Subject: $sv-ec alias proposal comments
From: Francoise Martinolle (fm@cadence.com)
Date: Mon Nov 11 2002 - 14:08:08 PST
Kevin,
I finally got around reading your alias proposal. I have found a couple of
potential issues
that we have to clarify.
First I would like to say that creating aliases is already possible in
Verilog by creating
a module called alias which short cicuit its ports.
Example:
module alias (a, a)
inout [WIDTH:0] a;
endmodule
Then instantiate it in module top to alias b and {c,d} together:
module top
wire [15:7] b;
wire [7:4] c;
wire [3:0] d;
alias #7 u1( b, {c, d});
end module
I agree that your proposal may be more convenient as it formally allows to
declare aliases but I
wanted to point out that currently users can do the same with the current
Verilog language.
The only thing I would like to specify in your proposal is what the
resolved net type when
the net aliased together have diffent net types.
For example, you could have a tri0 wire aliased to a wand wire. What is the
resulting inout
net type?
The net type resolution table in the Verilog 1364 LRM (section 12.3.10 in
the 1364 -2001 standard
only deals with resolution of net types across ports and resolves it to
either the external net type or
the internal net type and in some situations issue a warning. The
resolution as stated in the LRM is
not a symmetrical relationship between external and internal: for example
between tri0 and wand,
tri0 not always wins. The external type wins.
The problem in the ec proposal is that the net type resolution is not
specified. Also we cannot
use the table 45 in the Verilog 1364 LRM because in the current alias
proposal, nets can be aliased
at the same level of hierarchy.
I also feels that the rules for addressing specification errors when declaring
aliases are superfluous. The proposal states 2 rules: not allowed to alias
a net to a sub-element
of itself, not allowed to specify a given alias more than once. Why these
rules? I feel that
we should be able to short circuit whatever. Is there any reason for these
rules?
Francoise
'
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