Type Conversion in Procedure Call

[Anand P Paralkar]

Hi,

I want to include a type conversion in a procedure interface list as shown
below.

The procedure "proc" operates on integer formals (a, b and c) while the
process contains real actuals (a1, b1 and c1).

The procedure call contains type conversion in the association list.

entity test2 is
port (et1 : inout bit);
end entity test2;

architecture tarch of test2 is
procedure proc (variable a , b : in integer;
variable c : out integer) is
begin
c := a + b;
end procedure proc;
begin

testp : process is
variable a1, b1, c1 : real;
begin
proc (a => integer(a1), b => integer(b1), real(c) => c1);--**Type Con
end process testp;
end architecture tarch;

Is it legal to include a type conversion as shown above in a procedure
call? The compiler returns an error:

actual associated with variable parameter must be a variable

for the associations :

a => integer(a1), b => integer(b1)

What the error mean and why is it being reported?

Thanks,
Anand

 

[Paul Uiterlinden]

Anand P Paralkar wrote:
[snip]

procedure proc (variable a , b : in integer;
variable c : out integer) is [snip]
proc (a => integer(a1), b => integer(b1), real(c) => c1);--**Type Con [snip]
The compiler returns an error:

actual associated with variable parameter must be a variable

for the associations :

a => integer(a1), b => integer(b1)

What the error mean and why is it being reported?

It means that the item on the right hand side of the of the => must be a
variable. The object returned by a function is a constant, not a variable. The
reason why it must be a variable is because you said so! However, I think you
meant constant, because the mode of formals a and b is IN.

So your procedure declaration should be:

procedure proc (constant a , b : in integer;
variable c : out integer) is

Or even simpler (my preference):

procedure proc (a , b : in integer;
c : out integer) is

If the object class for a parameter of mode in is not specified, constant is
assumed. If the object class for a parameter of mode out or inout is not specified,
variable is assumed.

As said, normally I do not specify the object class, unless signal or file is
needed.

Paul.

 

[Jim Lewis]

Anand,

What the error mean and why is it being reported?
actual associated with variable parameter must be a variable

I think the error message is misleading.
I think it should have said that variables are not permitted
as a formal input to a subprogram

I have looked through the LRM, for functions, all formal parameters
are of mode in and the object class must be constant, signal, or file
(clause 2.1.1).

I could not find an explicit statement for procedures, but
my bet would be that this restriction also applies to
procedure inputs. Anyone know an LRM reference?

From a methodology point of view, even if variables were allowed
as inputs, I would never recommend them. Constant class inputs
are allowed to be associated with a value from an expression (a
variable name is a simple expression), while variable class objects
must be associated with a variable object. Hence, applying a
variable class to an input limits the usability of the procedure.

This is the only thing I see problematic with your code.
So, I reach the same conclusion that Paul U. did,
your procedure declaration should be either:

procedure proc (constant a , b : in integer;
variable c : out integer) is

Or (since constant is the default in and variable is the default out):
procedure proc (a , b : in integer;
c : out integer) is

Or (since "in" is the default mode):
procedure proc (a , b : integer;
c : out integer) is

For readability, I like the first one best.

These are the simplified rules I use for procedure IO:

In:
constant everything except objects that must be signal
signal Objects that use a signal attribute (such as 'event)
Objects in a wait statement (not recommended for synthesis)
Testbench: Objects from design under test (otherwise they
may not be updated)

Out, InOut:
variable Things that go to a process (such as an error count)

signal If procedure called concurrently (such as in synthesis)
Testbench: Objects to the design under test


Cheers,
Jim
--
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Jim Lewis
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SynthWorks Design Inc. http://www.SynthWorks.com
1-503-590-4787

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