On 16 Aug 2002 at 15:42, Constant Brouerius van Nidek wrote: >If I go into my Cmos setup I find that my hard discs can be >automatically detected. What I do not understand is why >there are three choices possible. >The choice which is highlighted (2) is labeled LBA. >The others are NORMAL and LARGE. >Can somebody tell me what the difference is. >What is the abbreviation LBA? >And why is it highlighted?
Well, this is explainable, though it may take a re-read or two to get through the paragraphs I've written, below. NORMAL mode is the original scheme for using the Cylinder/Head/Sector (C/H/S) type of addressing for locating information on a disk. It's straightforward; the OS supplies the C/H/S when it requests data, and the BIOS reads the drive, and passes the data back to the OS. This worked quite well for the original MFM and RLL hard drives. The only limit was that the C/H/S BIOS call was specified by IBM as comprising 10 / 8 / 6 bits, respectively. This still allowed for an 8GB drive, if you use 512 bits per sector, as the IBM PC does for hard drives. However, when IDE drives were introduced, the ATA specification parameters didn't match up with the IBM ones... ATA allowed for 16 / 4 / 8 bits , which gives an impressive max of 128GB... except that DOS (and later, Windows) and the IBM-PC BIOS couldn't handle those parameters. So, you were restricted to the lower limits of both: 10 / 4 / 6 bits - or 540MB, with 512 bit sectors. This is the infamous "540MB" limit that a number of older BIOSes have. LARGE (or Extended - CHS (ECHS)) was developed as a BIOS work-around. Through some bit-shifting, the number of cylinders is reduced by a factor of 16, and the number of heads is increased by a factor of 16, when reporting to the OS. So the OS receives a number of C/H/S it can handle, and the BIOS takes care of the simple bit-shifting involved. However, this simple method only gives a max drive size of 4.2GB under DOS / Windows. [ DOS-FAT16 is limited to 2GB partition sizes, for other reasons. ] LBA is a sort of "extended, extended CHS", and allows for drives of up to 8.4GB to be used under DOS / Windows, by adding yet another twist on the ECHS scheme, allowing one more bit to be packed in, thus doubling the size of drive which can be handled. [Though DOS-FAT16 still limits you to 2GB partitions.] For the most part, you should use LBA, if available, then use Large (or ECHS) if LBA isn't available . LBA will allow you to use the largest drives possible, under DOS/Windows, without having to install third- party software from the drive manufacturer to perform additional tricks to allow large drives to be accessed. For native support of larger drives, you need a BIOS which has the "Extended INT13 support", and an OS which will use that support. MS-DOS 6.22, for example, will not be natively be able to use a drive of more than 8.4GB, as it recognizes a partition table of four entries, no one of which may be more than 2GB. Hope this explains it fairly clearly. There are also WWW pages which describe it in excruciating detail, which you can find with a little WWW searching. (Try the hard drive manufacturer's WWW sites first.) Hope this helps, Anthony J. Albert =========================================================== Anthony J. Albert [EMAIL PROTECTED] Systems and Software Support Specialist Postmaster Computer Services - University of Maine, Presque Isle "I gots yer four basic food groups right here: bacon, beans, whiskey, and lard!" - Cookie, from Disney's _Atlantis_ To unsubscribe from SURVPC send a message to [EMAIL PROTECTED] with unsubscribe SURVPC in the body of the message. Also, trim this footer from any quoted replies. More info can be found at; http://www.softcon.com/archives/SURVPC.html
