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  Function 2EH (decimal 46) controls verification of disk-write operations.
  Call this function with AL = 01H to set DOS's internal verify flag and
  enable verification; call it with AL = 00H to turn off the flag and
  verification. Also, in DOS versions 1 and 2, you must zero DL before you
  call function 2EH.

  The disk-verify operation requires the disk controller to perform a
  cyclical redundancy check (CRC) each time it writes data to the disk. This
  process involves reading the data just written, which significantly
  decreases the speed of disk writes.

  With DOS versions 2.0 and later, function 54H can be used to report the
  current setting of the verify flag. (See page 379.)