Introduction to 8085 Microprocessor Architecture




  1. Central Processing Unit (CPU) - CPU, Brain of Computer is the most important part of a computer as it is responsible for performing all the arithmetic and logical operations and also controls the computer system. When CPU was fabricated on single chip then this was called MICROPROCESSOR. A computer with a microprocessor as its CPU is known as a Microcomputer. When CPU reads the bit patterns one at a time, it initiates appropriate micro programs through the control unit, and performs the task specified in the instructions.
  2. Input / Output Devices - Input devices are devices that bring information into a computer while output devices are devices that bring information out of a computer. Input devices accept data and instructions from the user or from another computer system. Examples of input devices are keyboard or switches or analog to digital converters. Output devices return processed data back to the user or to another computer system. Input / Output devices are also known as peripheral devices and the process of attaching the devices to microprocessor is known as interfacing. It helps in two way communication between microprocessor and peripheral devices.
  3. Bus Organisation - Bus can be defined as a group of wires which carry bits and are the communication path between the microprocessor and peripherals. Buses can be categorized into: 1. Address Bus : It is used to carry 16-bit address. It is a group of 16 lines for an 8-bit microprocessor. It is unidirectional as the information transfers from microprocessor to I/O elements or memory. 2. Data Bus : It can be defined as group of data lines used for data flow. Generally, they are eight in number. It is bi-directional in nature as data can flow is both directions, to or from the microprocessor. 3. Control Bus : It is a combination of control lines which carry synchronization signals and provide timing signals. It is basically responsible for receiving and generating various signals for coordination and control of various operations involving external devices.
  4. Buffers - Buffer is a logic device that is just responsible for transferring data from input to output and it amplifies the current or power. It has one input and one output line. Buffer is used primarily to increase the driving capability of a logic circuit and hence it is also called as driver.
  5. Encoders - An encoder is a combinational logic circuit that essentially performs a 'reverse' decoder function. It accepts an active level on one of its inputs representing a digit, such as a decimal or octal digit, and converts it to a coded output such as Binary or BCD.
  6. Decoders - Decoder is used to detect the presence of a specified combination of bits (code) on its inputs and to indicate that presence by a specified output level.
  7. Latches - Latch is the simplest kind of sequential circuits that has only 2 states. It is a memory cell, which is capable of storing one bit of information. When the microprocessor sends an output, data are available on the data bus for only a few microseconds, therefore a latch is used to hold data for display.
  8. Pins and signals - 8085 is 40 pin microprocessor.

Introduction to 8085 Microprocessor Architecture

Description of each pin is here as under:

  1. Pin 1,2 (X1, X2) - A crystal oscillator is connected between X1 & X2. The frequency of crystal oscillator is divided by two therefore a crystal of 6MHz is used//.
  2. Pin 3 (RESET OUT) - This signal can be used for resetting the peripheral devices connected to microprocessor.
  3. Pin 4,5 (SOD,SID) - SID is a data line for serial input whereas SOD is a data line for serial output.
  4. Pin 6 (TRAP) - This pin is used for vectored non-maskable Trap interrupt. When signal on this pin is high, the processor immediately calls the subroutine at 0024H.
  5. Pin 7 (RST7.5) - This pin is vectored interrupt. When signal on this pin is high, the processor immediately calls the subroutine at 003CH.
  6. Pin 8 (RST6.5) - This pin is vectored interrupt. When signal on this pin is high, the processor immediately calls the subroutine at 0034H.
  7. Pin 9 (RST5.5) - This pin is vectored interrupt. When signal on this pin is high, the processor immediately calls the subroutine at 002CH.
  8. Pin 10 (INTR) - This pin is used for interrupt request. When signal on this pin is made high then it means that microprocessor is being requested to interrupt the execution of program and to call subroutine.
  9. Pin 11 (INTA) - This pin is used for interrupt acknowledgement. The signal is generated in response to INTR request.
  10. Pin 12-19 (AD0-AD1) - These are address data lines. The lower order address lines are shared with data lines.
  11. Pin 20 (Vss) - Ground reference
  12. Pin 21-28 (A8-A15) - These are higher order address line. Address lines are used to identify the memory locations. Therefore, the maximum size of the memory which can be connected to 8085 is 65536.
  13. Pin 29,33 (S0 & S1) - These are status signals which indicates the status of operation being performed by microprocessor.
  14. Pin 30 (ALE) - It is address latch enable. When the signal on this pin is high the lower order address bus carries the lower order address and when the signal is low the lower order address bus carries 8-bit data.
  15. Pin 31 (WR) - It is active low signal. When signal is low, the write operation is performed with memory of I/O devices.
  16. Pin 32 (RD) - It is active low signal. When signal is low, the read operation is performed with memory of I/O devices.
  17. Pin 34 (IO/M) - When signal on this pin is high, the reading and writing operation is performed with I/O devices and when it is low, the reading and writing operation is performed with memory.
  18. Pin 35 (READY) - The ready signal is an active low signal. When this signal is high, it indicates that I/O or memory is ready to send or receive data
  19. Pin 36 (RESET IN) - When the signal on this pin is low the program counter is initiated with 0000H, address and data buses are twisted, interrupt system is doubled and Microprocessor Unit (MPU) is reset.
  20. Pin 37 (CLKOUT) - The signal on this pin can be used as system clock for peripherals to synchronize their operation with microprocessor.
  21. Pin 38 (HLDA) - It is Hold Acknowledge. The signal is generated in response to HOLD request. It is active high signal.
  22. Pin 39 (HOLD) - The signal is generated by the peripheral devices such as DMA (Direct Memory Access) which requests the microprocessor to relieve the address and data buses.
  23. Pin 40 (Vcc) - A power supply of +5V.
  24. Registers - Register is a group of predefined memory location which is used to store data and result.
  25. Interrupt - It is a process by which some external device or peripheral informs microprocessor to become ready for data communication with the help of accepting the request made.

