Re: VHDLisms

Discussion in 'VHDL' started by Jonathan Bromley, Aug 18, 2003.

  1. "William Wallace" <> wrote in message
    news:...
    > 1. Is there a general rule I can use to know when I need to use the
    > word "is" and when I do not? Also, what good does making somebody
    > type these two characters do?


    It means, for example, that the syntax doesn't need parentheses
    around the selector of a "case" statement, because "is" acts as
    a separator between the selector and the rest of the statement.

    > 2. Is there a general rule I can use to figure out when two words are
    > combined into one word (e.g., "elsif") versus being kept as two words
    > (e.g., "end if")?


    Yup. The only such elision in VHDL is "elsif" (a slightly mysterious
    spelling; "elif" or "elseif" would perhaps have made more sense).
    Contrast with Verilog's absurd statement bracketing syntax, with
    the huge collection of elided terminators (endmodule, endcase,
    endgenerate... and a scad more of them in SystemVerilog) and
    interminable repetition of "begin...end".

    > 3. Is there a general rule I can use to figure out when I need to put
    > a semicolon (e.g., after the generic section and also the port section
    > in the component declaration) and when I do not (e.g., not after the
    > generic section but only after a port section of an instantiation)?


    I agree this is fidgety syntax. Semicolon acts as:

    - a statement TERMINATOR (one after every statement or declaration)
    - a list SEPARATOR (one after every port specification in an
    entity, except after the last one)

    The second of these is likely to be regularised in VHDL-200x,
    so that you can use semicolon at the very end of a port list if
    you wish.

    As for the instantiation syntax, surely it makes sense that
    the port and generic lists of an entity are quite separate
    things, but the port and entity bindings on an instance
    are part of the same statement?

    > 4. Why if I pass a argument, say a std_logic_vector, to a function
    > that takes a std_logic_vector, do I need to use data_in(arg'low) in
    > the function as opposed to data_in(0). Why doesn't VHDL automatically
    > rejustify a function call, say, function(data(15 downto 8)) to
    > data_in(7 downto 0) in the function automatically?


    So you can find out what you were given. If you want to renormalise
    the input vector, you can easily do that using alias, constant or
    a variable:

    function f (a: in std_logic_vector) return stuff is
    constant norm_a: std_logic_vector(a'LENGTH-1 downto 0) := a;
    begin
    ...

    This is one of several areas where VHDL has immense superiority
    over Verilog. See my recently posted fixed-point arithmetic
    package for a more extended example.

    > 5. Is there a better way to bit reverse a bus than "new_bus(7 downto
    > 0) <= old_bus(0) & old_bus(1) & ... & old_bus (7);", which gives my
    > hands cramps for large busses.


    Yup; use a for loop or a generate loop, depending on the situation.
    Unlike Verilog, you can do it in a completely general way, thanks to
    exactly the thing you were complaining about with the function
    arguments. (Needs VHDL-93 for the "reverse_range" attribute;
    without that, a bit more work is required but it's still easy.)
    There is an appealing recursive formulation too, also impossible
    in Verilog, but this version is more practical.

    function reverse_any_bus (a: in std_logic_vector)
    return std_logic_vector is
    variable result: std_logic_vector(a'RANGE);
    alias aa: std_logic_vector(a'REVERSE_RANGE) is a;
    begin
    for i in aa'RANGE loop
    result(i) := aa(i);
    end loop;
    return result;
    end; -- function reverse_any_bus

    Show me how to do that in Verilog :)

    > 6. What advantage is there in not allowing an port output to not be
    > used on the rhs of an assignment in a module, forcing the RTL authors
    > to create internal versions of such signals (e.g., "signal oData_v
    > std_logic_vector(3 downto 0);") and later doing an assignment such as
    > "oData <= oData_v;"?


    It gives better consistency between entities and subprograms, but
    I agree that it's a bit of a nuisance. On the other hand, creating
    an internal signal is hardly a five-star headache.

    > I have actually seen RTL code from VHDL
    > engineers who did not know Verilog come up with even more ludicrous
    > ways of solving this problem (routing an output back into a module as
    > an input for use on the RHS of assigments inside the module).


    There are people writing execrably bad VHDL, just as there are people
    writing execrably bad Verilog. That's life.

    > I have concluded that the people who came up with VHDL syntax were not
    > working with each other, not referring back to previous decisions they
    > made when making new ones,


    Hmm. Maybe that's why the VHDL LRM is half the thickness of the Verilog
    LRM, but is considerably more precise and consistent.

    > and/or took the concept of "strongly typed"
    > to a ludicrous extreme.


    You mean, like, they chose to USE that concept? There's plenty that
    needs fixing about VHDL, but very little of it is stuff that the
    Verilog camp are likely to be able to teach us!

    > It is also my belief that if it weren't for the government mandate,
    > and the hordes of VHDL-only engineers these mandates created (VHDL
    > fanatics who refuse to learn a better HDL), VHDL would be dead.


    Believe away. You have many supporters.

    > Any VHDL apologists care to explain?


    I prefer to think of myself as an "HDL apologist", as someone who's
    been using both Verilog and VHDL for a decade and who is very aware
    that both have their considerable strengths and weaknesses.
    --

    Jonathan Bromley, Consultant

    DOULOS - Developing Design Know-how
    VHDL * Verilog * SystemC * Perl * Tcl/Tk * Verification * Project Services

    Doulos Ltd. Church Hatch, 22 Market Place, Ringwood, Hampshire, BH24 1AW, UK
    Tel: +44 (0)1425 471223 mail:
    Fax: +44 (0)1425 471573 Web: http://www.doulos.com

    The contents of this message may contain personal views which
    are not the views of Doulos Ltd., unless specifically stated.
     
    Jonathan Bromley, Aug 18, 2003
    #1
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