8085 Microprocessor Architecture Quiz

8085 Microprocessor Architecture

• Developed: Intel, 1977
• Technology: NMOS
• Addressable Memory: 64KB
• Data Bus: 8-bit
• Address Bus: 16-bit
• Power Supply: +5V
• Clock Speed: 3.2 MHz
• Internal Architecture: Consists of Timing & Control Unit, Interrupt Control, Serial I/O Control, Registers, and ALU

ALU (Arithmetic Logic Unit)

• Performs arithmetic and logical operations:
  • Addition: Adding two numbers
  • Subtraction: Subtracting one number from another
  • Increment: Increasing the value of a number by 1
  • Decrement: Decreasing the value of a number y 1.
  • Logical AND: Logical AND operation on two numbers
  • Logical OR: Logical OR operation on two numbers
  • Shift left: Shifting the bits to the left
  • Shift right: Shifting the bits to the right
  • Rotate left: Rotating the bits to the left
  • Rotate right: Rotating the bits to the right
• Word Length: 8-bit, determined by internal data bus

Timing and Control Unit

• Responsible for generating timing signals for the execution of instructions.
• Manages the data flow and memory management between peripherals and CPU.
• Provides status, timing, and control signals to manage I/O and memory devices.

System Bus

• Used for communication between the memory and I/O devices.
• Consists of:
  • Data Bus: Bidirectional, carries data between microprocessor and memory. Size determines the type of operations performed.
  • Address Bus: Unidirectional, sends instructions from microprocessor to memory. Size determines the amount of memory that can be addressed.
  • Control Bus: Unidirectional and Bidirectional signals, used for control and co-ordination.

Registers

• General Purpose Registers: B, C, D, E, H, and L, each 8-bit. Used as pairs (BC, DE, and HL) to perform 16-bit operations.
• Accumulator: 8-bit register within ALU, used to store data and operation results.
• Flag Register: 5 flags that track the status of operations:
  • Carry Flag (CF): Indicates if an operation produced a carry.
  • Parity Flag (PF): Indicates if the result has an even or odd number of set bits.
  • Zero Flag (ZF): Indicates if the result of an operation is zero.
  • Sign Flag (SF): Indicates if the MSB of the result is set.
  • Auxiliary Carry Flag (AC): Indicates if there's a carry from bit 3 to bit 4.
• Program Status Word (PSW): Combination of the Accumulator and Flag Register (16-bit). Used for stack operations.
• Stack Pointer (SP): 16-bit register that points to the stack in memory, acting as a memory pointer.
• Program Counter (PC): 16-bit register that stores the address of the next instruction to be executed.

8085 Pin Diagram

• Address/Data Bus (A0-A15): Unidirectional, sends signals to peripheral devices. Combined for additional control signals.
• Status and Control Signals:
  • ALE (Address Latch Enable): Enables the lower 8-bits of the address bus during T1.
  • IO/M: Determines if the current operation is memory or I/O related.
  • S0, S1: 4 combinations determine operation status:
    • Opcode Fetch: Fetching instruction from memory.
    • Memory Read: Reading data from memory.
    • Memory Write: Writing data to memory.
    • I/O Read: Reading data from an I/O device.
    • I/O Write: Writing data to an I/O device.
    • Interrupt Acknowledge: Acknowledging an interrupt request.
    • Halt: Indicates the microprocessor is stopped.
• Clock Frequency & Power Supply
  • Vcc: +5V power supply
  • Vss: Ground reference
  • CLK (OUT): Clock signal for other devices
  • X1, X2: Used to determine the clock frequency (6 MHz for 3 MHz operation)
• Reset Signals
  • RESET IN: Resets the microprocessor (PC = 0)
  • RESET OUT: Used to reset other devices
• Serial I/O Ports:
  • SID (Serial Input Data): Input line for serial data.
  • SOD (Serial Output Data): Output line for serial data.
• Interrupts and Peripheral Signals
  • INTR: Interrupt request signal
  • INTA: Interrupt acknowledgment
  • RST 7.5, RST 6.5, RST 5.5: Maskable interrupts with varying priorities
  • TRAP: Non-maskable interrupt
  • HOLD: Signal to request a hold operation.
  • HLDA: Hold acknowledge
• DMA Signals:
  • HOLD: Requests control of the bus for DMA transfers.
  • HLDA: Acknowledges the HOLD request.

8085 Operations

• Memory Read: Reads data from memory.
• Memory Write: Writes data to memory.
• I/O Read: Reads data from an I/O device.
• I/O Write: Writes data to an I/O device.

8085 Addressing Modes

• Direct Addressing: Uses a specific memory address.
• Register Addressing: Operand is in a general-purpose register.
• Immediate Addressing: Operand is included in the instruction directly.
• Indirect Addressing: The instruction specifies the register containing the address of the operand.
• Implicit Addressing: No operands are explicitly specified.

8085 Interrupts

• External signals: Trigger interrupt handling routines.
• 5 interrupt signals:
  • TRAP: Non-maskable interrupt, highest priority.
  • RST 7.5, RST 6.5, RST 5.5: Maskable interrupts with decreasing priorities.
  • INTR: Maskable interrupt, lowest priority.
• Interrupt Service Routine (ISR): Code executed when an interrupt occurs.

8085 Applications

• Instrumentation: Digital kiosks, security systems, printing press control panels
• Entertainment: Gaming consoles, DVD Players
• Communication: Digital telephone sets, satellite communication
• Home Appliances: Washing machines, refrigerators, ovens
• Military: Various applications
• Traffic Light Control
• Data Acquisition Systems

Machine Cycles & Bus Timings

• Machine Cycle: Time required for a single instruction execution. Four types:
  • Opcode Fetch Cycle: Retrieves the instruction opcode from memory.
  • Memory Read Cycle: Reads data from memory if needed.
  • Memory Write Cycle: Writes data to memory if needed.
  • I/O Read/Write Cycle: Performs I/O operations.
• Clock Speed: Determines the rate of data processing (typically 3 MHz).
• Bus Cycle Timing: Timing diagram that illustrates signal transitions over time during each machine cycle.
• Control Signals: Manage data flow and device operation.
• Memory & I/O Timings: Specify access times, setup & hold times for data and address signals.

Instruction Set of 8085

• Data Transfer Instructions: Move data between registers and memory locations.
• Arithmetic Instructions: Perform arithmetic operations (add, subtract, increment, decrement).
• Logical Instructions: Perform logical operations (AND, OR, XOR, NOT).
• Control Transfer Instructions: Alter the flow of program execution (jumps, calls, returns).
• Input/Output Instructions: Interact with input/output devices.