Adder

Preparations

Please navigate to $LJHOME/chisel-tutorial/src/main/scala/problems and open the Adder.scala. This is where the problem is set and where you will implement your solution.

Also, please keep handy the Chisel Introductory Examples page.

Understanding what's already there

In this problem you are required to implement an adder in such a way that: out should be the sum of in0 and in1. So let's do that first and then we will speak about the second line of the problem (parameterization).

Here are the lines provided by the tutorial:

class Adder(val w: Int) extends Module {
  val io = IO(new Bundle {
    val in0 = Input(UInt(1.W))
    val in1 = Input(UInt(1.W))
    val out = Output(UInt(1.W))
  })

  io.out := 0.U
}

The lines defining the Scala class (which extends Module), creating and collecting the module interfaces within an instance of a Bundle and designating the interface directions, have been explained.

What has not been mentioned until now is the line:

io.out := 0.U

To understand the operator := for re-assignment (in contrast to the operator = for assignment), let's take a look how nodes are dealt with in Chisel.

Nodes in Chisel - wire and val

When a val is declared in Scala, it creates a node that represents the data that it is assigned to. This tells the Chisel compiler to treat the value as wire¹.

In order to tell the compiler to allocate the value for the first time, i.e. upon the definition, the = operator is used. On every subsequent re-assignment to the value, we must use the operator :=.

An important fact to note here is that Chisel, unlike Verilog, is compiled sequentially. Therefore, re-assignment might be necessary in cases when a value or connection is unknown until later part of the Chisel code. The most obvious example of such necessity, is in the construction of the top level I/O for the module shown above; the value of out is not immediately known at the time of declaration and, thus, it must be re-assigned later in the code, i.e. in the line:

io.out := 0.U

Thinking about the implementation

The line re-assigning io.out, at the moment, is actually only a placeholder - as the assigned value is always the same and nowhere near the sum of in0 and in1. We need to, somehow, re-assign the value obtained from the addition of these two inputs to io.out. After a brief peak into the Chisel Cheat-sheet, we note that the addition in Chisel is performed as follows:

in0 + in1

However, re-assigning that value to io.out would cause an error - because in0 and in1 are members of io instance of a Bundle, just as out is. While talking about out, notice how it is referenced from outside of io. The correct way to access the inputs as analogous: io.in0 and io.in1.

Please take a minute to think and then write down the line that adds these two inputs and re-assigns the value obtained from that addition to the output and then continue to the next subsection.

Solution

The correct solution to this part of the problem is:

io.out := io.in0 + io.in1

Testing

In another window, please open ../../AdderTests.scala. At a glance, everything is familiar, as it was already briefly covered - which is enough for now. Just take a moment to recognize poke, step and expect. Close this file and navigate to $LJHOME/chisel-tutorial and run the appropriate test as follows:

./run-problem.sh Adder

and the standard output should provide:

...
================================================================================
Errors: 1: in the following tutorials
Tutorial Adder: exception error: ConcreteUInt(12, 1) bad width 1 needs 4
================================================================================

Exception: sbt.TrapExitSecurityException thrown from the UncaughtExceptionHandler in thread "run-main-0"
java.lang.RuntimeException: Nonzero exit code: 1
	at scala.sys.package$.error(package.scala:27)
[trace] Stack trace suppressed: run last test:runMain for the full output.
[error] (test:runMain) Nonzero exit code: 1
[error] Total time: 9 s, completed Jan 3, 2018 10:48:28 AM

However, the error you are seeing is due to the second line of the problem: Adder width should be parametrized and we will take care of that in the next step of the Learning Journey

---

1) ...as long as the value is not assigned to be a register type - more details in a later section.