1. When the computer is powered on, the internal power supply initializes. The power supply doesn't immediately provide power to the rest of the computer. First, it determines whether it can supply the proper voltages that the computer's components require. The power supply sends out a POWER GOOD signal when it determines that it can supply reliable power to the rest of the computer. When the chipset receives this signal, it issues a SYSTEM RESET signal to the processor.
2. When the processor receives the SYSTEM RESET signal, it accesses the jump address for the start of the BIOS boot program at its hard-wired preset address and loads it into RAM. The jump address contains the actual address of the BIOS boot program on the ROM BIOS chip. The jump address is typically located at address FFFF0 (hexadecimal) or 1,048,560 (decimal), which is at the end of the first megabyte of system memory.
3. With the primary part of the BIOS now loaded into RAM, the POST (Power On Self Test) process begins. If any fatal errors happen during the POST process, the appropriate error beep code sounds or sometimes an error message displays, and the boot process stops. At this point in the boot process only the system speaker can notify the user of errors.
4. If all is well, the boot sequence continues and the system BIOS loads the device BIOS of the video adapter and loads into memory. As your computer boots, the video adapter's information displays on the monitor.
5. Any other device-specific BIOS routines, such as those for the hard disks or SCSI devices are loaded. Information, usually including the manufacture and the BIOS version, displays. The BIOS begins a series of tests on the system, including a run-up count of the amount of memory detected on the system. Because the display is no available, any errors found in this process are displayed on the monitor as an error message instead of a beep code played through the system speaker.
6. The system determines if the devices listed in the CMOST configuration data are present and functioning, including test for device speeds and access modes.
7. The serial and parallel ports are assigned their identities (COM1, COM2, LPT1, and so on), and a message is displayed for each device found, configured, and tested. If the BIOS program supports Plug and Play, any PnP devices detected are configured. Although it usually goes by much too fast to read, the BIOS displays a message for each device it finds and configures.
8. The configuration is confirmed. The BIOS displays a summary screen that details the computer as the BIOS sees it. This summary screen signals that the system is verified and ready for use.
9. The BIOS looks in the CMOS data to determine which disk drive to use for the operating system. If the boot device is the hard drive, the BIOS looks for the master boot record. If the boot device is a floppy disk or a CD-ROM, it looks at the first sector of the disk for the operating system's boot program. If the boot program is not found on the first device listed, the next device indicated is searched, and then the third, and so on until the boot program is found. If no boot device is found, the boot sequences stop and an error message is displayed.
Cold and Warm Booting
A cold boot makes the computer go through the entire boot sequence. From the POWER GOOD signal to POST to loading the operating system. A warm boot does not run the POST and re-establishes the operating system and drivers on the computer.
[b]POST[//b]
The POST is a hardware diagnostic routine built into the BIOS that checks the computer's hardware to make sure that everything that is supposed to be there is present, and that everything is working properly. The POST process ensures that the system is ready to begin the boot sequence.
If the POST process detects errors, it generates a signal to indicate where in the process the error occurred and which device had the error. Not all POST errors are fatal; the POST process generally continues past non-fatal problems. If a fatal error is detected, the POST process signals its error code and halts the boot process immediately. If the OST detect an error before the device drivers for the monitor are loaded, then it must signal an error the only way it can: using sounds, actually beeps, issued through the computer's speaker.
The meaning of a beep code depends on the manufacturer of the BIOS. Each BIOS maker has its own set of beep codes, which can also vary from one version to the next.
Just about all BIOS programs will sound a single beep right before displaying the BIOS start-up screen. As long as the boot sequence continues, the beep doesn't indicate a problem. BIOS beep codes can be used to troubleshoot hardware failures occurring in the POST procedure.
This article was originally published by CyberArmy.net in the CyberArmy Library.
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