OSDev.org

         I would like to know where the chips such as 8259 interrupt controller is situated in the PC motherboard.

In these days of large scale integration, these chips no longer exist as discreet components, rather there are integrated into the board "chipset", which typically consists of two generic chips (available from multiple manufacturers) called north bridge and south bridge. North bridge is closest to the CPU and typically handles RAM and AGP (sometimes PCI too). South bridge is really where your interest as an OS programmer lies, this controls PCI (usually - sometimes in north bridge), DMA, interrupts, timers, floppy and IDE controllers etc, the specfics of programming these directly is well contained in documents relating to 8259 etc, the entire register set and functionality of each discreet chip is integrated into the south bridge.

Detailed datasheets on the actual chipset can be hard to get. If your chipset is Intel, you can download the datasheets directly from their website. If your chipset is SIS, the documents are probably not on the website, but they will usually supply datasheets if you ask for them. If your chipset is VIA, then good luck, in my experience VIA have never even had the courtesy to reply to my emails, and I am an electronic design engineer with a genuine commercial interest !

Note that many motherboards do have more than just northbridge and southbridge, but it is very common to find just these two.