What is a Microprocessor? Block Diagram of 8085 Microprocessor

A Microprocessor is a programmable digital electronic chip that functions as the Central Processing Unit (CPU) of a computer. It fetches, decodes, and executes instructions from memory to perform arithmetic, logical, and control operations.

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Definition

A microprocessor is a single VLSI (Very Large Scale Integration) chip that contains the entire CPU — including the ALU, control unit, and registers. The Intel 8085 is an 8-bit microprocessor introduced in 1976, operating at a clock speed of 3-5 MHz with 40 pins and a 16-bit address bus capable of addressing 64 KB of memory.

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Block Diagram of 8085 Microprocessor

The block diagram consists of the following major blocks connected via internal data bus (8-bit):

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Explanation of Each Block

A. Accumulator (A Register)

• It is an 8-bit register that stores one operand and the result of ALU operations.
• It is the most frequently used register in data processing.

B. ALU (Arithmetic and Logic Unit)

• Performs arithmetic operations (addition, subtraction, increment, decrement) and logical operations (AND, OR, XOR, complement).
• It receives data from the accumulator and temporary register.
• Results are stored back in the accumulator.

C. Flag Register (Status Register)

• A special 5-bit register with flags: S (Sign), Z (Zero), AC (Auxiliary Carry), P (Parity), CY (Carry).
• These flags are set or reset based on the result of ALU operations.

D. Register Array (General Purpose Registers)

• Contains six 8-bit registers: B, C, D, E, H, L.
• They can be used as register pairs (BC, DE, HL) to hold 16-bit data or addresses.
• H-L pair is commonly used as a memory pointer.

E. Program Counter (PC)

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

F. Stack Pointer (SP)

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

G. Timing and Control Unit

• Generates timing and control signals (like RD', WR', ALE, IO/M') to synchronize all operations.
• Controls the flow of data between microprocessor, memory, and I/O devices.

H. Instruction Register and Decoder

• The instruction register holds the opcode of the current instruction being executed.
• The decoder interprets the opcode and activates appropriate control signals.

I. Address Buffer and Data Buffer

• Address buffer provides the 16-bit address (A0–A15) to the address bus.
• Data buffer is an 8-bit bidirectional buffer connected to the data bus (AD0–AD7, which is multiplexed with lower address lines).

J. Interrupt Control

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

K. Serial I/O Control

• SID (Serial Input Data) and SOD (Serial Output Data) pins allow serial communication one bit at a time.

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Conclusion

The 8085 microprocessor is a well-structured 8-bit processor with a clean architecture consisting of ALU, registers, timing & control unit, and interrupt mechanism. Its simple design makes it ideal for learning the fundamentals of microprocessor architecture and forms the basis for understanding advanced processors.