Passthrough
Modules are the building blocks of digital circuits. We will start by showing how to define a module in Virdant.
Passthrough is a module which takes in some data and immediately sends it back out unchanged.
mod Passthrough {
incoming inp : Word[8]
outgoing out : Word[8]
out := inp
}
Modules are declared using the mod keyword, followed by the name of the module.
The body of the module consists of a set of declarations about what the module contains. The order these declarations appear in does not matter.
Ports
Ports are signals that allow communication between the module and the outside world.
incoming inp : Word[8]
outgoing out : Word[8]
In Virdant, each port has a direction, either incoming or outgoing, a name, and a type.
Note
Verilog uses the keywords input and output rather than incoming and outgoing. Virdant chooses the keywords it does because it makes the ports line up nicely. This makes the code much easier to read, without asking the designer to add extra spaces or tabs into their code.
Our example has two ports: inp and out.
The first port is an incoming port named inp. It has type Word[8]. This is how we say “an 8-bit value” in Virdant. The value of inp is given to the module from the outside world.
The second port is an outgoing port named out. It also has type Word[8]. It is a value we are sending out to the outside world. The module is obligated to drive the value of out using a driver statement.
Drivers
The last statement in our module is a driver statement:
out := inp
A driver statement tells Virdant what value a component will have. The left hand side is the component we are driving. The right hand side is an expression, which determines the value we drive.
In our case, we are driving out using the expression inp. In other words, we are taking the input to our module and simply sending it back out, unchanged. So while our Passthrough module doesn’t do very much, it does pass data through.
Note
The := driver in Virdant is analogous to the assign keyword in Verilog.
So how do you feel about the simplicity of Virdant so far? It turns out that Simplicity is one of the Principles of Virdant!