Karbosguide.com - Module 2a.4.

The system software of hardware


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  • The BIOS in adapter ROM

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    During the start-up process the BIOS programs are read from the ROM circuits. BIOS stands for Basic Input Output System and it is small program routines which controls specific hardware units.

    For instance you have a BIOS routine which reads the keyboard:

    The BIOS is a part of the modular design of the IBM Compatible PC. The OS and other programs access the hardware units by making requests to the BIOS routines.

    BIOS typically occupies 64 KB, and the programs are stored in ROM chips on the motherboard.


    The reserved areas

    In the original PC design we only had 1 MB of RAM. This memory was adressed using hex numbers, so each byte had its own address going from 00000h to FFFFFh.

    Important parts of the system software is mapped into this range, where we also find two reserved areas:

    Hex address Kilobytes Occupied by
    C0000-C8000 768-800 BIOS from the video card
    F0000 - FFFFF 960-1024 BIOS from the Motherboard

    These two ranges are reserved for this special adapter ROM. Other adapters cannot map their BIOS routines into these addresses.

    If it is setup to shadowing ("Shadow RAM" in the Setup utility), then this BIOS code is copied into RAM. If not, it has to be read directly from the ROM circuit. The last access is slower.



    BIOS on many adapters

    There are BIOS codes on many adapters (expansion cards). The adapters are external hardware, which are connected to and “integrated” with the motherboard during the hardware configuration and internalizing.

    The adapters hold their own BIOS code making them functional. This BIOS must be included during the configuration. Therefore, the adapter ROM is read during start-up, and the program code is “woven” together with other BIOS programs and the CMOS data. It is all written into RAM, where it is ready for the operating system, as you can see here:

    The BIOS routines are not always in use. They can be regarded as basic program layers in the PC, giving it a simple functionality.

    Many programs routinely bypass BIOS. In that case, they "write direct to hardware", as we say. Windows contains program files, which can be written directly to all kinds of hardware - bypassing BIOS routines. One example is the COM ports. If you use the BIOS routines connected with them, you can transmit only at max. 9600 baud on the modem. That is insufficient. Therefore, Windows will assume control over the COM port.

    BIOS update

    BIOS programs can be updated . The modern motherboard has the BIOS instructions in flash ROM, which can be updated. You can get new BIOS software from your supplier or on the Internet, which can be read onto the motherboard. The loading is a special process, where you might need to change a jumper switch on the motherboard. Usually, you do not need to do this, but it is a nice available option.


    ATX motherboards

    The latest PC electronic standard is called ATX. It consists of a new type motherboard with a specific physical design like the traditional board (30.5 cm X 19 cm). However the board has been shifted 90 degrees for a better placing of the units.

    The I/O connectors COM1, COM2 and LPT, keyboard, mouse and USB are mounted directly on the motherboard. The ATX board requires specifically designed chassises with an I/O access opening measuring 1¾ by 6¼ inch. ATX is designed by Intel, but has gained general acceptance.

    The ATX motherboard is more ”intelligent” than the ordinary type. In a few years, it will be wide spread. It includes advanced control facilities, where the BIOS program continually checks the CPU temperature and voltages, the cooling fans RPM, etc. If over heating occurs, the PC will shut down automatically. The PC can also be turned on by for example modem signals, since the power supply is controlled by the motherboard. 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.


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    Learn more

    Module 2b. About the boot process and system bus

    Read more about I/O buses in module 2c

    Read more about the motherboard chip set in module 2d

    Read more about RAM in module 2e

    Read about EIDE in module 5b

    I also recommend two books for further studies. Gunnar Forst: "PC Principals", from MIT is excellent. Also "The Winn L. Rosch Hardware Bible" from Brady covers the same subjects. Also "PC Intern" from Abacus is fine.

    Links to BIOS information:

    BIOS Guide

    Mr BIOS FAQ

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    [Karbo's Dictionary]
    [The Windows 98 pages]



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