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  Function 3FH (decimal 63) reads the file or device associated with the
  handle in BX. The CX register specifies the number of bytes to read; DS:DX
  points to the buffer where data that is read will be placed. If the read
  operation is successful, function 3FH clears the carry flag and returns
  the number of bytes read in AX. If this value is 0, the function has tried
  to read from the end of a file. If the read operation fails, this function
  sets the carry flag and leaves an error code in AX. The possible error
  codes are 05H (access denied) and 06H (invalid handle).