The 4096 Cylinder BIOS Barrier

 
 

Summary: Some BIOS-- developed before the second half of 1996-- limited the physical drive Cylinders to a 12 bit binary number, which supports a maximum of 4096 Cylinders or Tracks. (CMOS values 0 to 4095)

 

    This problem amounts to poor programming, that was short sighted, by some BIOS manufactures. As both the BIOS Int 13h system call, and the ATA (EIDE) standard do not provide a technical reason for this limit. Rather some BIOS, which were developed before the second half of 1996, limited the P-CHS Cylinders to a 12 bit binary number. 212 = 4096, which means that any drive that requires more than 4096 Cylinders (more than 4095 in the CMOS settings) is unsupported.

   Possible consequences of this limit includes:

  • The BIOS will only recognize a maximum Cylinder value of 4095. And this truncates the capacity of a larger drive to only 1.97GB (2.11 billion Byte).
     
  • The BIOS only uses the lower 12 bits of the (ATA) 16 bit binary Cylinder number. So after subtracting the largest multiple of 1.97GB (2.11 billion Bytes) that is still smaller than the drive capacity, the remainder is the capacity the BIOS recognizes.
     
    Example: A 4GB drive happens to have 8322 Cylinders, or the CMOS value for the last Cylinder is 8321. (8322 = [4096 * 2] + 30) But the BIOS only recognizes this as a 30 Cylinder drive, having a capacity of about 14.8MB
     
  • When a cylinder value over 4095 is entered in the BIOS setup, the computer will lock-up (become totally unusable) when trying to boot.
 
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Page Content Updated: 10 June 2005
   

    When the size of a hard drive is auto-detected, the returned result might wrongly suggest the BIOS is either truncating the drive capacity, or only using the lower bits of the Cylinder value. However, sometimes it is only the routine that calculates and displays the detected size for the hard drive that has a problem. So except for this "cosmetic defect", the drive is fully supported by the BIOS.