From: "Jack R Payton" :Sent: Wednesday, May 30, 2001 1:11 AM
Subject: Re: [PCBUILD] Replacing Hard Drive


> Mary:
> 
> Depite my abject admiration for your computer knowledge, I question the
> idea that a harddrive, no matter how large, cannot be partitioned - with
> Windows or a program such as Partition Magic - into chunks of readable
> size by the OS and/or BIOS. My understanding is (was?) that one way or
> another, a harddrive can be partitioned into chunks readable by the
> OS/BIOS. Please elucidate.
> 
> YOU:
> "Jack - For you to be able to partition the hard drive into sizes equal
> or less than the BIOS limitation the BIOS must be able to recognize the
> entire drive.  The BIOS limitation of 8.4 g means that the BIOS can't
> recognize any part of the drive past the 8.4 g, therefore you would not
> be able to access it to partition it.
>    "If there is not a BIOS upgrade, you could also add a Promise hard
> drive controller, Ultra 100 or Ultra 66, that would support the faster
> speeds of the larger hard drives now."
> 
> Mary Wolden
> 

Jack,

Here is a quote from Western Digital explaining hard drive limitations.  I hope this helps you understand.

"The amount of space available to store information on hard drives is constantly increasing. Computer operating systems and system BIOSs have limitations that are related to specific hard drive capacities. Three capacity points that can affect how your operating system and system BIOS support your hard drive are 8.4 GB, 2.1 GB, and 528 MB.

The limitations of your system BIOS and your operating system combined determine your overall system limitation. For example, if your operating system recognizes extended BIOS functions, but your system BIOS has a 2.1 GB drive barrier, you are limited to your system BIOS's 2.1 GB drive barrier. Conversely, if your operating system does not recognize extended BIOS functions, but your system BIOS supports 8.4 GB hard drives, you are limited to your operating system's capability. Your system limitation is based on the lowest functioning barrier."

These limitations apply to any hard drive that reports an EIDE capacity of 16320 cylinders x 16 heads x 63 sectors per track or larger.  INT 13 Function 8 is the traditional method used by the system BIOS to access hard drives using CHS (cylinders, heads and sectors per track).  The maximum values the BIOS can report (1024 cylinders, 255 heads and 63 sectors) have been exceeded by the 8.4 GB and larger hard drives.  Since conventional INT 13 functions use 24 bits to represent addresses, this traditional CHS method permits 8.4GB ( 2(24th)* 512 bytes/sector) to be addressed. To utilize the full capacity of hard drives larger than 8.4 GB, the system BIOS must use extended INT 13 functions and LBA method of addressing. Extended INT 13 functions use 32 bits to represent addresses.  LBA addressing passes a single parameter to represent the address.  An ATA hard disk can accept a single address of 28 bits, so the LBA  method of addressing allows addressing of 137.4 GB.

If both your BIOS and motherboard do not support extended BIOS functions your drive, no matter how large it is, the drive will only be recognized at the lowest limit of either the BIOS or the OS. 

I believe what you are referring to is neither the correct BIOS nor drivers can overcome limitations inherent in operating systems. For example, in Windows 3.1x and early versions of Windows 95, 16 bits are available for cluster addressing in the file allocation table (thus the term, FAT16). Since each address specifies a cluster of up to 32KB, the maximum partition size is 2.1GB. So while hard disk drives larger than 8.4GB can be used with these FAT16 operating systems, multiple partitions no larger than 2.1GB each are required. 

Mary Wolden

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