8085 Instruction Cycle

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The 8085 microprocessor instruction cycle is the process by which the microprocessor fetches, decodes, and executes an instruction. Each instruction cycle consists of several phases and understanding this cycle is crucial for grasping how the microprocessor operates. Here's a detailed explanation of the 8085 instruction cycle:

1. Instruction Fetch (IF):

• Purpose: To fetch the instruction from memory.
• Steps:
• Addressing: The microprocessor places the address of the instruction on the Address Bus (Using the Program Counter).
• Read Operation: The microprocessor sends a Read signal to the memory. The instruction is read from the memory location specified by the Address Bus.
• Loading: The fetched instruction is placed int Instruction Register (IR).
• Program Counter Update: The Program Counter (PC) is incremented to point to the address of the next instruction.

2. Instruction Decode (ID);

• Purpose: To decode the fetched instruction and prepare for execution.
• Steps:
• Decoding: The Instruction Register (IR) holds the fetched instruction, which is decoded by the Instruction Decoder. The decoder translates the instruction into control signals.
• Control Signal Generation: The decoded instruction generates specific control signals needed for execution. This may involve reading or writing data, or performing arithmetic operations.

3. Execution (EX):

• Purpose: To execute the decoded instruction.
• Steps:
• Execution of Operation: The microprocessor performs the operation specified by the instruction. This might involve arithmetic operations (e.g. addition, subtraction), data transfer (e.g. moving data between register), or logical operations (e.g. AND, OR).
• Addressing: If the instruction involves memory or I/O operations, the microprocessor may place addresses on the Address Bus and perform read/write operations.

4. Memory Read/Write (if needed):

• Purpose: To read data from or write data to memory or I/O ports.
• Steps:
• Read Operations: For instructions that require reading data from memory, the microprocessor places the address on the address Bus and sends a Read signal. The data is the read from memory and placed onto the Data Bus.
• Write Operation: For instructions that require writing data to memory, the  microprocessor places the address and data on the Address and Data Buses, respectively and sends a Write signal.

5. Interrupt Handling (if applicable):

• Purpose: To handle any pending interrupts.
• Steps:
• Checking: The microprocessor checks if there are any interrupts pending.
• Service Routine: If an interrupt is pending, the microprocessor saves he current state and jumps to the appropriate interrupt service routine (ISR) to handle the interrupt.

Cycle Timing:

The timing for these phases is governed by the clock signal provided to the microprocessor. Each phase of the instruction cycle is synchronized with clock cycles, ensuring that operations are completed in a systematic and orderly manner.

Instruction Cycle Timing Diagram

Here's a simplified textual timing diagram for better understanding:

+--------+--------+--------+--------+--------+--------+
| T1     | T2     | T3     | T4     | T5     | T6     |
+--------+--------+--------+--------+--------+--------+
| IF     | IF     | ID     | ID     | EX     | EX     |
|        |        |        |        |        |        |
|        |        |        |        |        |        |
+--------+--------+--------+--------+--------+--------+
T1, T2: Instruction Fetch Phase - Fetching the instructions from memory.
T3, T4: Instruction Decode Phase - Decoding the fetched instruction.
T5, T6: Execution Phase - Executing the instruction, possibly including memory or I/O operations.