File System Recognizers

First, a file system recognizer is a standard Windows NT kernel mode device driver which serves a single simple task—it looks at physical media devices and, if it recognizes the media format, it loads the full file system driver.

Why not just load the entire file system?  Since the full file system driver might never be required on the system, using a little driver can save several hundred kilobytes of precious system memory.  For example, on a system where the CD-ROM is never accessed, the CDFS file system need never be loaded—saving that memory for more important operations.

How does a file system recognizer determine that its file system is present on a disk? Typically, this is done by examining a signature on the disk. This signature might include information within the partition table, a unique value located at a particular byte offset from the beginning of the partition, and the serial number or whatever other clever mechanism the developer might have defined.

An FSR  analyzes the media to determine if it contains a recognized file system. However, instead of mounting the drive, the FSR loads the FSD. Once the FSD is loaded, the FSR can unload since its task is done.When the File System Driver (FSD) is loaded, it is responsible for analyzing the disk and determining if the media contains a recognized file system. If it does, the FSD will then mount the drive.