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Current Head. This is another field typically used when doing INT 13 BIOS calls. The value would originally have been used to store the track on which the boot record was located, but the value stored on disk is not currently used as such. Therefore, operating systems such as Windows NT uses this field to store two flags: The low order bit is a "dirty" flag, used to indicate that autochk should run chkdsk against the volume at boot time. The second lowest bit is a flag indicating that a surface scan should also be run.
Computer Boot Sequence - The Mossywell Web Site
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