Architecture of 8085 -

Introduction to 8085 Microprocessor Architecture

 

Architecture of 8085 consists of following components:

  1. Arithmetic and Logic Unit (ALU)
  2. Register Array
  3. Timing and Control Unit
  4. Instruction Register and Instruction Decoder
  5. Interrupt Controller
  6. Serial I/O Ports
  • ALU - This unit includes the accumulator, the temporary register, the arithmetic and logic circuits and five flags.
    a. Accumulator - It is an 8 bit programmable register of 8085. All the arithmetic and logical operations are performed with the contents of accumulator and the result is stored in accumulator only.
    b. Temporary Register - It is an 8 bit non-programmable register used to store 8 bit data temporarily during an arithmetic and logical operation.
    c. Arithmetic and Logic Circuits - This unit performs all the arithmetic and logical operations on data coming from accumulator and temporary register.
    d. Flags - 8085 has 5 flags which reflect the status of arithmetic and logical operations performed on the contents of accumulator with some exceptions.
    The different flags are as follows:
    i. Sign Flag (S): This flag is used for operation on signed numbers. It is set when the result of an arithmetic operation is a negative number.
    ii. Zero Flag (Z): This flag is set when the result of an arithmetic or logical operation is zero.
    iii. Carry Flag (CY): Carry flag is set when carry is generated during an arithmetic operation.
    iv. Auxiliary Carry (AC): This flag is set when a carry is generated during the addition of D3 bits of two numbers and passed to D4 bit.
    v. Parity Flag (P): Parity flag is set when the result of an arithmetic or logical operation has even parity and it is reset if result carries odd parity.
  • Register Array - 8085 consists of six general purpose 8 bit programmable registers called B,C,D,E,H and L. These registers can be used in pairs as BC, DE and HL for 16 bit operations. There are two 8 bit non-programmable temporary registers called W and Z. There are 2 special purpose 16 bit programmable registers called stack pointer (SP) and program counter (PC). The program counter is used to store 16 bit address of the next byte to be fetched from the memory or address of next instruction to be executed. The SP is a 16 bit programmable register which is used to store the address of top of stack.
  • Timing and Control Unit - This unit synchronizes all operations of microprocessor with clock and generates control signal necessary for communication between the microprocessor and peripherals.
  • Instruction Register and Instruction Decoder - Instruction register is a 8 bit non-programmable register used to store 8 bit instruction code when Opcode is fetched from memory. Instruction decoder decodes the instruction codes and establishes the sequence to be followed.
  • Interrupt Controller - This unit accepts the different interrupt and generates the interrupt acknowledgement if required.
  • Serial I/O Ports - This unit communicates the data serially between microprocessor and the peripherals.


Frequently Asked Questions

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