8085 Microprocessor

Microprocessor 8085 | University Notes | Physics Notes| RRB JE | RRB Technician Grade 1 | Basic Science & Engineering | B.Sc. Physics Notes by Study Buddy Notes
Microprocessor 8085 | University Notes | Physics Notes| RRB JE | RRB Technician Grade 1 | Basic Science & Engineering | B.Sc. Physics Notes by Study Buddy Notes


The Intel 8085 microprocessor is an 8-bit microprocessor introduced by Intel in 1976. It was widely used in early personal computers and embedded systems due to its simplicity and effectiveness in handling various tasks. Named "8085" due to its 8-bit data bus and 5-volt power requirement, it is one of the most well-known processors in microprocessor history and remains a fundamental topic in electronics and microprocessor studies.

1. Overview of the 8085 Microprocessor

The Intel 8085 is a general-purpose, 8-bit microprocessor that is capable of addressing up to 64KB of memory. It is designed using NMOS technology, has a single power supply of +5V, and includes a 16-bit address bus and 8-bit data bus.

  • Data Bus Width: 8 bits, meaning it can process 8 bits of data simultaneously.
  • Address Bus Width: 16 bits, allowing it to address up to 64 KB of memory space.
  • Clock Speed: Operates at 3 MHz in typical applications.
  • Instruction Set: Contains 74 basic instructions with support for arithmetic, logic, branching, and control operations.

2. Architecture of the 8085 Microprocessor

The architecture of the 8085 microprocessor consists of several essential components, including the ALU, control unit, registers, and buses. Let’s break down each of these components:

a. Arithmetic and Logic Unit (ALU)

The ALU is the heart of the 8085 microprocessor. It performs all arithmetic operations (addition, subtraction) and logical operations (AND, OR, NOT, XOR). The ALU in the 8085 also has additional functionality for incrementing and decrementing operations.

b. Control Unit (CU)

The control unit manages and coordinates the operations of the microprocessor. It interprets instructions fetched from memory and generates signals to operate other components within the CPU, such as fetching, decoding, and executing instructions.

c. Registers

Registers in the 8085 are small, fast storage locations within the CPU, holding data temporarily during execution. The main registers include:

  • Accumulator (A): An 8-bit register that is primarily used for arithmetic and logical operations. Most operations in the 8085 involve the accumulator.

  • General Purpose Registers (B, C, D, E, H, L): These are 8-bit registers that can be used individually or as register pairs (BC, DE, HL) for 16-bit operations.

  • Special Purpose Registers:

    • Program Counter (PC): Holds the address of the next instruction to be executed.
    • Stack Pointer (SP): Points to the top of the stack in memory.
    • Flag Register: Contains five status flags that reflect the outcome of operations: Sign (S), Zero (Z), Auxiliary Carry (AC), Parity (P), and Carry (CY).

d. Buses

Buses are communication channels that facilitate data transfer among the different components of the microprocessor:

  • Address Bus: A 16-bit bus that specifies memory addresses, allowing the processor to access 64 KB of memory.
  • Data Bus: An 8-bit bidirectional bus that transfers data to and from memory or I/O devices.
  • Control Bus: Carries control signals generated by the control unit, managing various operations like memory read/write and I/O.

3. Instruction Set of the 8085 Microprocessor

The 8085 microprocessor supports a diverse instruction set, categorized as follows:

  • Data Transfer Instructions: Move data between registers, memory, and I/O ports (e.g., MOV, MVI, LDA, STA).
  • Arithmetic Instructions: Perform mathematical operations like addition, subtraction, increment, and decrement (e.g., ADD, SUB, INR, DCR).
  • Logical Instructions: Execute bitwise operations, complement, rotate, etc. (e.g., ANA, XRA, CPI).
  • Branching Instructions: Change the sequence of execution (e.g., JMP, JC, CALL, RET).
  • Control Instructions: Control operations related to processor control (e.g., NOP, HLT, DI, EI).

4. Working of the 8085 Microprocessor

The 8085 microprocessor follows a repetitive Fetch-Decode-Execute cycle for each instruction:

  1. Fetch: The microprocessor fetches the instruction from memory. The address of the instruction is placed on the address bus, and the data (instruction) is fetched over the data bus.

