Computer Organization Chapter 14.4: Instruction Pipelining

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Questions and Answers

What is the first stage of pipelining, where the CPU retrieves an instruction from memory?

Fetch Instruction (FI)

What is the purpose of the Calculate Operands (CO) stage in pipelining?

To calculate the effective address of the operands if required

What is the primary function of the Decode Instruction (DI) stage in pipelining?

To decode the fetched instruction and determine the operation and operands needed

What is the final stage of pipelining, where the results of the execution are stored?

Write Operands (WO) Signup and view all the answers

What is the benefit of using a pipeline architecture in computer organization?

Reduced execution time for multiple instructions Signup and view all the answers

What is a conditional branch instruction in computer programming?

An instruction that may or may not cause branching, depending on a condition Signup and view all the answers

What is the main difference between an unconditional branch and a conditional branch?

An unconditional branch always results in branching, while a conditional branch may or may not Signup and view all the answers

What is the purpose of the Execute Instructions (EI) stage in pipelining?

To perform the actual operation indicated by the instruction Signup and view all the answers

What is the primary consideration for designers to achieve high-performance in pipelining?

To consider the overhead involved in moving data from buffer to buffer and in performing various preparation and delivery functions at each stage of the pipeline. Signup and view all the answers

What is the consequence of increasing the number of stages in a pipeline?

The logic controlling the gating between stages becomes more complex than the stages being controlled. Signup and view all the answers

What is the cycle time of an instruction pipeline?

The time needed to advance a set of instructions one stage through the pipeline. Signup and view all the answers

What happens to instructions I4 through I7 in the pipeline at time 8?

They are flushed from the pipeline, leaving only instructions I3 and I15 in the pipeline. Signup and view all the answers

What is the impact of latching delay on instruction cycle time?

It adds to the instruction cycle time, increasing the overall execution time. Signup and view all the answers

What is the total time required to execute n instructions in a pipeline with k stages, assuming no branches?

The total time can be determined by the pipeline's architecture and design, considering the cycle time and number of stages. Signup and view all the answers

What is the primary function of branch instructions in program execution?

To implement control flow in program loops and conditionals. Signup and view all the answers

What happens to the pipeline when a conditional branch instruction is executed?

The pipeline is cleared of instructions that are not useful. Signup and view all the answers

What is the performance penalty incurred due to conditional branches?

The pipeline is cleared, and no instructions complete during the clearing process. Signup and view all the answers

What is the role of the execute stage in the pipeline?

To execute instructions, including conditional branches. Signup and view all the answers

What is the purpose of the instruction decoding stage in the pipeline?

To determine the operation and operands of an instruction. Signup and view all the answers

How are operands calculated in the pipeline?

During the operand calculation stage. Signup and view all the answers

What is the benefit of a pipelined architecture?

It allows for the simultaneous execution of multiple stages of instruction processing. Signup and view all the answers

What is the significance of the time unit 7 in the pipeline operation?

It is when the conditional branch outcome is determined. Signup and view all the answers

Study Notes

Instruction Pipelining

The CPU fetches instructions from memory in the Fetch Instruction (FI) stage.
In the Decode Instruction (DI) stage, the instruction is decoded to determine the operation and operands needed.
If required, effective address calculation occurs in the Calculate Operands (CO) stage.
The CPU retrieves operands from memory or registers in the Fetch Operands (FO) stage.
The actual operation is performed in the Execute Instructions (EI) stage.
Results are stored back into memory or registers in the Write Operands (WO) stage.

Two Stage Instruction Pipeline

A six-stage pipeline can reduce execution time for 9 instructions from 54 time units to 14 time units.

Conditional Branch

A branch instruction can be either unconditional (always results in branching) or conditional (may or may not cause branching, depending on some condition).
Branch instructions are used to implement control flow in program loops and conditionals.

Effect of Conditional Branch on Instruction Pipeline Operation

When a conditional branch is executed, the pipeline must be cleared of instructions that are not useful.
This can result in a performance penalty due to the uncertainty of which instruction will come next.

Six Stage Instruction Pipeline

The pipeline can be depicted as a sequence of events, with time progressing vertically down the figure.

Alternative Pipeline Depiction

The pipeline can be depicted as a series of stages, with each row showing the state of the pipeline at a given point in time.

Pipeline Performance

The cycle time of an instruction pipeline is the time needed to advance a set of instructions one stage through the pipeline.
The cycle time can be determined by considering the number of stages and the number of instructions being processed.
Factors that affect pipeline performance include:
- Overhead involved in moving data between stages
- Control logic required to handle memory and register dependencies
- Latching delay (time taken for pipeline buffers to operate)