if-then vs. if-generate

Why lables are mandatory for generate statemente? Why *else* cannot be used
with *generate*? For example, it should be quite typical situation, you have
two different implementations of the same block depending on a generic.

B1: if Param1 generate
...
end generate;

B2: if not Param1 generate
...
end generate;



Wouldn't it be more neat to allow the following implementation of generate
block:

B1: if Param1 generate
...
else
...
end generate;

???? Thanks.

valentin tihomirov

On Sat, 21 Feb 2004 14:11:46 +0200, "valentin tihomirov"
<> wrote:

> Why *else* cannot be used
>with *generate*?

This is a mistake in the language, and will be fixed in VHDL 200x.
You will also be able to use 'case-generate'.

Expect tool support sometime this decade.

Regards,
Allan.

Allan Herriman

Hi,

valentin tihomirov wrote:
> Why lables are mandatory for generate statemente?

I am not sure about the motivation for this restriction
but I think the reason might be as follows:

Any components instantiated in the design must get a unique
instance name. E.g., the VHDL code

B1: if Param1 generate
mycomp: entity test port map (....);
end generate;

mycomp: entity test port map (....);

will generate two components named ":B1:mycomp" and ":mycomp".
If you were allowed to omit "B1" then both components would
receive the same instance name.

Of course, one might argue that the LRM could forbid creating
the same instance name for both components. However, note that
the generate statement creates a new declarative region.
E.g., one can write


architecture test of test is
constant myconst : integer := 11;
begin

B1: if Param1 generate
-- declares a new constant myconst that
-- is only visible within the "generate"
-- region (and overloads the previously
-- declared constant "myconst").
-- This is legal in VHDL.
constant myconst : bit := '1';
begin
mycomp: entity test port map (....);
end generate;

mycomp: entity test port map (....);

end test;

I.e., within the generate statement, new objects/types/... may
be created that "overload" other declarations with the same
name. So, defining "mycomp" to be illegal while declaration of
"myconst" is ok would create some kind of inconsistency.
Perhaps, this is the reason to make a label mandatory for the
generate statement.

Please note that I wasn't involved in the definition of
the VHDL LRM. So, I'm just guessing...

--
Edwin


>
> B1: if Param1 generate
> ...
> end generate;
>
> B2: if not Param1 generate
> ...
> end generate;
>
>
>
> Wouldn't it be more neat to allow the following implementation of generate
> block:
>
> B1: if Param1 generate
> ...
> else
> ...
> end generate;
>
> ???? Thanks.
>
>

Edwin Naroska