MECH 2301 Lecture 30: MECH 2301 Lecture 30 Notes
22 May 2018
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MECH 2301 Lecture 30 Notes
Introduction
FAT Table
• To locate a particular block in a particular file, the file manager goes to the directory
entry and finds the starting block for the file.
• It then follows the links through the FAT until the desired block is reached.
• Since the FAT is stored in memory, access to a particular block is fast.
• For example, for the file STRANGE.DAT shown the directory entry indicates that the first
block of the file is stored in block number 42.
• Successive blocks of this file are stored in blocks 48, 70, and 16.
• Confirm your understanding of the FAT method by finding the third block of the
fileWORK.TXT in the figure.
• A major disadvantage of the FAT approach is that it becomes unacceptably inefficient
for devices with large amounts of storage
• The FAT itself requires a large amount of memory.
• Observe that the FAT table requires an entry for every block on the device, even if the
block is unused.
• If a disk is broken into 216, or 65,536 clusters, with a corresponding 2-byte entry for
each cluster, the FAT will require 128 KB of memory.
• A 1 GB disk would require a cluster size of 16 KB.
• If most of the files on the disk are small, then much of the capacity of the disk is wasted.
• A 1 KB file stored in a single cluster would waste more than 90 percent of the capacity of
the cluster.
• Alternatively, the number of blocks in the table can be increased with a corresponding
increase in the memory requirements to hold the table.
• FAT32 allows as many as 228 or 256 million clusters.
• Each entry requires 4 bytes of storage.
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