On Fri, Dec 24, 2010 at 1:23 PM, Ankit Chaturvedi < ankit.chaturv...@gmail.com> wrote:
> > > On Thu, Dec 23, 2010 at 10:28 PM, Rajveer Singh > <torajveersi...@gmail.com>wrote: > >> On Wed, Dec 22, 2010 at 11:20 PM, Chirag Anand <anand.chi...@gmail.com >> >wrote: >> >> > On Dec 22 10:43PM, Rajveer Singh wrote: >> > > Hello Guys, >> > > >> > > I've a confusion related to hard drive detection process by system >> BIOS. >> > > This question may sound strange to some of you but I'll appreciate if >> I >> > can >> > > get any link or lead to explore it further. >> > > >> > > As all of us know, to detect hardrive, we must need a suitable driver >> for >> > > hard disk in kernel. When we buy a new hard drive, if it's driver is >> not >> > in >> > > the kernel, it doesn't detect but system BIOS can read it's MBR. So >> I'm >> > just >> > > wondering, What machenism or techniques are used by BIOS so it doesn't >> > > require any additional drivers to detect hard drives. >> > >> > Talking about programming, the BIOS first reads the 1 sector of any hard >> > disk by a BIOS interrupt 0x80 and tries to locate the byte "0xAA55" at >> > the 512th byte, which confirms that the starting 512 bytes are bootable >> > code. >> > >> > So, if your hard disk is detected inside the BIOS, I think it will be >> > able to do the above procedure. After the kernel is loaded, the generic >> > drivers for IDE/SATA etc. should do the needful. >> > >> > According to me, there is no need for drivers at the BIOS level, they >> > are only required while/after the kernel is loaded. >> > >> >> Thanks guys for your quick responses, >> >> Yes, I agree, drivers for disk(IDE/SATA/SCSI/SAS chipsets, thanks to >> Mahesh >> for correcting me) are required by kernel not by BIOS but I'm wondering, >> why >> BIOS doesn't need drivers to access it. What special techniques are used >> in >> BIOS, so it can detect disks without it's drivers. If it's true, why >> kernel can not use the same technique to detect the new disks. >> > > BIOS is not a singular entity, there are several BIOS' on any system. The > motherboard bios is the master bios however. Here is a quick run down at how > BIOS detects and loads OS from disk: > > 1. On start, the processor looks at FFF0h location for code to execute. But > since there is no code yet, this location is usually the start of mapped m/b > bios. > > 2. After POST, the m/b bios issues an Interrupt 09h which inits the > IDE/SATA controller. This init is different from driver loading as it does > not support UDMA modes, caching or paged r/w. In this restricted mode the > drive is only able to access specific tracks and sectors. > > 3. After this, bios searches for other h/w rom's that may be needed to init > the devices. For example, video bios is located at C000h, floppy controller > at 7C00h and IDE hdd at C8000h. These addresses are reserved and hard-mapped > by devices. Once it discovers all the bios' the main bios executes all of > them and inits the devices. So now our hdd has been initialised and is ready > to read write (in restricted mode, only 16bit txfers), and so is the video > controller ready to display in a text only mode. > > 4. Next Int 19h is issued which is an interrupt for Bootstrap Loader. This > call searches for a bootable MBR in the order specified in bios settings. > MBR in hdd is always at cylinder 0, head 0, sector 1. This is where it looks > for 0xAA55 pattern to detect if it is loadable or not. If it is, it is > loaded into location 0x7C00h and then executed from there. > > > Remember that once the IDE drivers are loaded the ide controller as well as > hdd itself is reset and put to udma mode for faster transfers. But since the > mbr is in ram, it can still be executed to load the initial kernel or ntldr > or whatever bootstrap you are using. > > Hope this helps. > > >> Thank you so much, Ankit and all of you who have shared so valuable information here. By going through the above points, I understood that kernel uses the drivers which gives features like udma, pagged r/w, caching etc. whereas the bios way to access the disk is restricted to specific track and sectors. So if we don't require these features like udma, pagged r/w, caching etc. and only needed the restricted access to specific track and sectors then kernel may detect the new disk(SATA/ATA/SCSI/SAS etc) the same way as the bios without specific drivers. Correct me if I'm wrong. Thank you all guys for helping me to overcome this doubt. Regards, Rajveer Singh _______________________________________________ Ilugd mailing list Ilugd@lists.linux-delhi.org http://frodo.hserus.net/mailman/listinfo/ilugd
