequential order.
    • The microprocessor fetches those instructions from the memory,
    • Then decodes it and Executes those instructions till STOP instruction is reached
    • Later, it sends the result in binary to the output port.
    • Between these processes, the register stores the temporarily data and ALU performs the computing functions.

List of Terms Used in a Microprocessor
  • Instruction Set − It is the set of instructions that the microprocessor can understand.
  • Bandwidth − It is the number of bits processed in a single instruction.
  • Clock Speed − It determines the number of operations per second the processor can
  • perform. It is expressed in megahertz (MHz) or gigahertz (GHz).It is also known as Clock Rate.
  • Word Length − It depends upon the width of internal data bus, registers, ALU, etc. An 8-bit microprocessor can process 8-bit data at a time. The word length ranges from 4 bits to 64 bits depending upon the type of the microcomputer.
  • Data Types − the microprocessor has multiple data type formats like binary, BCD, ASCII, signed and unsigned numbers.
Features of a Microprocessor
  • Cost-effective − the microprocessor chips are available at low prices and results its low cost.
  • Size − the microprocessor is of small size chip, hence is portable.
  • Low Power Consumption − Microprocessors are manufactured by using metal oxide semiconductor technology, which has low power consumption.
  • Versatility − the microprocessors are versatile as we can use the same chip in a number of applications by configuring the software program.
  • Reliability − the failure rate of an IC in microprocessors is very low, hence it is reliable.
Microprocessor 8085 Architecture
  • 8085 is pronounced as "eighty-eighty-five" microprocessor. It is an 8-bit microprocessor designed by Intel in 1977 using NMOS technology.
It has the following configuration 
  • 8-bit data bus
  • 16-bit address bus, which can address upto 64KB
  • A 16-bit program counter
  • A 16-bit stack pointer
  • Six 8-bit registers arranged in pairs: BC, DE, HL
  • Requires +5V supply to operate at 3.2 MHZ single phase clock
  • It is used in washing machines, microwave ovens, mobile phones, etc.
8085 consists of the following functional units
  • Accumulator
    • It is an 8-bit register used to perform arithmetic, logical, I/O & LOAD/STORE operations.
    • It is connected to internal data bus & ALU.
  • Arithmetic and logic unit
    • As the name suggests, it performs arithmetic and logical operations like Addition, Subtraction, AND, OR, etc. on 8-bit data.
  • General purpose register
    • There are 6 general purpose registers in 8085 processor, i.e. B, C, D, E, H & L. Each register can hold 8-bit data.
    • These registers can work in pair to hold 16-bit data and their pairing combination is like B-C, D-E & H-L.
  • Program counter
    • It is a 16-bit register used to store the memory address location of the next instruction to be executed. Microprocessor increments the program whenever an instruction is being executed, so that the program counter points to the memory address of the next instruction that is going to be executed.
  • Stack pointer
    • It is also a 16-bit register works like stack, which is always incremented / decremented by 2 during push & pop operations.
  • Temporary register
    • It is an 8-bit register, which holds the temporary data of arithmetic and logical operations.
  • Flag register
    • It is an 8-bit register having five 1-bit flip-flops, which holds either 0 or 1 depending upon the result stored in the accumulator.
    • These are the set of 5 flip-flops −
      •  Sign (S)
      •  Zero (Z)
      •  Auxiliary Carry (AC)
      •  Parity (P)
      •  Carry (C)
  • Instruction register and decoder
    • It is an 8-bit register. When an instruction is fetched from memory then it is stored in the Instruction register. Instruction decoder decodes the information present in the Instruction register.
  • Timing and control unit
    • It provides timing and control signal to the microprocessor to perform operations.
    • Following are the timing and control signals, which control external and internal circuits
      •  Control Signals: READY, RD’, WR’, ALE
      •  Status Signals: S0, S1, IO/M’
      •  DMA Signals: HOLD, HLDA
      •  RESET Signals: RESET IN, RESET OUT
  • Interrupt control
    • As the name suggests it controls the interrupts during a process. When a microprocessor is executing a main program and whenever an interrupt occurs, the microprocessor shifts the control from the main program to process the incoming request. After the request is completed, the control goes back to the main program.
    • There are 5 interrupt signals in 8085 microprocessor: INTR, RST 7.5, RST 6.5, RST 5.5, TRAP.
  • Serial Input/output control
    • It controls the serial data communication by using these two instructions: 
    • SID (Serial input data) and SOD (Serial output data)
  • Address buffer and address-data buffer
    • The content stored in the stack pointer and program counter is loaded into the address buffer and address-data buffer to communicate with the CPU. The memory and I/O chips are connected to these buses; the CPU can exchange the desired data with the memory and I/O chips.
  • Address bus and data bus
    • Data bus carries the data to be stored. It is bidirectional, whereas address bus carries the location to where it should be stored and it is unidirectional. It is used to transfer the data & Address I/O devices.
Microprocessor 8085


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