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.
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.
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.
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.
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:
Mr BIOS FAQ
Copyright (c) 1996-2005 by Michael B. Karbo. www.karbosguide.com.