Improve descriptiveness of GCD example (#113)

Author: edwardcwang

src/main/scala/examples/GCD.scala

@@ -3,23 +3,33 @@ package examples
 
 import chisel3._
 
+/**
+ * Compute the GCD of 'a' and 'b' using Euclid's algorithm.
+ * To start a computation, load the values into 'a' and 'b' and toggle 'load'
+ * high.
+ * The GCD will be returned in 'out' when 'valid' is high.
+ */
 class GCD extends Module {
   val io = IO(new Bundle {
-    val a  = Input(UInt(16.W))
-    val b  = Input(UInt(16.W))
-    val e  = Input(Bool())
-    val z  = Output(UInt(16.W))
-    val v  = Output(Bool())
+    val a     = Input(UInt(16.W))
+    val b     = Input(UInt(16.W))
+    val load  = Input(Bool())
+    val out   = Output(UInt(16.W))
+    val valid = Output(Bool())
   })
+  val x = Reg(UInt())
+  val y = Reg(UInt())
 
-  val x  = Reg(UInt())
-  val y  = Reg(UInt())
-  when   (x > y) {
-    x := x - y
+  when (io.load) {
+    x := io.a; y := io.b
+  } .otherwise {
+    when (x > y) {
+      x := x - y
+    } .elsewhen (x <= y) {
+      y := y - x
+    }
   }
-    .elsewhen (x <= y) { y := y - x }
 
-  when (io.e) { x := io.a; y := io.b }
-  io.z := x
-  io.v := y === 0.U
-}
+  io.out := x
+  io.valid := y === 0.U
+}

src/test/scala/examples/GCDTests.scala

@@ -13,18 +13,18 @@ class GCDTests(c: GCD) extends PeekPokeTester(c) {
     poke(c.io.a, inputs(i)._1)
     poke(c.io.b, inputs(i)._2)
 
-    poke(c.io.e, 1)
+    poke(c.io.load, 1)
     step(1)
-    poke(c.io.e, 0)
+    poke(c.io.load, 0)
 
     var ready = false
 
     do {
-      ready = peek(c.io.v) == 1
+      ready = peek(c.io.valid) == 1
       step(1)
     } while (t < 100 && ! ready)
 
-    expect(c.io.z, outputs(i))
+    expect(c.io.out, outputs(i))
     i += 1
   } while (t < 100 && i < 3)