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From Power-up to Shut-down: BIOS
Now that you are familiar with the parts of a PC, let's see what happens in a typical computer session, from the moment you turn the computer on until you shut it down:

You press the "On" button on the computer and the monitor.

You see the BIOS software doing its thing, called the power-on self-test (POST). On many machines, the BIOS displays text describing such data as the amount of memory installed in your computer and the type of hard disk you have. During this boot sequence, the BIOS does a remarkable amount of work to get your computer ready to run.

The BIOS determines whether the video card is operational. Most video cards have a miniature BIOS of their own that initializes the memory and graphics processor on the card. If they do not, there is usually video-driver information on another ROM on the motherboard that the BIOS can load.

The BIOS checks to see if this is a cold boot or a reboot. It does this by checking the value at memory address 0000:0472. A value of 1234h indicates a reboot, in which case the BIOS skips the rest of POST. Any other value is considered a cold boot.

If it is a cold boot, the BIOS verifies RAM by performing a read/write test of each memory address. It checks for a keyboard and a mouse. It looks for a PCI bus and, if it finds one, checks all the PCI cards. If the BIOS finds any errors during the POST, it notifies you with a series of beeps or a text message displayed on the screen. An error at this point is almost always a hardware problem.

The BIOS displays some details about your system. This typically includes information about the following:

Processor

Floppy and hard drive

Memory

BIOS revision and date

Display

Any special drivers, such as the ones for SCSI adapters, are loaded from the adapter and the BIOS displays the information.

The BIOS looks at the sequence of storage devices identified as boot devices in the CMOS Setup. "Boot" is short for "bootstrap," as in the old phrase "Lift yourself up by your bootstraps." Boot refers to the process of launching the operating system. The BIOS tries to initiate the boot sequence from the first device using the bootstrap loader.

The bootstrap loader loads the operating system into memory and allows it to begin operation. It does this by setting up the divisions of memory that hold the operating system, user information and applications. The bootstrap loader then establishes the data structures that are used to communicate within and between the sub-systems and applications of the computer. Finally, it turns control of the computer over to the operating system.

From Power-up to Shut-down: Operating System
Once loaded, the operating system's tasks fall into six broad categories:

Processor management - Breaking the tasks down into manageable chunks and prioritizing them before sending to the CPU

Memory management - Coordinating the flow of data in and out of RAM and determining when virtual memory is necessary

Device management - Providing an interface between each device connected to the computer, the CPU and applications

Storage management - Directing where data will be stored permanently on hard drives and other forms of storage

Application Interface - Providing a standard communications and data exchange between software programs and the computer

User Interface - Providing a way for you to communicate and interact with the computer

  • You open up a word processing program and type a letter, save it and then print it out. Several components work together to make this happen:

    • The keyboard and mouse send your input to the operating system.
    • The operating system determines that the word-processing program is the active program and accepts your input as data for that program.
    • The word-processing program determines the format that the data is in and, via the operating system, stores it temporarily in RAM.
    • Each instruction from the word-processing program is sent by the operating system to the CPU. These instructions are intertwined with instructions from other programs that the operating system is overseeing before being sent to the CPU.
    • All this time, the operating system is steadily providing display information to the graphics card, directing what will be displayed on the monitor.
    • When you choose to save the letter, the word-processing program sends a request to the operating system, which then provides a standard window for selecting where you wish to save the information and what you want to call it. Once you have chosen the name and file path, the operating system directs the data from RAM to the appropriate storage device.
    • You click on "Print." The word-processing program sends a request to the operating system, which translates the data into a format the printer understands and directs the data from RAM to the appropriate port for the printer you requested.
  • You open up a Web browser and check out any internet site. Once again, the operating system coordinates all of the action. This time, though, the computer receives input from another source, the Internet, as well as from you. The operating system seamlessly integrates all incoming and outgoing information.

  • You close the Web browser and choose the "Shut Down" option.

  • The operating system closes all programs that are currently active. If a program has unsaved information, you are given an opportunity to save it before closing the program.

  • The operating system writes its current settings to a special configuration file so that it will boot up next time with the same settings.

  • If the computer provides software control of power, then the operating system will completely turn off the computer when it finishes its own shut-down cycle. Otherwise, you will have to manually turn the power off.

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