关于MIPS正确计算方法的技术咨询：两种计算示例结果差异解惑

Hey there, let's get to the bottom of this MIPS calculation mix-up—your first example is totally correct, and the second one has a straightforward arithmetic mistake that's throwing everything off.

First, let's lock down what MIPS (Million Instructions Per Second) actually means: it measures how many millions of instructions a processor can execute in one second. The core formula you can always rely on is:

MIPS = (Clock Cycles Per Second) / (CPI × 1,000,000)

Or put more simply: since Instructions Per Second = Clock Frequency / CPI, then MIPS = Instructions Per Second / 1,000,000. That's the foundation of all correct calculations.

Your First Example (100% Correct) #

Let's recap to confirm:

• Clock frequency: 1.5 GHz = 1.5 × 10⁹ Hz
• CPI: 0.6
• Instructions Per Second = (1.5 × 10⁹) / 0.6 = 2.5 × 10⁹
• MIPS = (2.5 × 10⁹) / 10⁶ = 2500 MIPS
  This follows the formula perfectly—no missteps here.

The Confusing Second Example (Where It Went Wrong) #

The other source you found made a critical error in its calculation steps. Let's fix it:

• Clock frequency: 600 MHz = 600 × 10⁶ Hz
• CPI: 3

Your approach to this case was right:

Instructions Per Second = (600 × 10⁶) / 3 = 200 × 10⁶
MIPS = (200 × 10⁶) / 10⁶ = 200 MIPS

The wrong calculation from that source (600/3 = 200; 200/10^6 = 0.0002 MIPS) messed up by double-counting the million factor. When using 600 (MHz), that value already represents 600 × 10⁶ cycles per second. Dividing the resulting 200 (which is 200 × 10⁶ instructions per second) by 10⁶ again is unnecessary and leads to the absurdly low number.

Quick Simplified Trick for MHz #

Here's a handy shortcut when your clock frequency is in MHz: since MHz is already in millions of cycles per second, the 10⁶ terms cancel out. So you can just use:

MIPS = Clock Frequency (MHz) / CPI

For the 600 MHz, 3 CPI example, that's 600 / 3 = 200 MIPS—exactly the same result as the full calculation. The key is to pick one unit system (Hz or MHz) and stick with it without extra, unnecessary divisions.

内容的提问来源于stack exchange，提问作者Kasun Sampath Weligamage