\chapter{Introduction to \myhdl\ }

\section{A basic \myhdl\ simulation}

Let's introduce \myhdl\ with a classical \code{Hello World} example
under the form of a \myhdl\ simulation run.  Here are the contents of
a \file{Hello1.py} script:

\begin{verbatim}
from myhdl import delay, now, Simulation

def sayHello():
    while 1:
        yield delay(10)
        print "%s Hello World!" % now()

gen = sayHello()
sim = Simulation(gen)
sim.run(30)

\end{verbatim}

When we run this script, we get the following output: 

\begin{verbatim}
% python Hello1.py
10 Hello World!
20 Hello World!
30 Hello World!
StopSimulation: Simulated for duration 30

\end{verbatim}

To explain what happened, we will go through the script line by
line. The first line imports a number of objects from the \myhdl\
package. In good Python style, and unlike most other languages, we can
only use identifiers that are \emph{literally} defined in the source
file \footnote{I don't want to explain the \code{ from package import
* } syntax}.

Next, we define a generator function called
\code{sayHello}. This is a generator function (as opposed to
a classic Python function) because it contains a \code{yield}
statement (instead of \code{return} statement). In \myhdl\, a
\code{yield} statement has a similar purpose as a \code{wait}
statement in VHDL: the statement suspends execution of the function,
and its clauses specify when the function should resume. In this case,
there is a \code{delay} clause, that specifies the required delay.

To make sure that the generator runs ``forever'', we wrap its behavior
in a \code{while 1} loop. This is as standard Python idiom, and it is
the \myhdl\ equivalent to a Verilog \code{always} block or a
VHDL \code{process}.

In hardware language terms, the generator function corresponds to a
hardware module. To simulate a module, we need to pass an
\emph{instance} of it to a simulator. In \myhdl{}, the equivalence of
instantiating is calling a generator function to create an actual
generator. For example, variable \code{gen} in the script refers a
generator that can be simulated. To do this, we first create a
\code{Simulation} object that takes the generator as its argument. We
then run the simulation for the desired amount of time.

\section{Concurrent generators and signals}



Then, we create a clock \code{Signal} named \code{clk}, with initial
value \code{0}. We also define a generator function \code{clkGen},
that operates on clock \code{clk}. 

After each delay, the clock is assigned a new value, by
assigning to its \code{next} attribute. Assigning to the 
\code{next} attribute of a signal is the \myhdl\ equivalent to signal
assignments in VHDL and non-blocking assignments in Verilog.