Home > Articles > Programming > Games

  • Print
  • + Share This
This chapter is from the book

The Game Loop

The game loop is the overall flow control for the entire game program. It’s a loop because the game keeps doing a series of actions over and over again until the user quits. Each iteration of the game loop is known as a frame. Most real-time games update several times per second: 30 and 60 are the two most common intervals. If a game runs at 60 FPS (frames per second), this means that the game loop completes 60 iterations every second.

There can be many variations of the game loop, depending on a number of factors, most notably the target hardware. Let’s first discuss the traditional game loop before exploring a more advanced formulation that’s designed for more modern hardware.

Traditional Game Loop

A traditional game loop is broken up into three distinct phases: processing inputs, updating the game world, and generating outputs. At a high level, a basic game loop might look like this:

while game is running
    process inputs
    update game world
    generate outputs

Each of these three phases has more depth than might be apparent at first glance. For instance, processing inputs clearly involves detecting any inputs from devices such as a keyboard, mouse, or controller. But those aren’t the only inputs to be considered; any external input must be processed during this phase of the game loop.

As one example, consider a game that supports online multiplayer. An important input for such a game is any data received over the Internet, because the state of the game world will directly be affected by this information. Or take the case of a sports game that supports instant replay. When a previous play is being viewed in replay mode, one of the inputs is the saved replay information. In certain types of mobile games, another input might be what’s visible by the camera, or perhaps GPS information. So there are quite a few potential input options, depending on the particular game and hardware it’s running on.

Updating the game world involves going through everything that is active in the game and updating it as appropriate. This could be hundreds or even thousands of objects. Later in this chapter, we will cover exactly how we might represent said game objects.

As for generating outputs, the most computationally expensive output is typically the graphics, which may be 2D or 3D. But there are other outputs as well—for example, audio, including sound effects, music, and dialogue, is just as important as visual outputs. Furthermore, most console games have “rumble” effects, where the controller begins to shake when something exciting happens in the game. The technical term for this is force feedback, and it, too, is another output that must be generated. And, of course, for an online multiplayer game, an additional output would be data sent to the other players over the Internet.

We’ll fill in these main parts of the game loop further as this chapter continues. But first, let’s look at how this style of game loop applies to the classic Namco arcade game Pac-Man.

The primary input device in the arcade version of Pac-Man is a quad-directional joystick, which enables the player to control Pac-Man’s movement. However, there are other inputs to consider: the coin slot that accepts quarters and the Start button. When a Pac-Man arcade cabinet is not being played, it simply loops in a demo mode that tries to attract potential players. Once a quarter is inserted into the machine, it then asks the user to press Start to commence the actual game.

When in a maze level, there are only a handful of objects to update in Pac-Man—the main character and the four ghosts. Pac-Man’s position gets updated based on the processed joystick input. The game then needs to check if Pac-Man has run into any ghosts, which could either kill him or the ghosts, depending on whether or not Pac-Man has eaten a power pellet. The other thing Pac-Man can do is eat any pellets or fruits he moves over, so the update portion of the loop also needs to check for this. Because the ghosts are fully AI controlled, they also must update their logic.

Finally, in classic Pac-Man the only outputs are the audio and video. There isn’t any force feedback, networking, or anything else necessary to output. A high-level version of the Pac-Man game loop during the gameplay state would look something like what is shown in Listing 1.1.

Listing 1.1 Theoretical Pac-Man Game Loop

while player.lives > 0
   // Process Inputs
   JoystickData j = grab raw data from joystick

   // Update Game World
   update player.position based on j
   foreach Ghost g in world
      if player collides with g
         kill either player or g
         update AI for g based on player.position

   // Pac-Man eats any pellets

   // Generate Outputs
   draw graphics
   update audio

Note that the actual code for Pac-Man does have several different states, including the aforementioned attract mode, so these states would have to be accounted for in the full game’s code. However, for simplicity the preceding pseudo-code gives a representation of what the main game loop might look like if there were only one state.

Multithreaded Game Loops

Although many mobile and independent titles still use a variant of the traditional game loop, most AAA console and PC titles do not. That’s because newer hardware features CPUs that have multiple cores. This means the CPU is physically capable of running multiple lines of execution, or threads, at the same time.

All of the major consoles, most new PCs, and even some mobile devices now feature multicore CPUs. In order to achieve maximum performance on such systems, the game loop should be designed to harness all available cores. Several different methods take advantage of multiple cores, but most are well beyond the scope of this book. However, multithreaded programming is something that has become prevalent in video games, so it bears mentioning at least one basic multithreaded game loop technique.

Rendering graphics is an extremely time-consuming operation for AAA games. There are numerous steps in the rendering pipeline, and the amount of data that needs to be processed is rather massive; some console games now render well over a million polygons per frame. Suppose it takes 30 milliseconds to render the entire scene for a particular game. It also takes an additional 20 milliseconds to perform the game world update. If this is all running on a single thread, it will take a total of 50 milliseconds to complete a frame, which will result in an unacceptably low 20 FPS. But if the rendering and world update could be completed in parallel, it would only take 30 milliseconds to complete a frame, which means that a 30 FPS target can be achieved.

In order for this to work, the main game loop thread must be changed so it processes all inputs, updates the game world, and outputs anything other than the graphics. It then must hand off any relevant data to a secondary rendering thread, which can then draw all the graphics.

But there is a catch to this: What should the main thread do while the rendering thread is drawing? We don’t want it to simply wait for the drawing to finish, because that would be no faster than performing all the actions on one thread. The way this problem is solved is by having the rendering thread always lag one frame behind the main thread. So every frame, the main thread is updating the world while the rendering thread draws the results of the last main thread update.

One big drawback of this delay is an increase of input lag, or how long it takes for a player’s action to be visible onscreen. Suppose the “jump” button is pressed during frame 2. In the multithreaded loop, the input will not get processed until the beginning of frame 3, and the graphics will not be visible until the end of frame 4. This is illustrated in Figure 1.1.

Figure 1.1

Figure 1.1 The jump is delayed a couple of frames due to input lag.

If a particular game relies on extremely quick response time, including fighting games such as Street Fighter, this increased input lag may be deemed unacceptable. But for most other genres, the increased lag may not be particularly noticeable. Several other factors increase input lag. The game loop can be one of these factors, and some, like the display lag most LCD panels have, might be out of the programmer’s control. For more information on this topic, check the references at the end of this chapter, which includes an interesting set of articles on the topic of measuring and solving input lag in games.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020