What happens from the moment you press the on button on a computer to the moment the screen goes on?

Answer 1

What happens When a PC Powers-up?

When you turn it on, a computer's power supply must bring all the separate voltages required by the Motherboard and peripheral components to an acceptable level with enough current to power the whole system. These voltages include both + and - (plus and minus) 12.0 V (volts), + and - 5.00 V and most likely +3.30 V (which may be reduced even further by circuits on the motherboard for the latest power efficient processors). [ Note: All these voltages will vary slightly for each combination of power supply, motherboard, disk drives and various peripherals (since all electronic components drawing current will alter the power supply's load), but they should never fluctuate by much (if at all) upon reaching their operating levels. If a power supply isn't quiet enough (i.e, it has too much AC noise on any of its outputs), the motherboard's BIOS chip may not function correctly; or more likely, the Memory portion of the Power-On Self Test will fail. Either way, the system will never boot up completely! In locations where the AC input from the power company experiences occasional problems or total loss thereof, devices which store up power for a some minutes, allowing you to save your data and properly shut-down the system, should be used. ]

Due to the sensitive nature of some IC chips (Integrated Circuits inside small metal, ceramic or plastic packages), especially the CPU (Central Processing Unit; which produces a great deal of heat, thus the reason for a large metal heatsink and fan on it), the motherboard may never be allowed to power up until a power good signal is received from the power supply. This may take over half a second; a tremendously long time when compared to the speed of a CPU that can step through millions of instructions per second (MIPS), yet it often seems almost instantaneous to its human operator.

By the time you notice the sound of a fan rotating or drive motor spinning up, the voltages have already stabilized, power has been applied to all the chips (the interface chips that facilitate communications between the CPU and the Memory modules; along with any audio, video or LAN cards plugged into bus connectors, and the controllers for peripheral devices such as keyboard, mouse, hard disk and CD/DVD drives) and the motherboard's Clock-pulses are already synchronizing all those interactions. In what feels like a fleeting moment to us, the PC is already executing instructions from the BIOS (Basic Input/Output System) chip.

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. See here for more details on how PnP detects devices and assigns resources.

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.