63 Sectors BIOS limit

 
  BIOS Int 13  
a.k.a. Logical CHS  		Drive CHS  
a.k.a. Physical CHS  
This Limit  
Maximum Cylinders  1024 65536  65536
Maximum Heads  256 16 16
Max. Sectors/track  63 256 63
Maximum Capacity  7.88GB 128GB 31.5GB
 

    For any drive up to 7.88GB (The BIOS Int 13h CHS barrier), the physical drive CHS is supposed to use the same 63 sector maximum as the BIOS Int 13h system call. For LBA to/from L-CHS translation, the logical CHS is supposed to always uses 63 sectors. And this capacity limitation occurs when the BIOS just uses 63 sectors for all situations.

    That for the Physical CHS (P-CHS), using the following calculation for Cylinders will result in a value over 65536, if the drive capacity exceeds the 31.5GB (33.8 billion Bytes) limit:

Drive Capacity / 16 Heads / 63 Sectors = Calculated Cylinders.

 
 
[WWW Link]Ontrack: How do I use the Set Drive Size function ?
http://support.ontrack.com/cgi-bin/ontrack.cfg/php/enduser/std_adp.php?p_faqid=77    [New Window]
A work-around for system lockup during drive auto-detect, if an attached drive is larger than 31.5GB (33.8 billion Bytes).
 

Ontrack DiskManager workaround for BIOS lockup
 
 Based on: Ontrack Data International solution for 33GB BIOS lockup
(. . . an article that is no longer available from IBM Storage.)

    The drive size auto-detection in the Award 4.5x BIOS, and possibly other makes of BIOS, will cause a computer to lock up at boot time when a drive with a capacity over 33.8 billion Bytes (32GB) is installed.

    If this happens, use the Disk Manager utility "Set Drive Size" to command the drive to report its size (in total sectors) as less than 33.8 billion Bytes (each boot). This allows the computer to boot with the drive installed, but only the reported drive size will be supported by the BIOS.

  1. Do not jumper the drive to limit the drive capacity to 32GB! Install the drive into you system as you intend to use it.
     
  2. Boot the system, and enter the System BIOS. This is often accomplished by pressing the F1, F2, Delete, or Escape key-- as prompted by the computer during the boot process. For details, consult with the documentation or technical support for your computer.
     
  3. For the just installed drive, select None, or the equivalent setting for your BIOS, instead of Auto detection.
     
     
    If you have a computer that the BIOS forces drive auto-detect (as most PC's from IBM do), then the drive may instead need to be put into another computer to permit setting the drive size.
     
     
  4. Save the settings and exit the BIOS set-up.
     
  5. Reboot the system using the DiskManager diskette. The Disk Manager menu should appear.
     
  6. At the menu, select:
    2 - Disk Manager v9.xx: Hard Drive Installation.
     
  7. From the main program menu select:
    Advanced Options >> Maintenance Options >> Utilities >> Your Drive >> Set Drive Size
     
  8. At the Set Drive Size Menu, press Enter to accept the suggested Total Sectors value.
     
  9. Reboot and reenter the System BIOS.
     
  10. Change the settings for the drive back to Auto.

    Windows 2000 and Windows XP scans the drive bus itself, ignoring the BIOS, so your Operating System should now see the full size of the drive. For other Operating systems, a program like DiskManager may need to be used to provide a Dynamic Drive Overlay, so the full capacity of the drive can be used.

 
Back UP To: Specific Drive Capacity Limits
Next Page: Windows 95 Drive Size barrier
Previous: BIOS Int 13h CHS barrier
 

 

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Page Content Updated: 1 July 2005
   

Hard drives over 7.88GB are supposed to report their geometry as 16383/16/63 (C/H/S). This in effect means that "geometry" cannot be used to reliably calculate the size of a hard drive. Instead, the drive size is found in the LBA capacity field, returned by the ATA IDENTIFY command. Under the new ATA-6 specification, drives over 128GB are supposed to report an LBA capacity of #FFFFFFF (all 28 bits are 1's). Which comes to 268435455 sectors or 137,438,952,960 Bytes. And the actual drive size will now be reported by the new 48 bit capacity field.