Int 13 Fn 04  - Disk - Verify Disk Sector(s)                               [B]

   AH = 04h
   AL = number of sectors to verify (must be nonzero)
   CH = low eight bits of cylinder number
   CL = sector number 1-63 (bits 0-5)
        high two bits of cylinder (bits 6-7, hard disk only)
   DH = head number
   DL = drive number (bit 7 set for hard disk)
   ES:BX -> data buffer (PC,XT,AT with BIOS prior to 11/15/85)

Return: CF set on error
   CF clear if successful
   AH = status (see AH=01h)
   AL = number of sectors verified

Notes: errors on a floppy may be due to the motor failing to spin up quickly
     enough; the write should be retried at least three times, resetting
     the disk with AH=00h between attempts
   this function does not compare the disk with memory, it merely
     checks whether the sector's stored CRC matches the data's actual CRC
   the IBM AT BIOS and many other BIOSes use only the low four bits of
     DH (head number) since the WD-1003 controller which is the standard
     AT controller (and the controller that IDE emulates) only supports
     16 heads
   AWARD AT BIOS and AMI 386sx BIOS have been extended to handle more
     than 1024 cylinders by placing bits 10 and 11 of the cylinder number
     into bits 6 and 7 of DH

See Also: AH=02h