SYSTEM STARTUP PROCESSES

System Boot Sequence      

The system BIOS is what starts the computer running when you turn it on. The following are the steps that a typical boot sequence involves. Of course this will vary by the manufacturer of your hardware, BIOS, etc., and especially by what peripherals you have in the PC. Here is what generally happens when you turn on your system power:

1. The internal power supply turns on and initializes. The power supply takes some time until it can generate reliable power for the rest of the computer, and having it turn on prematurely could potentially lead to damage. Therefore, the chipset will generate a reset signal to the processor (the same as if you held the reset button down for a while on your case) until it receives the Power Good signal from the power supply.
2. When the reset button is released, the processor will be ready to start executing. When the processor first starts up, it is suffering from amnesia; there is nothing at all in the memory to execute. Of course processor makers know this will happen, so they pre-program the processor to always look at the same place in the system BIOS ROM for the start of the BIOS boot program. This is normally location FFFF0h, right at the end of the system memory. They put it there so that the size of the ROM can be changed without creating compatibility problems. Since there are only 16 bytes left from there to the end of conventional memory, this location just contains a "jump" instruction telling the processor where to go to find the real BIOS startup program.
3. The BIOS performs the power-on self test (POST). If there are any fatal errors, the boot process stops. POST beep codes can be found in this area of the Troubleshooting Expert.
4. The BIOS looks for the video card. In particular, it looks for the video card's built in BIOS program and runs it. This BIOS is normally found at location C000h in memory. The system BIOS executes the video card BIOS, which initializes the video card. Most modern cards will display information on the screen about the video card. (This is why on a modern PC you usually see something on the screen about the video card before you see the messages from the system BIOS itself).
5. The BIOS then looks for other devices' ROMs to see if any of them have BIOSes. Normally, the IDE/ATA hard disk BIOS will be found at C8000h and executed. If any other device BIOSes are found, they are executed as well.
6. The BIOS displays its startup screen.
7. The BIOS does more tests on the system, including the memory count-up test which you see on the screen. The BIOS will generally display a text error message on the screen if it encounters an error at this point; these error messages and their explanations can be found in this part of the Troubleshooting Expert.
8. The BIOS performs a "system inventory" of sorts, doing more tests to determine what sort of hardware is in the system. Modern BIOSes have many automatic settings and will determine memory timing (for example) based on what kind of memory it finds. Many BIOSes can also dynamically set hard drive parameters and access modes, and will determine these at roughly this time. Some will display a message on the screen for each drive they detect and configure this way. The BIOS will also now search for and label logical devices (COM and LPT ports).
9. If the BIOS supports the Plug and Play standard, it will detect and configure Plug and Play devices at this time and display a message on the screen for each one it finds.
10. The BIOS will display a summary screen about your system's configuration. Checking this page of data can be helpful in diagnosing setup problems, although it can be hard to see because sometimes it flashes on the screen very quickly before scrolling off the top.
11. The BIOS begins the search for a drive to boot from. Most modern BIOSes contain a setting that controls if the system should first try to boot from the floppy disk (A:) or first try the hard disk (C:). Some BIOSes will even let you boot from your CD-ROM drive or other devices, depending on the boot sequence BIOS setting.
12. Having identified its target boot drive, the BIOS looks for boot information to start the operating system boot process. If it is searching a hard disk, it looks for a master boot record at cylinder 0, head 0, sector 1 (the first sector on the disk); if it is searching a floppy disk, it looks at the same address on the floppy disk for a volume boot sector.
13. If it finds what it is looking for, the BIOS starts the process of booting the operating system, using the information in the boot sector. At this point, the code in the boot sector takes over from the BIOS. The DOS boot process is described in detail here. If the first device that the system tries (floppy, hard disk, etc.) is not found, the BIOS will then try the next device in the boot sequence, and continue until it finds a bootable device.
14. If no boot device at all can be found, the system will normally display an error message and then freeze up the system. What the error message is depends entirely on the BIOS, and can be anything from the rather clear "No boot device available" to the very cryptic "NO ROM BASIC - SYSTEM HALTED". This will also happen if you have a bootable hard disk partition but forget to set it active.

This process is called a "cold boot" (since the machine was off, or cold, when it started). A "warm boot" is the same thing except it occurs when the machine is rebooted using {Ctrl}+{Alt}+{Delete} or similar. In this case the POST is skipped and the boot process continues roughly at step 8 above.

BIOS Power-On Self Test (POST)
The first thing that the BIOS does when it boots the PC is to perform what is called the Power-On Self-Test, or POST for short. The POST is a built-in diagnostic program that checks your hardware to ensure that everything is present and functioning properly, before the BIOS begins the actual boot. It later continues with additional tests (such as the memory test that you see printed on the screen) as the boot process is proceeding.
The POST runs very quickly, and you will normally not even noticed that it is happening--unless it finds a problem. You may have encountered a PC that, when turned on, made beeping sounds and then stopped without booting up. That is the POST telling you something is wrong with the machine. The speaker is used because this test happens so early on, that the video isn't even activated yet! These beep patterns can be used to diagnose many hardware problems with your PC. The exact patterns depend on the maker of the BIOS; the most common are Award and AMI BIOSes. This part of the Troubleshooting Expert will help you figure out what the POST beep codes mean and what to do about them, if you are having this problem.
Note: Some POST errors are considered "fatal" while others are not. A fatal error means that it will halt the boot process immediately (an example would be if no system memory at all is found). In fact, most POST boot errors are fatal, since the POST is testing vital system components.

Many people don't realize that the POST also uses extended troubleshooting codes that you can use to get much more detail on what problem a troublesome PC is having. You can purchase a special debugging card that goes into an ISA slot and accepts the debugging codes that the BIOS sends to a special I/O address, usually 80h. The card displays these codes and this lets you see where the POST stops, if it finds a problem. These cards are obviously only for the serious PC repairperson or someone who does a lot of work on systems.

Steps in Booting

Steps:

1. computer BIOS runs self-tests
2. if no problems, BIOS tries to boot an operating system from a drive. Older computers usually try a: drive first, c: drive second. Newer computers have a BIOS setting to tell BIOS where to look first, usually c:
3. BIOS looks for bootup files (usually on drive c:
4. most common bootup files are the Microsoft bootup files: io.sys, msdos.sys, and command.com
5. next, DOS looks for config.sys file and loads drivers line by line.
6. next, DOS looks for autoexec.bat file and executes DOS commands or runs given programs line by line