VHDL component and outputs based on generic - vhdl

Question

I have a system consisting out of several components that has to be attached to a bus. However to keep the system bus independent, I gave the system a generic bus port that I run through a bus specific module that translates between my system and a specific bus. Thus it is easy to connect the entire system to different buses by switching the translating module.
However I don't want to go through the hassle of connecting the system with the translation module every time. Thus I wonder if it is possible to instantiate a module from a generic parameter, and use its types for the outputs of the architecture.
The whole becomes clearer with a little illustration, my translator modules have the following "signature".
entity translate<BusName>
port(toSystem: out toSystem_t,
fromSystem: in fromSystem_t,
toBus: out to<BusName>_t,
fromBus: in from<BusName>_t
end entity;
I now want to build an entity containing the system and a translator, based on a generic, somewhat like so:
entity entireSystem is
generic(busType := translate<BusName>)
port(toBus: out to<BusName>_t,
fromBus: in from<BusName>_t)
end entity
architecture genArc of entireSystem is
signal toTrans: fromSystem;
signal fromTrans: toSystem;
begin
system: system(toTrans,fromTrans)
translator: translate<BusName>(
fromTrans,
toTrans,
toBus,
fromBus
)
end architecture;
My problems are:
Can I use a generic parameter to directly instantiate a component, or do I have to go the if generic=xxx generate path? Can I derive the type of ports from a generic parameter.
It would be best if I can use one generic parameter to determine the ports and the entity, so that one can not pick wrong types for an entity by accidents. It would be fine to derive the types from the generic parameter with a function.

Answer 1

I don't think so, no.
If you want to instantiate different things based on a generic, you have to use if..generate.
The only influence that generics can have on the types of ports is that of changing the width. You can't switch between (say) an integer and a boolean based on a generic.

Answer 2

Can I use a generic parameter to directly instantiate a component,
or do I have to go the if generic=xxx generate path?
You can use the generic map syntax in module instantiation to pass the bus size to your sub-components. i.e.:
u_system: system
generic map ( INPUT_WIDTH => INPUT_WIDTH, OUTPUT_WIDTH => OUTPUT_WIDTH )
port map ( ... )
In the top level, you will need to have two generics instead of one.
Alternatively, assuming that your top level component must have the same bus size as your sub-components, you can try to do this in a package file and define the bus size by using function call. i.e.:
package pack is
-- # Define support bus name here
constant BusA : integer := 0;
...
constant BusZ : integer := 25;
-- # Bus name here
constant busname : integer := BusA;
-- # Input and Output Width
constant IWIDTH : integer := getIWIDTH( busname )
constant OWIDTH : integer := getOWIDTH( busname )
-- # Function declaration
function getIWIDTH( ii: integer ) return integer;
function getOWIDTH( ii: integer ) return integer;
end pack;
package body pack is
function getIWIDTH( ii: integer ) return integer is
variable oo : integer;
begin
-- # get iwidth here
return oo;
end function;
function getOWIDTH( ii: integer ) return integer is
variable oo : integer;
begin
-- # get owidth here
return oo;
end function;
end package body;