what's the differences between the behavioral model and the RTLmodel?

Discussion in 'VHDL' started by risingsunxy@googlemail.com, Mar 9, 2006.

  1. Guest

    Hallo everyone,now I have a question,what's the differences between the
    behavioral model and the RTLmodel?

    I am reading some source codes of a simple processor,it is a behavioral
    model.
    As I understand ,the behavior model just describes how the processor
    behaves,
    so it is not the description of the real circuits.Can this behavioral
    model be synthesize?

    And in these codes ,i see a lot of procedures are used.Are they typical
    style of the behavioral descriptions?

    Ok,waiting for your answers : )

    Thanks
    , Mar 9, 2006
    #1
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  2. Mike Treseler, Mar 9, 2006
    #2
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  3. It is totally like your going to a restaraunt and give an order for one
    dish.

    If you order French Chicken, then the waiter will bring you a French
    Chicken. That is behavioral model. You don't have to know all details.
    Because its making is well known before you make an order.

    If you describe a detailed list on what an chef needs to do or prepare
    the dish:
    1. Buy a new chicken;
    2. Mix a French wine with it for a few minutes;
    ....
    That is a RTL model.

    What is the difference?

    For a RTL model, you must be a professional chef who knows everything
    to make a dish.
    For a behavioral model, you don't have to know every details, but know
    its name.

    With a behavioral model of CPU, you can immediately learn a taste how a
    CPU is designed, but miss every details of the design. That is
    important to a beginner.

    What an chip designer is doing everyday in a company is most likely a
    RTL model.

    Weng
    Weng Tianxiang, Mar 9, 2006
    #3
  4. Guest

    Thank you very much !
    so I think the behavioral model can not be syntheiszed ,right?
    Or,if i want to design a CPU,just a behavioral model is not enought,
    and i should consentrate on the RTL model .right?
    : )
    , Mar 9, 2006
    #4
  5. All behaviral models must be there for your to use. Especially they are
    compiled in a function or a procedure form and incooperated in
    synthesizer. If they are not ready, they are not available and you
    cannot use it.

    For example, when you multiply 2 16-bit integers, by using the
    behaviral model, what you need to do is to write
    C <= A*B; if they are defined properly. You don't have to care how it
    is implemented in details.

    And the synthizer will generate code for you. There is a liberary that
    does it for you.

    The library may be working, but not in the most efficient form.

    For your situation as a beginner, that you start with behaviral model
    is appropriate.

    All behaviral models, if they pass the compilation, can be synthesized
    without any problem.

    Weng
    Weng Tianxiang, Mar 10, 2006
    #5
  6. Hello,

    Weng Tianxiang schrieb:
    > For your situation as a beginner, that you start with behaviral model
    > is appropriate.
    >
    > All behaviral models, if they pass the compilation, can be synthesized
    > without any problem.


    I disagree. Behavioral means you describe what to do on a abstract
    level. This might be synthesisable but is not allways synthesisable. In
    fact behavioral means you didn't care, if its synthesisable. There
    exist tools to help you synthesising behavioral code, but they are
    limited.

    out<=a+b;

    is behavioral[1], but synthesisable with every tool, if a and b are
    integer or std_logic_vector. The code is hardly synthesisabel if a and
    b are complex type useing a special function for "+".

    The following behavioral model would pass every compilation but could
    never be sythesised:

    var a, b : integer
    for i in 1 to 10 loop
    a := a *b;
    b := b+i;
    end loop
    wait for 1 us;

    bye Thomas

    [1] and is also accepted as rtl code for integer or std_logic_vector.
    Thomas Stanka, Mar 10, 2006
    #6
  7. I agree with your this opinion:
    "Behavioral means you describe what to do on a abstract level."

    I like saying that in another way as a everyday life thing:
    Behaviral means something other people have already help build, either
    in a library or in a module.

    If they are available, they are synthesisable, otherwise they are not.

    Here is the function to do complex addition in IEEE Standard VHDL
    Mathematical Packages
    function "+" ( L: in COMPLEX; R: in COMPLEX ) return COMPLEX;

    Weng
    Weng Tianxiang, Mar 10, 2006
    #7
  8. Actually I started using VHDL about 6 years ago. As a starter, I was
    really confused with the behavioral and RTL model definitions. Later I
    learned that you don't have to understand it and can safely put it
    aside without any harm. Even their concepts are not only useless, but
    also harmful and meaningless.

    Do you hear the same things in software? No. it is not because the same
    things don't happen with software, but because in software industry, no
    body talks about the concept. When you call a subroutine in software,
    in VHDL, it is called behavioral, when you design it with assembly
    language, in VHDL, it is called RTL.

    When you hold a party, do you really care the dishes are made by
    yourselves or by order?

    The decisive thing about a code is that it is reliable and error free.

    Every beginner with VHDL or Verilog has to face the same dillema: what
    are the behavioral and RTL model definitions?

    Weng
    Weng Tianxiang, Mar 10, 2006
    #8
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