8085 Microprocessor and its Operation

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The 8085 microprocessor is an 8 bit microprocessor designed by Intel and introduced in 1976. It is a widely used microprocessor in embedded systems and educational settings due to its simplicity and effectiveness. Here's an overview of its architecture and operation:

• Data Bus:
• 8-bit: It can handle 8 bits of data at a time, which means it processes 1 byte of data in a single operation.
• Address Bus:
• 16-bit: It can address up to $2^{16}$ = 65,536 memory locations, which translates to 64KB of addressable memory.
• Registers:
• General Purpose Registers: 6 registers (B, C, D, E, H, L) which can be used individually or in pairs (BC, DE, HL) for 16-bit operations.
• Accumulator: An 8-bit register (A) used for arithmetic and logic operations.
• Flag Register: Contains flags that indicate the status of the processor (Zero, Carry, Sign, Parity, and Auxiliary Carry Flags).
• Program Counter (PC):
• 16-bit register that holds the address of the next instruction to be executed.
• Stack Pointer (SP):
• 16-bit register that points to the current top of the stack in memory.
• Instruction Register (IR):
• Holds the current instruction being executed.
• Instruction Decoder:
• Decodes the instruction fetched from memory and generates the appropriate control signals.

Operations

• Fetch:
• The microprocessor fetches the instruction from memory using the address stored in the Program Counter (PC). The fetched instruction is then loaded into the Instruction Register (IR).
• Decode:
• The Instruction Decoder interprets the instruction and generates the necessary control signals to execute it.
• Execute:
• The microprocessor performs the operation specified by the instruction. This could involve arithmetic operations, data movement, logical operations or control operations.
• Write Back:
• Results of the operation, if any are written back to the designated registers or memory locations.

Instruction Set:

The 8085 has a rich set of instructions categorized into several types:

• Data Transfer Instructions: Move data between registers, memory and I/O ports (e.g. MOV, MVI, LXI, STA, LDA).
• Arithmetic Instructions: Perform arithmetic operations (e.g. ADD, SUB, MUL, DIV).
• Logic Instructions: Perform logical operations (e.g. AND, OR, XOR, CMP).
• Branch Instructions: Control the flow of the program (e.g. JMP, CALL, RET, NOP).
• Control Instructions: Control the operation of the processor (e.g. RIM, SIM, DI, EI).

Timing and Control:

The 8085 operates in a synchronous manner, meaning it relies on a clock signal to synchronize its operations. The clock speed of the 8085 is typically 3 MHz to 5 MHz. The timing of read and write operations is managed through a series of control signals generated by the microprocessor based on the clock signal.

Example Operation

Let's consider an example where the 8085 is executing a simple instruction:

• Instruction: MOV A, B
• Fetch: The microprocessor fetches the MOV A, B instruction from memory using the address in the PC.
• Decode: The Instruction Decoder identifies the MOV instruction and prepares to move the contents of register B into the Accumulator (A).
• Execute: The microprocessor copies the data from register B to the Accumulator.
• Write Back: The contents of register B are now in the Accumulator and the PC is updated to point to the next instruction.