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various limitations for IDE disks

don't we need this put somewhere in docs/faqs? it looks cool and explains 
roots of many ide/bios issues... 


stolen from http://sunsite.unc.edu/pub/Linux/docs/HOWTO/mini/Large-Disk 
section 4.2 "History of BIOS and IDE limits"


 ATA Specification (for IDE disks) - the 137 GB limit
        At most 65536 cylinders (numbered 0-65535), 16 heads (numbered
        0-15), 255 sectors/track (numbered 1-255), for a maximum total
        capacity of 267386880 sectors (of 512 bytes each), that is,
        136902082560 bytes (137 GB).  This is not yet a problem (in
        1999), but will be a few years from now.

     BIOS Int 13 - the 8.5 GB limit
        At most 1024 cylinders (numbered 0-1023), 256 heads (numbered
        0-255), 63 sectors/track (numbered 1-63) for a maximum total
        capacity of 8455716864 bytes (8.5 GB). This is a serious
        limitation today.  It means that DOS cannot use present day
        large disks.

     The 528 MB limit
        If the same values for c,h,s are used for the BIOS Int 13 call
        and for the IDE disk I/O, then both limitations combine, and one
        can use at most 1024 cylinders, 16 heads, 63 sectors/track, for
        a maximum total capacity of 528482304 bytes (528MB), the
        infamous 504 MiB limit for DOS with an old BIOS.  This started
        being a problem around 1993, and people resorted to all kinds of
        trickery, both in hardware (LBA), in firmware (translating
        BIOS), and in software (disk managers).  The concept of
        `translation' was invented (1994): a BIOS could use one geometry
        while talking to the drive, and another, fake, geometry while
        talking to DOS, and translate between the two.

     The 2.1 GB limit (April 1996)
        Some older BIOSes only allocate 12 bits for the field in CMOS
        RAM that gives the number of cylinders. Consequently, this
        number can be at most 4095, and only 4095*16*63*512=2113413120
        bytes are accessible.  The effect of having a larger disk would
        be a hang at boot time.  This made disks with geometry
        4092/16/63 rather popular. And still today many large disk
        drives come with a jumper to make them appear 4092/16/63.  See
        also over2gb.htm.

     The 3.2 GB limit
        There was a bug in the Phoenix 4.03 and 4.04 BIOS firmware that
        would cause the system to lock up in the CMOS setup for drives
        with a capacity over 3277 MB. See over3gb.htm.

     The 4.2 GB limit (Feb 1997)
        Simple BIOS translation (ECHS=Extended CHS, sometimes called
        `Large disk support' or just `Large') works by repeatedly
        doubling the number of heads and halving the number of cylinders
        shown to DOS, until the number of cylinders is at most 1024.
        Now DOS and Windows 95 cannot handle 256 heads, and in the
        common case that the disk reports 16 heads, this means that this
        simple mechanism only works up to 8192*16*63*512=4227858432
        bytes (with a fake geometry with 1024 cylinders, 128 heads, 63
        sectors/track).  Note that ECHS does not change the number of
        sectors per track, so if that is not 63, the limit will be
        lower.  See over4gb.htm.

     The 7.9 GB limit
        Slightly smarter BIOSes avoid the previous problem by first
        adjusting the number of heads to 15 (`revised ECHS'), so that a
        fake geometry with 240 heads can be obtained, good for
        1024*240*63*512=7927234560 bytes.

     The 8.4 GB limit
        Finally, if the BIOS does all it can to make this translation a
        success, and uses 255 heads and 63 sectors/track (`assisted LBA'
        or just `LBA') it may reach 1024*255*63*512=8422686720 bytes,
        slightly less than the earlier 8.5 GB limit because the
        geometries with 256 heads must be avoided.  (This translation
        will use for the number of heads the first value H in the
        sequence 16, 32, 64, 128, 255 for which the total disk capacity
        fits in 1024*H*63*512, and then computes the number of cylinders
        C as total capacity divided by (H*63*512).)


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