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The xblock dialog interprets an xblock and it's various fields.

xblock.png

At the top of this dialog is the "File Range Control". The file range control gives you a quick graphical view of where in the file this block is located.

Next comes the xblock header. This is always located as the start of an xblock.
  • btype The type of block. This should always be btypeXB for a vaild xblock. If you see a different value, you are probably not actually looking at an xblock.
  • cLevel The level of an xblock. A level 1 xblock points to external blocks. A level 2 xblock (sometimes called an xxblock) points to xblocks.
  • cEnt The number of blocks referenced in this xblock
  • lcbTotal The total logical size of this xblock

The block trailer dialog includes an interpretation of the fields of the block trailer structure. They include:
  • cb (Disk Aligned) The size of the block, and the aligned version (rounded up)
  • wSig The block signature. This is based on the block id and location of the block on disk
  • dwCRC The CRC of the block. This is validated everytime the block is read from disk.
  • bid The block id of the block. This is the key into the BBT to find the block.

Below the block trailer is the contents of the xblock shown as a list of BIDs of the blocks this xblock points to. Double clicking on these blocks will open the block inspector for that BID.

As an exercise, try opening the same block as Binary data, a block, and an xblock all side by side. See if you can point out how the raw binary data translates into the block trailer fields, and how the body of the block interpretation translates into the list of BIDs in the xblock interpretation.

Last edited Mar 12, 2010 at 10:12 PM by terrymah, version 3

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