What is 8085 Microprocessor? Explain the Functional Units of 8085 Microprocessor

The 8085 microprocessor is an 8-bit general-purpose microprocessor designed by Intel in 1976. It operates on a single +5V power supply, has 8-bit data bus, 16-bit address bus (can address 64KB of memory), and operates at a maximum clock frequency of 3.2 MHz.

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Functional Units of 8085 Microprocessor

The 8085 microprocessor can be divided into the following functional units:

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A. Accumulator and Register Unit

• Accumulator (A) — An 8-bit register used to store data for arithmetic and logical operations. The result of every ALU operation is stored here.
• General Purpose Registers — Six 8-bit registers: B, C, D, E, H, L. They can be used individually (8-bit) or as register pairs BC, DE, HL (16-bit).
• Temporary Register — Holds data temporarily during ALU operations (not accessible to programmer).

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B. Arithmetic and Logic Unit (ALU)

• Performs arithmetic operations (addition, subtraction, increment, decrement) and logical operations (AND, OR, XOR, complement).
• Operates on 8-bit data.
• The result is stored in the Accumulator and appropriate flags are set.

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C. Flag Register

• A special 5-bit register that indicates the status of the result after ALU operations.

• The five flags are:

• S (Sign Flag) — Set if result is negative

• Z (Zero Flag) — Set if result is zero

• AC (Auxiliary Carry Flag) — Set if carry from lower nibble to upper nibble

• P (Parity Flag) — Set if result has even number of 1s

• CY (Carry Flag) — Set if there is a carry out of MSB

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D. Program Counter (PC)

• A 16-bit register that holds the memory address of the next instruction to be fetched.
• It is automatically incremented after every instruction fetch.

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E. Stack Pointer (SP)

• A 16-bit register that points to the top of the stack in memory.
• Used during PUSH, POP, CALL, and RET operations.

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F. Timing and Control Unit

• Generates timing and control signals to coordinate all operations of the microprocessor.
• Important control signals include: RD̅ (Read), WR̅ (Write), ALE (Address Latch Enable), IO/M̅, READY, HOLD, HLDA.
• Synchronizes data transfer between microprocessor and peripherals.

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G. Instruction Register and Decoder

• Instruction Register (IR) — Stores the opcode of the current instruction fetched from memory.
• Instruction Decoder — Decodes the opcode and generates internal signals to execute the instruction.

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H. Interrupt Control Unit

• Handles 5 hardware interrupts: TRAP, RST 7.5, RST 6.5, RST 5.5, and INTR.
• TRAP is the highest priority (non-maskable), INTR is the lowest priority (maskable).

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I. Serial I/O Control

• Has two pins: SID (Serial Input Data) and SOD (Serial Output Data).
• Used for serial communication using RIM and SIM instructions.

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J. Address and Data Bus

• Address Bus — 16-bit unidirectional bus (A0–A15), can address $2^{16}$ = 64KB memory.
• Data Bus — 8-bit bidirectional bus (AD0–AD7), lower 8 address lines are multiplexed with data bus to reduce pin count.

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Conclusion

The 8085 microprocessor is a well-structured 8-bit processor with clearly defined functional units including ALU, registers, control unit, interrupt system, and buses — all working together to fetch, decode, and execute instructions efficiently.