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  Service 00H resets the disk controller and drive. This service does not
  affect the disk itself. Instead, a reset through service 00H forces the
  ROM BIOS disk-support routines to start from scratch for the next disk
  operation by recalibrating the disk drive's read/write head--an operation
  that positions the head on a certain track. This reset service is normally
  used after an error in any other drive operation.

  When you call service 00H for a fixed-disk drive, the ROM BIOS also resets
  the diskette-drive controller. If you want to reset the fixed-disk
  controller only, use service 0DH. (See page 207.)