Concept of pipelining

  

Concept of Pipelining in Microprocessors

Pipelining is a technique used in modern microprocessors to improve their performance by allowing multiple instruction phases to be processed simultaneously. It is similar to an assembly line in a factory, where different stages of a product are worked on concurrently, increasing the overall throughput and efficiency.

In a pipelined processor, the execution of an instruction is broken down into several stages, and different instructions are at different stages of execution at the same time. This allows the processor to work on multiple instructions in parallel, thereby increasing the overall processing speed.

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How Pipelining Works

A typical instruction cycle consists of several stages:

1. Instruction Fetch (IF): The processor fetches the instruction from memory.
2. Instruction Decode (ID): The instruction is decoded to determine what operation needs to be performed.
3. Execute (EX): The operation specified by the instruction is carried out (e.g., arithmetic operations, address calculation).
4. Memory Access (MEM): If needed, the instruction accesses memory (load or store operations).
5. Write Back (WB): The result of the instruction is written back to a register or memory.

In a non-pipelined processor, these stages would occur sequentially, one after the other. For example, the processor would first fetch the instruction, then decode it, execute it, etc. This would mean that every instruction has to wait for the previous one to finish all stages before starting the next.

In a pipelined processor, multiple instructions are processed at different stages simultaneously. The pipeline allows the processor to execute one instruction per cycle, assuming there are no dependencies between the instructions.

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Pipelining Stages in 8086 Microprocessor

In the case of the 8086 microprocessor, the Bus Interface Unit (BIU) and the Execution Unit (EU) work in parallel to implement pipelining.

1. BIU (Bus Interface Unit):
• This unit is responsible for fetching instructions and calculating addresses. It operates independently of the Execution Unit.
• The BIU uses a 6-byte instruction queue, which holds the next few instructions so that the EU can always have an instruction ready to execute.
2. EU (Execution Unit):
• This unit decodes and executes instructions fetched by the BIU.
• While the BIU fetches the next instruction, the EU executes the current one.

This parallel operation between the BIU and the EU ensures that multiple instructions are processed at different stages, which improves overall throughput.

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Example: Instruction Pipelining

Let’s look at a simplified example with four instructions:

Instruction	IF (Fetch)	ID (Decode)	EX (Execute)	MEM (Memory)	WB (Write Back)
I1	Stage 1	Stage 2	Stage 3	Stage 4	Stage 5
I2	Stage 2	Stage 3	Stage 4	Stage 5	Stage 6
I3	Stage 3	Stage 4	Stage 5	Stage 6	Stage 7
I4	Stage 4	Stage 5	Stage 6	Stage 7	Stage 8

• I1: The first instruction starts by fetching in stage 1. It moves to decode (stage 2), execute (stage 3), and so on.
• I2: The second instruction starts fetching in stage 2, while I1 is decoding in stage 2. As I1 moves to the next stage, I2 moves to the next one, and so forth.

By the time the processor completes the execution of I1, it has already fetched and decoded I2, and so on. This parallel processing of instructions leads to faster execution times.

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Advantages of Pipelining

1. Increased Throughput:
• Pipelining increases the throughput of the processor, as multiple instructions can be processed simultaneously in different pipeline stages.
2. Improved CPU Utilization:
• All parts of the processor are used efficiently, as the execution unit and the bus interface unit can work concurrently.
3. Faster Execution:
• Instructions can be processed faster, as the processor doesn’t have to wait for one instruction to complete before starting the next one.

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Challenges in Pipelining

1. Data Hazards:
• Occur when an instruction depends on the result of a previous instruction, causing delays.
• Example: If instruction 2 is trying to use the result of instruction 1, the pipeline has to wait for instruction 1 to finish.
2. Control Hazards:
• Occur when instructions depend on branches (like conditional jumps), causing uncertainty in the instruction sequence.
• Example: If instruction 1 is a conditional jump, the processor may need to determine the target of the jump before continuing to the next instruction.
3. Structural Hazards:
• Occur when hardware resources are not available to handle all the instructions in the pipeline simultaneously.
• Example: If there aren’t enough memory ports to fetch instructions and access memory at the same time.

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Conclusion

Pipelining is a technique that significantly boosts the performance of microprocessors by allowing multiple instruction stages to run concurrently. While it introduces some complexities, such as data hazards and control hazards, the benefits of improved throughput and faster processing make it an essential feature in modern processors.