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  Service 04H (decimal 4) verifies the contents of one or more disk sectors.
  This operation is not what many people think it is: No comparison is made
  between the data on the disk and the data in memory. The verification
  performed by this service simply checks that the sectors can be found and
  read and that the cyclical redundancy check (CRC) is correct. The CRC acts
  as a sophisticated parity check for the data in each sector and will
  detect most errors, such as lost or scrambled bits, very reliably.

  Most programmers use the verify service to check the results of a write
  operation after using service 03H, but you can verify any part of a disk
  at any time. The DOS FORMAT program, for example, verifies each track
  after it is formatted. However, many people regard verification as an
  unnecessary operation because the disk drives are so reliable and because
  ordinary error reporting works so well. Even DOS doesn't verify a write
  operation unless you ask it to with the VERIFY ON command.

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  NOTE:
    It's worth pausing here to note that there is nothing unusual or
    alarming about having "bad tracks" marked on a disk, particularly a
    fixed disk. In fact, it is quite common for a fixed disk to have a few
    bad patches on it. The DOS FORMAT program notices bad tracks and marks
    them as such in the disk's file-allocation table. Later, the bad-track
    marking tells DOS that these areas should be bypassed. Bad tracks are
    also common on diskettes; with a diskette, unlike a fixed disk, you have
    the option of throwing away the defective media and using only perfect
    disks.
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  The verify service operates exactly as do the read and write services and
  uses the same registers. The only difference between them is that the
  verify operation does not use any memory area and therefore does not use
  the register pair ES:BX.