OFFSET           Offset of Expression                           MASM Operator

  OFFSET expression

    Returns the number of bytes between the expression and the beginning
    of the segment it is defined in.

       Notes:     Expression can be a label, segment name, variable, or
                  other symbol.

                  In order to generate valid offsets when accessing
                  variables in groups or segments, use the segment-
                  override operator (':') to force OFFSET to calculate the
                  offset relative to the start of the group or segment.