Question

What is the difference between clock speed and instruction cycle time?

• A. Clock speed is faster than instruction cycle time.
• B. Instruction cycle time is faster than clock speed.
• C. They are the same thing.
• D. Clock speed indicates the number of cycles per instruction.

Hint:

Think of it like a factory assembly line. The "clock speed" is how fast the conveyor belt is moving (e.g., 1 meter per second). An "instruction" is like building a complete product, which requires it to pass through multiple stations on the belt. The time to build the product (instruction cycle time) will be much longer than the time it takes for the belt to move one meter (clock cycle time).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Clock Speed: This is a measure of the rate at which a CPU's internal clock generates pulses, or ticks. It's measured in Hertz (Hz), typically Gigahertz (GHz), meaning billions of cycles per second. Instruction Cycle: This is the process a CPU goes through to execute a single machine-level instruction (e.g., fetch, decode, execute, write-back). Instruction Cycle Time: This is the time it takes to complete one instruction cycle. A single instruction almost always requires multiple clock cycles to complete. The number of cycles needed is measured by a metric called CPI (Cycles Per Instruction).
Step 2: Detailed Explanation:
Let's analyze the relationship and the options:
- Time for one clock cycle = \(1 / \text{Clock Speed}\). For a 3 GHz CPU, one clock cycle takes \(1 / (3 \times 10^9)\) seconds. - Time for one instruction = \(\text{CPI} \times (\text{Time for one clock cycle})\). Since CPI is typically greater than 1, the time to execute one instruction (instruction cycle time) is longer than the time for one clock cycle. Now let's evaluate the options, keeping in mind that "faster" is used informally.
Clock speed is faster than instruction cycle time: This compares a rate (speed) to a time. While dimensionally inconsistent, the intended meaning is that the rate of clock ticks is much faster than the rate of instruction completions. This is correct. Many clock ticks occur during the time it takes to execute one instruction.
Instruction cycle time is faster than clock speed: This is incorrect. The time for an instruction is longer than the time for a clock cycle.
They are the same thing: This is incorrect.
Clock speed indicates the number of cycles per instruction: This is incorrect. That metric is CPI. Clock speed is cycles per second.
Step 3: Final Answer:
The best description, despite the imprecise language, is that the clock speed (rate) is faster than the instruction completion rate, meaning many clock cycles occur within one instruction cycle time.