How  do  I  prevent  selected  parameters  of  a  module  from  being  overridden  during instantiation?

If a specific parameter within a module must be prevented from being overridden, then it must be  declared  using  the  localparam  construct,  rather  than  parameter  construct.  Localparam construct has been introduced from Verilog-2001. Note that a localparam variable is fully identical to being  defined  as  a  parameter,  too.  In  the  below example,  localparam  construct  is  used  to specify num_bits, and thus trying to override it directly gives an error message.

Consider that, though, that, since width and depth are specified using the parameter construct, they can be overridden at the time of instantiation or using defparam, and thus will indirectly override the num_bits values. Usually, localparam constructs are useful in defining new and localized identifiers whose values are derived from regular parameters.