> What is the exact work of a BIOS chip residing on the motherboard ?
Getting the mainboard and the most critical devices (memory, keyboard,
display) to work in a foreseeable way. That means initializing these
devices, together with the chipset the mainboard is based on.
Apart from that, it provides some crucial routines needed to do basic stuff,
e.g. reading a sector of the disk or displaying a character. These, as you
know, work only in real mode, so that in protected mode a driver for the
interface is needed.
DOS uses the BIOS for almost everything.
> does it do the wellness checking of the attached devices basically
> keyboard, monitor, ram only or all the devices attached ?
That depends if you have your BIOS set up in PnP or Non-PnP mode. In PnP
mode, only the crucial devices (chipset, memory, display, keyboard, disk)
are initialized and assigned ressources, while in Non-PnP mode every device
is initialized (by jumping into a special ROM on that particular device,
e.g. SCSI-adaptor which is not used to boot, sound card, nic if not used to
book). PnP mode leaves any additional devices to the operating system, which
may (or may not) assign ressources based on different needs (like
IRQ-sharing or whatever).
In e.g. Windows 98 conflicts can come up, because the ressources are not
assigned properly by the OS; then you should change your CMOS to Non-PnP
mode. The BIOS then assigns these ressources and Windows simply uses the
already-assigned ressources and doesn't bother to initialize again. This, of
course, only works with devices which are (a) present at boot-time and (b)
have the intelligence to tell the BIOS what how to do or the code to do it