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There are two "standard" ways to deal with the geometry of a hard drive:

  • CHS (Cluster Head Sector Addressing) -- the older method, and (in the Intel world, only (??)), it creates a 2 GB limit on the size of a hard drive (partition??) (Oops, I guess that't the 500 MB limit, and also the cause of the no booting beyond the 1024th cluster -- should make a short bulleted list (page) of the various hard drive limits (should I start with FAT12??)
  • LBA (Logical Block Addressing) -- the newer method (it, or something else creates an 8 GB limit on the size of a hard drive




I should explain more later, including the size calculations that result in unit size (where it should be explained that the units are cylinders, and thus unit size is the number of bytes per cylinder.

Also, get all the terminology and synonyms squared away:

  • block and cluster (in most filesystems, the smallest amount of space allocated to a single file (it's tempting to also call this units ;-))
  • sector (always 512 bytes)
  • track
  • cylinder
  • head
  • platter

Explain the addressing on the first sector of the hard drive -- some say 0,0,1; others say 0,0,0 (IIRC). I assume they are the same physical location.

LBA probably had two driving forces:

  • overcoming the 2 GB limit
  • dealing with (what is it called) variable length sectors depending on their radial location on the disk

Question: Learn more about dd and how much it depends on geometry -- if you dd a disk with CHS geometry (i.e., without the LBA flag set) to a disk with CHS geometry, then look for data "under a microscope" is the data in the same locations on both disks? (I think not, and I think I proved that to myself -- I had a bootable Win95 partition without the LBA set -- I set it, and then the partition was no longer bootable.)

Along the same lines, if I dd a partition from one LBA disk to another, but of different sizes, is the result readable (and "logically" the same?). (Does dd care about LBA vs CHS -- maybe it sits one level "above them"? Read man dd, I guess.)


  • () RandyKramer - 10 Mar 2003
  • <If you edit this page: add your name here; move this to the next line; and include your comment marker (initials), if you have created one, in parenthesis before your WikiName.>

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Topic revision: r1 - 2003-03-10 - RandyKramer
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