Understanding CPUs

More detail on CPUs. “Guy in Box” analogy, Clock Speed, Codebook, Databuses explained.

Published

September 16, 2024

Modified

August 8, 2025

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What if all computers contained some guy, trapped in a box? Something to consider. Image source: unknown

Introduction

Last week one component we learned was the CPU (Central Processing Unit): A complex collection of electronic circuits on one or more integrated circuits (chips).

We learned that the CPU is responsible for:

  • Executing the instructions in a software program.
  • Communicating with other parts of the computer system

Today we’re going to take a deep dive into how the CPU carries out those responsibilities.

Lesson Overview

More detail on CPUs. “Guy in Box” analogy, Clock Speed, Codebook, data buses explained.

CPU Analogy: Guy in a Box

Let’s start with an analogy: CPUs are a “Guy-in-a-box”. This guy:

  • can perform any mathematical function quickly.
  • lives in a closed-up box so direct communication is impossible.

Let’s say have 16 light bulbs: 8 for the guy, and 8 for us.

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  • The light bulbs:
    • can be controlled (turned on or off) by us and by the guy-in-the-box
    • Are synchronized (bulb 1 for us has the same value as bulb 1 for the guy)
  • We can share messages by setting the light bulbs on/off, as long as we:
    • Agree on schedule (not changing the lightbulbs at the same time)
    • Agree on meaning (share the same interpretation of lightbulb being on/off)
  • This arrangement is called a data bus: a method for transporting data from one component to another using electrical circuits

Registers

So far, we can send/receive “messages” from the light bulbs. But, in order to “remember” a message, the guy needs at least a few workstations ( registers ) to copy our messages, otherwise the messages will be lost every time the data bus updates.

  • Registers are places to store a very small amount of data temporarily
  • Registers are usually the around the 1-4x the size of the data bus
  • We can give the registers names to distinguish them (AX, BX, CX, DX for example)

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Codebook: Agreeing on meaning

It’s important to have common definitions for the meaning of the lightbulbs being on/off to understand messages sent by this method

CPUs are designed with a codebook (instruction set) that defines the commands that we can give to the guy in the box

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Clock: Agreeing on schedule

Now we are ready to communicate with the guy in the boxWe can send them the first command by lighting up the bulbs on the data bus.

But how do they know when we have finished setting up the light bulbs?

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  • We can use a bell activated by a button.
  • A real computer uses a special wire called the clock wire (CLK). The CLK wire is turned on and off on a precise and regular schedule. This is called a clock cycle.
  • Each time the clock turns on, the state of our lightbulbs is shared with the guy in the box
  • The guy then has until the clock turns off to send a message back
  • The next time the clock turns on, a new message is sent to the CPU
  • The time between each clock cycle is the speed that a CPU can perform a given task
  • The maximum number of clock cycles a CPU can handle in given period of time is called the clock speed.

Sample Guy in a Box Problem

Using the following codebook, ask the “Guy in a box” to calculate 2+3

You must send a series of commandsto the CPU. The CPU will act on each command and give you an answer.

Your answer should be a sequence of numbers you could send to the guy in the box to get the right answer.

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Solution

10000000  The next line is a number, put it in AX
00000010  The number 2
10010000  The next line is a number, put it in BX
00000011  The number 3
10110000  Add AX to BX and put the result in AX
11000000  Place the value of AX on the external data bus

A set of commands such as this is called a program. A program is a series of command sent to a CPU in a specific order for the CPU to perform work.

Pipelining

  • Almost all CPU instructions feature a set of 4 stages:
    • Fetch : retrieve the instruction, address, and data, write them to registers
    • Decode : search for instruction code in codebook, verify it is a real instruction, and that data/address are valid
    • Execute : perform the instruction, update registers with results
    • Write: write to databus to share results
  • Rather than have each instruction repeat all of these steps, we can organize CPUs further by specializing our “guy in a box” for each stage
  • This is called pipelining , and it allows for greater efficiency in CPU programming

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CPU Properties

The most important properties that define CPU behavior are speed and bus width: that is, how fast the clock cycle repeats, and how much data per clock cycle the CPU can process.

Clock Speed

We measure CPU speed : in cycles per second, e.g. megahertz (MHz) or gigahertz (GHz).

Unit of Measurement: Hertz, usually written as Hz, denote frequency. 1Hz is “one time per second”, 10Hz is “ten times per second”.

We can use SI Prefixes (International System of Unit Prefixes) to compare magnitudes easily.

You will see many of these in computer science over and over again – get to know them well!

SI Prefixes for Frequencies
Unit Meaning Definition Examples
Hz Once per second 1/s, or 1Hz 1-2Hz: human heart-rate at rest (1Hz = 60 beats per minute, 2Hz = 120 bpm)
24Hz: framerate of historic cinema
20-60Hz: frequency range for bass notes (C0 = 16.35Hz to C2 = 65.4Hz)
60-144Hz: framerate of most monitors
kHz Thousand times per second 1000 Hz 0.1-2kHz: frequency range for midrange notes (A2=0.110kHz, C7=2kHz)
20kHz: maximum frequency audible to human ear
740 kHz: Intel 4004 processor clock speed (1971)
MHz Million times per second 1000000 Hz 8MHz: fastest CPU clock speed in the 70s
35MHz: fastest CPU clock speed in the 80s
500-1000MHz: fastest CPU clock speed in the 90s)
GHz Billion times per second 1000000000 Hz 2-5GHz: Almost all consumer CPUs in 2024 have base clock speeds in this range
9.117 GHz: fastest CPU clock speed ever recorded (2024)

Bus width

The amount of data processed at a time depends on the “number of lightbulbs” or the width of the data bus

Bus width is measured in bits, e.g. 8-bits, 16-bit, 32-bit, 64-bit, etc.

Question: how many bits do you need in bus width to represent the Guy in the Box and their codebook from the previous problems?

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In actual computers, the CPU receives instructions , addresses memory , and data on separate buses.

This allows the CPU to send AND receive the following information at the same time:

What command to perform

Where in memory the result should be stored

What data to use to perform the command

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Figure from In One Lesson on Youtube (nice visualization)

Clock Speed vs bus width compared

  • You can double check that you understand a scientific concept by checking what units of measurement that concept is measured by:
  • Highway
    • Number of lanes: (a number)
    • Speed limit: (unit distance per time elapsed, e.g. km/h)
  • System bus
    • Bus width: (a number of bits, eg a 32-bit processor)
    • Bus speed: (unit information per time elapsed, e.g. bits/second)

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Exercises

Knowledge check

Additional references

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