What is the term for the process of starting up when a computer is initially turned on?

Learn more about the startup sequence of a computer.

In this step, you’ll learn how the components of a computer work together from the moment you press the “on” button.

The startup sequence

From the moment you press the power button, a whirlwind of tasks happen inside your computer.

Let’s have a look at each of the components and systems that work together to start up your computer.

CPU

The CPU, or central processing unit, is a large chip inside the computer. This is the brains of the computer: it controls everything. It works by reading instructions and data from the random access memory (RAM), performing an instruction, and then writing the data back to RAM. Some of the instructions may involve other components like the hard drive, but the CPU is in control.

What is the term for the process of starting up when a computer is initially turned on?

RAM

RAM temporarily stores data while your computer is running.

  • RAM is both readable and writable. You can add, change, and delete data stored in RAM.
  • It is volatile. When the computer is switched off, all the data stored in RAM is lost.
  • It is fast.

What is the term for the process of starting up when a computer is initially turned on?

ROM

ROM stands for read-only memory. It is a chip containing data installed by the manufacturer that is not typically replaced or upgraded. It stores the BIOS.

  • ROM is read-only. It is usually programmed by the computer manufacturer, and cannot be changed or overwritten. (There is a process for overwriting the ROM, called “flashing” it, but it’s difficult and can lead to a completely broken and unrecoverable computer.)
  • ROM is non-volatile memory, which means it does not need power to keep the data inside it.
  • It is fast.

What is the term for the process of starting up when a computer is initially turned on?

Hard Drive

The hard drive (sometimes called the hard disk) is the main storage device in your computer. Like RAM, it can be added to and changed, and like ROM it is non-volatile, but it is slow. If you have files and folders on your computer, are stored on the hard drive. The operating system is also stored on the hard drive.

What is the term for the process of starting up when a computer is initially turned on?

BIOS stands for Basic Input Output System. The BIOS is stored in the ROM. It contains all the basic code for controlling your computer hardware (such as keyboards, mice, monitors and hard drives). After the startup sequence is complete, and control has gone to the operating system, the BIOS does very little.

When you start up your computer, you may see a black screen displaying “Press F2 for Setup”. This is the BIOS. By pressing F2, you enter a setup screen where you can change where the BIOS loads the operating system from.

The operating system is normally stored on the hard drive, but you can load an operating system from a USB drive or a CD instead.

Startup sequence

So, how are these components used in the startup sequence?

  • The CPU starts and fetches instructions into RAM from the BIOS, which is stored in the ROM.
  • The BIOS starts the monitor and keyboard, and does some basic checks to make sure the computer is working properly. For example, it will look for the RAM.
  • The BIOS then starts the boot sequence. It will look for the operating system.
  • If you don’t change any of the settings, the BIOS will fetch the operating system from the hard drive and load it into the RAM.
  • The BIOS then transfers control to the operating system.

That’s a lot of information and acronyms!

Hopefully, this step has helped you understand what happens underneath the bonnet of a computer when you turn it on. Once you know this sequence, you can create some really fun lesson plans based on it…

An example lesson

Give students or teams of students a component each:

  • CPU
  • BIOS
  • ROM
  • RAM
  • Operating system

Place them at different tables around the room with some paper. There should be some distance between the students so that everyone can clearly see what’s happening.

Get each team to discuss the key features of their component and write them down on a large piece of paper. They could use their notes, or this could be a test of what they’ve learnt so far.

Run through a simple scenario. For example, “I’m writing a document in Word and my computer crashes. What does RAM do?”

Get the students to physically throw a piece of paper that was on the RAM table into the bin. It’s lost without power. Throw ROM’s data in the bin, and ask the students: is this correct?

Run through the more complicated startup sequence. Have a piece of card with the word CONTROL on it to indicate who has control. Each team has to say what they’re doing as they’re doing it. For example:

I am the BIOS. I’m just going to go over here and check that I have some RAM.

Give a high five to the RAM team

OK, RAM is there. Now I’m going to look for the operating system.

Go over and give a thumbs up to the OS team

I’m going to pick the OS up and put it in the RAM.

The OS team follows BIOS over to the RAM table

Go through several iterations of the startup sequence until the students understand it without looking at their notes. If you can, get them to perform the startup sequence for another class.

Physically modeling the startup sequence is a great learning opportunity for students. Once they can visually see what is happening with the components during startup, they will remember the sequence more easily.

For more detailed descriptions of what’s happening inside some of these components, check out our How Computers Work course.

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 initialises. 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 initialises 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.
The BIOS displays its startup screen.

6. 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.

7. 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).

8. 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.
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.

9. 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.

10. 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.

11. 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.
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.

12. 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 10 above.

see also POST

Source Scott Muller's Guide to repairing and upgrading PC's

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