Copyright Goodheart-Willcox Co., Inc. 67 Chapter 2 Operating Systems POST All operating systems start with a power-on self-test. Th e power-on self-test (POST) is a simple diagnostic program that is initiated when electrical power is applied to the computer system. It is common to all operating systems, even to macOS and all versions of Linux. Th e POST does a quick system check to determine if all major hardware components, such as the CPU, RAM, keyboard, mouse, video system, and storage devices, are in proper working order. Th e devices checked may vary slightly from computer to computer depending on the fi rmware. Th e test it performs is not as sophisticated as diagnostic software, but it will check for major problems. When the POST is fi nished, it usually makes one “beep” sound to let you know that the POST is complete and everything is in working order. If an error is detected during the POST, an error code is usually displayed on the screen and a series of beeps is heard that match the code. Th e codes and beep pattern vary according to the diff erent BIOS or UEFI chip manufacturers. A list of error codes and beep codes can be obtained from the website of the manufacturer. BIOS In a BIOS-based system, the POST is initiated by the bootstrap program in the fi rm- ware. Th e bootstrap program is a short program that runs the POST searches for the Master Boot Record (MBR), which is typically located on the fi rst section of the hard drive loads into memory some basic fi les and then turns the boot operation over to the operating system. Th e word bootstrap comes from the expression “to pull oneself up by one’s bootstraps.” UEFI Th e boot sequence is slightly diff erent for a computer equipped with UEFI or EFI. Th e Unifi ed Extensible Firmware Interface (UEFI) is a modern approach to the BIOS system and an enhancement to the boot process. When a device equipped with UEFI completes POST, control is turned over to a built-in boot manager program rather than the MBR, which contains the bootstrap program. Th e UEFI fi rmware uses an improved hard drive technology known as the GUID partition table. Th ere will be more about UEFI and GUID later in this textbook when you have a better under- standing of data storage and system fi les. Some computers have a UEFI boot manager and an MBR to ensure backward compatibility. UEFI is backward compatible with devices and software that require BIOS fi rmware. Th e UEFI fi rmware directly supports BIOS features using a compati- bility support module (CSM), which is programmed into the UEFI. Th e CSM emulates the BIOS features. For example, when a computer system that has BIOS fi rmware is 1001: 3.5 1002: 1.8 1001: 3.5 Goodheart-Willcox Publisher Figure 2-26 The Restart command can be used to perform a warm boot.