  2. Decode: The fetched instruction is decoded in the control unit to determine what operation needs to be performed.

  3. Execute: The decoded instruction is then executed by the microprocessor, which may involve performing arithmetic, logical, or data transfer operations.

  4. Store/Result: The result, if any, is stored in the accumulator or specified registers, and the status flags are updated accordingly.

5. Pin Diagram and Signals of 8085

The 8085 microprocessor has a total of 40 pins, each serving specific functions:

  • Power Supply:

    • Vcc: +5V power supply.
    • GND: Ground.

  • Data and Address Lines:

    • A8–A15: Higher order address bus (used for memory addressing).
    • AD0–AD7: Multiplexed address/data bus (lower order address and data lines).

  • Control and Status Signals:

    • ALE (Address Latch Enable): Activates the address latch, differentiating address and data.
    • RD: Indicates a memory read operation.
    • WR: Indicates a memory write operation.
    • IO/M: Distinguishes between memory and I/O operations.

  • Interrupts and Externally Initiated Signals:

    • INTR, RST7.5, RST6.5, RST5.5: Maskable interrupts for interrupt-based processing.
    • TRAP: A non-maskable interrupt.
    • RESET IN and RESET OUT: Used to reset the microprocessor.

  • Serial I/O Ports:

    • SID (Serial Input Data): Used to accept serial input data.
    • SOD (Serial Output Data): Used to transmit serial output data.

  • Clock Signals:

    • X1, X2: Crystal or oscillator connections for clock signal generation.
    • CLK OUT: Used to provide a clock signal to external devices.

6. Interrupts in the 8085 Microprocessor

The 8085 has a set of interrupts that allow the CPU to pause its current operations and execute a special service routine (ISR or Interrupt Service Routine). These interrupts are crucial for real-time systems.

  • TRAP: A non-maskable, highest priority interrupt.
  • RST 7.5, RST 6.5, RST 5.5: Maskable vectored interrupts.
  • INTR (Interrupt Request): A general-purpose interrupt that the processor can ignore if needed.

7. Applications of 8085 Microprocessor

The 8085 microprocessor is used in several applications due to its simplicity and versatility:

  • Educational Purposes: Often used in teaching microprocessor fundamentals.
  • Embedded Systems: Early embedded systems used 8085 microprocessors to perform control functions.
  • Instrumentation: Used in basic instrumentation for data acquisition and control.
  • Early Personal Computers: 8085 was a popular choice for early computing systems and home computers.

8. Advantages and Disadvantages of 8085

Advantages:

  • Simple Design: Easy to understand, making it suitable for educational purposes.
  • Low Power Consumption: Operates with a 5V power supply.
  • Good Support for Interrupts: The 8085 has an extensive interrupt system.

Disadvantages:

  • Limited Speed: The 3 MHz clock speed is slow compared to modern microprocessors.
  • Memory Limitations: Can only address up to 64 KB of memory, which is inadequate for advanced applications.
  • 8-bit Architecture: Limited processing power and data handling compared to 16-bit or 32-bit processors.

9. Diagram of 8085 Microprocessor Architecture

Microprocessor 8085 | University Notes | Physics Notes| RRB JE | RRB Technician Grade 1 | Basic Science & Engineering | B.Sc. Physics Notes by Study Buddy Notes
Microprocessor 8085 | University Notes | Physics Notes| RRB JE | RRB Technician Grade 1 | Basic Science & Engineering | B.Sc. Physics Notes by Study Buddy Notes


Diagram Description:

The architecture of the 8085 includes the following key components, typically illustrated as follows:

  • Central Rectangle (ALU): Positioned at the center of the diagram, the ALU performs arithmetic and logic operations.

  • Registers:

    • Located around the ALU, representing the Accumulator, General Purpose Registers (B, C, D, E, H, L), Stack Pointer, and Program Counter.

  • Control Unit (CU): Adjacent to the ALU, managing data flow between components.

  • Buses:

    • Data Bus: Connecting the registers, ALU, and memory for data transfer.
    • Address Bus: A separate bus from the ALU to memory, specifying the address location for reading/writing data.
    • Control Bus: Running across the control unit, directing operations based on the instruction set.

  • Clock Signal: Illustrated as a waveform symbol, providing timing for all operations.

The diagram offers a top-level view of how data moves within the 8085, highlighting the interaction between components.