What Is POST? (Power On Self Test Definition) GA S REGULAR Menu Lifewire Tech for Humans Newsletter! Search Close GO Software & Apps > Windows 288 288 people found this article helpful
What Is POST?
Definition of POST and an explanation of different types of POST errors
By Tim Fisher Tim Fisher Senior Vice President & Group General Manager, Tech & Sustainability Emporia State University Tim Fisher has more than 30 years' of professional technology experience. He's been writing about tech for more than two decades and serves as the VP and General Manager of Lifewire. lifewire's editorial guidelines Updated on March 18, 2022 Tweet Share Email Tweet Share Email Windows The Ultimate Laptop Buying Guide POST, short for Power On Self Test, is the initial set of diagnostic tests performed by the computer right after it's powered on, with the intent to check for any hardware related issues. Computers aren't the only devices that run a POST. Some appliances, medical equipment, and other devices also run very similar self-tests after being powered on. The unrecognized ordinary / Getty Images You might also see POST abbreviated as P.O.S.T., but probably not too often anymore. The word "post" in the technology world also refers to an article or message that's been posted online. POST, as it's explained in this article, has nothing at all to do with the internet-related term. The Role of POST in the Startup Process
A Power On Self Test is the first step of the boot sequence. It doesn't matter if you've just restarted your computer or if you've just powered it on for the first time in days; the POST is going to run, regardless. POST doesn't rely on any specific operating system. In fact, there doesn't even need to be an OS installed on a hard drive for it to run. This is because the test is handled by the system's BIOS, not any installed software. If an OS is installed, POST runs before it has a chance to start up. This test checks that basic system devices are present and working properly, like the keyboard and other peripheral devices, and other hardware elements like the processor, storage devices, and memory. The computer will continue to boot after the POST, but only if it was successful. Problems can certainly appear afterward, like Windows hanging during startup, but most of the time those can be attributed to an operating system or software problem, not a hardware one. If the POST finds something wrong during its test, you'll usually get an error of some kind, and hopefully, one clear enough to help jump-start the troubleshooting process. Problems During the POST
Remember that the Power On Self Test is just that: a self-test. Just about anything that might prevent the computer from continuing to start will prompt some kind of error. Errors might come in the form of flashing LEDs, audible beeps, or error messages on the monitor, all of which are technically referred to as POST codes, beep codes, and on-screen POST error messages, respectively. For example, one of the AMIBIOS beep codes is three short beeps, which means there's a memory read/write error. If some part of the test fails, you'll know very soon after powering on your computer, but how you find out depends on the type, and severity, of the problem. For example, if the issue lies with the video card, and therefore you can't see anything on the monitor, then looking for an error message wouldn't be as helpful as listening for a beep code or reading a POST code with a POST test card. On Mac computers, these errors often appear as an icon or another graphic instead of an actual error message. For example, a broken folder icon after starting up your Mac may mean that the computer can't find a suitable hard drive to boot from. Certain types of failures during the POST might not produce an error at all, or the error may hide behind a computer manufacturer's logo. Since issues during the POST are so varied, you may need a troubleshooting guide specific to them—see How to Fix Stopping, Freezing, and Reboot Issues During the POST. Was this page helpful? Thanks for letting us know! Get the Latest Tech News Delivered Every Day Subscribe Tell us why! Other Not enough details Hard to understand Submit More from Lifewire What Is a Power on Self Test Card? How to Fix Problems that Occur During the POST What Is a POST Code? (Power on Self Test Code) CMOS: What It Is and What It's For What Are Beep Codes? (BIOS Beep Code Definition) Phoenix Beep Code Error Troubleshooting What Is a POST or BIOS Error Message? How to Fix a Computer That Turns On and Then Off How to Fix a Computer That Turns on but Displays Nothing 4 Best Free Memory Test Programs (October 2022) MemTest86 v10 Free Memory Testing Tool Review What Does It Mean to 'Boot' a Computer? How to Access BIOS Setup Utility for Major BIOS Manufacturers How to Fix NTLDR Is Missing and Related NTLDR Errors Windows Memory Diagnostic Review (A Free RAM Test Tool) How to Fix a Computer That Won't Turn On Newsletter Sign Up Newsletter Sign Up Newsletter Sign Up Newsletter Sign Up Newsletter Sign Up By clicking “Accept All Cookies”, you agree to the storing of cookies on your device to enhance site navigation, analyze site usage, and assist in our marketing efforts. Cookies Settings Accept All Cookies