The PC start-up process

The PC start-up process

When you turn power on, several things happen in the PC:
  • You hear the fan motor starting. There are one or more cooling fans in the PC. They produce a whirring sound.

  • After a few seconds, text starts to scroll on the screen.

  • Now the PC tests and counts the RAM. You see a number on the screen. It increases in size.

  • To understand the working of the PC, it is useful to study the PC start-up process. Those are events, which take place from
    power-on until the PC is ready to work. Remember, the PC can do nothing without receiving instructions. These instructions
    are commands, which are sent to the CPU. During start-up, the PC reads the commands in this sequence:
  • First it receives commands from the ROM chips. Those chips are inherent in any computer. They contain the POST and
    BIOS instructions, which we will look at shortly.

  • Next, the operating system is read from the hard disk (or from floppy drive A). This is called the boot process.
    The ROM chips

ROM (Read Only Memory). The ROM chips are on the system board. They contain. system software. System software are instructions, which enable the PC to coordinate the functions of various computer components.
The ROM chips contain instructions, which are specific for that particular system board. Those instructions will remain in the PC throughout its life. They will usually not be altered. Primarily, they are start-up instructions. There are different parts in the start-up instructions. For most users, they are all woven together. You can differentiate between:
  • POST (Power On Self Test)

  • The Set-up instructions, which connect with the CMOS instructions

  • BIOS instructions, which connect with the various hardware peripherals

  • The Boot instructions, which calls the operating system (DOS, OS/2, or Windows)

All these instructions are in ROM chips, and they are activated on by one during start-up. Let us look at each part.


Power On Self Test is the first instruction executed during start-up. It checks the PC components and that everything works.
You can recognize it during the RAM test, which occurs as soon as you turn power on.
As users, we have only limited ability to manipulate the POST instructions. But certain system boards enable the user to order a quick system check. Some enable the user to disable the RAM test, thereby shortening the duration of the POST. The duration of the POST can vary considerably in different PC's. On the IBM PC 300 computer, it is very slow. But you can
disrupt it by pressing [Esc].

If POST detects errors in the system, it will write error messages on the screen. If the monitor is not ready, or if the error is inthe video card, it will also sound a pattern of beeps (for example 3 short and one long) to identify the error to the user. If you want to know more of the beeps, you can find explanations on the Award, AMI and Phoenix web sites. POST also reads those user instructions, which are found in CMOS:


CMOS (Complimentary Metal Oxide Semiconductor) is a small amount of memory in a special RAM chip. Its memory is
maintained with electric power from a small battery. Certain system data are stored in this chip. They must be read to make
the PC operable. There may be 100 to 200 bytes of data regarding date, time, floppy and hard disk drives, and much more.
CMOS data can be divided in two groups:
  • Data, which POST can not find during the system test.

  • Data, which contain user options.

For example, POST cannot by itself find sufficient information about the floppy drive(s). Floppy drives are so "dumb," that POST cannot read whether they are floppy drives or not, nor what type. About the same goes for IDE hard disks, while EIDE hard disks are a little more "intelligent," However, POST still needs assistance to identify them 100% correctly.

The same goes for RAM: POST can count how much RAM is in the PC. However, POST cannot detect whether it is FPM, EDO or SD RAM. Since the CPU and BIOS reads data from RAM chips differently, depending on the RAM type, that typemust be identified.


The PC must be configured, be supplied with this information. That is done in the factory or store, where it is assembled. This information is stored in CMOS, where they stay. CMOS data only need to be updated, when different or additional hardware components are installed. This could be a different type hard disk or floppy disks or an new RAM type, Often he user can do this.

Other data in CMOS contain various user options. Those are data, which you can write to CMOS. For example, you can adjust date and time, which the PC then adjusts every second. You can also choose between different system parameters.

Maybe you want a short system check instead of a long one. Or if you want the PC to try to boot from hard disk C beforetrying floppy disk A, or vice versa. These options can be written to CMOS. Many of the options are of no interest to the ordinary user.

These are options, which regard controller chips on the system board, which can be configured in different ways. Ordinarily, there is no need to make such changes. The system board manufacturer has already selected the optimal configurations. They recommend in their manuals, that you do not change these default settings.
We can conclude, that CMOS data are essential system data, which are vital for operation of the PC. Their special feature is,
An illustrated Guide to Motherboards

that they are user adjustable. Adjustments to CMOS are made during start-up.
Suppliers of system software
All PC's have instructions in ROM chips on the system board. The ROM chips are supplied by specialty software manufacturers, who make BIOS chips. The primary suppliers are:

  • Phoenix

  • AMI (American Megatrends)

  • Award

You can read the name of your BIOS chip during start-up. You can also see the chip on the system board. Here is a picture
(slightly blurred) of an Award ROM chip:

Here is an AMI chip with BIOS and start-up instructions:

the power supply is controlled by the system board. The on/off button will turn the PC "down" without turning it completely off. If you want a PC designed for the future, the ATX layout is what you should go for.