first commit

.gitignore

+### Project Specific stuff
+test_run_dir/*
+### XilinxISE template
+# intermediate build files
+*.bgn
+*.bit
+*.bld
+*.cmd_log
+*.drc
+*.ll
+*.lso
+*.msd
+*.msk
+*.ncd
+*.ngc
+*.ngd
+*.ngr
+*.pad
+*.par
+*.pcf
+*.prj
+*.ptwx
+*.rbb
+*.rbd
+*.stx
+*.syr
+*.twr
+*.twx
+*.unroutes
+*.ut
+*.xpi
+*.xst
+*_bitgen.xwbt
+*_envsettings.html
+*_map.map
+*_map.mrp
+*_map.ngm
+*_map.xrpt
+*_ngdbuild.xrpt
+*_pad.csv
+*_pad.txt
+*_par.xrpt
+*_summary.html
+*_summary.xml
+*_usage.xml
+*_xst.xrpt
+
+# project-wide generated files
+*.gise
+par_usage_statistics.html
+usage_statistics_webtalk.html
+webtalk.log
+webtalk_pn.xml
+
+# generated folders
+iseconfig/
+xlnx_auto_0_xdb/
+xst/
+_ngo/
+_xmsgs/
+### Eclipse template
+*.pydevproject
+.metadata
+.gradle
+bin/
+tmp/
+*.tmp
+*.bak
+*.swp
+*~.nib
+local.properties
+.settings/
+.loadpath
+
+# Eclipse Core
+.project
+
+# External tool builders
+.externalToolBuilders/
+
+# Locally stored "Eclipse launch configurations"
+*.launch
+
+# CDT-specific
+.cproject
+
+# JDT-specific (Eclipse Java Development Tools)
+.classpath
+
+# Java annotation processor (APT)
+.factorypath
+
+# PDT-specific
+.buildpath
+
+# sbteclipse plugin
+.target
+
+# TeXlipse plugin
+.texlipse
+### C template
+# Object files
+*.o
+*.ko
+*.obj
+*.elf
+
+# Precompiled Headers
+*.gch
+*.pch
+
+# Libraries
+*.lib
+*.a
+*.la
+*.lo
+
+# Shared objects (inc. Windows DLLs)
+*.dll
+*.so
+*.so.*
+*.dylib
+
+# Executables
+*.exe
+*.out
+*.app
+*.i*86
+*.x86_64
+*.hex
+
+# Debug files
+*.dSYM/
+### SBT template
+# Simple Build Tool
+# http://www.scala-sbt.org/release/docs/Getting-Started/Directories.html#configuring-version-control
+
+target/
+lib_managed/
+src_managed/
+project/boot/
+.history
+.cache
+### Emacs template
+# -*- mode: gitignore; -*-
+*~
+\#*\#
+/.emacs.desktop
+/.emacs.desktop.lock
+*.elc
+auto-save-list
+tramp
+.\#*
+
+# Org-mode
+.org-id-locations
+*_archive
+
+# flymake-mode
+*_flymake.*
+
+# eshell files
+/eshell/history
+/eshell/lastdir
+
+# elpa packages
+/elpa/
+
+# reftex files
+*.rel
+
+# AUCTeX auto folder
+/auto/
+
+# cask packages
+.cask/
+### Vim template
+[._]*.s[a-w][a-z]
+[._]s[a-w][a-z]
+*.un~
+Session.vim
+.netrwhist
+*~
+### JetBrains template
+# Covers JetBrains IDEs: IntelliJ, RubyMine, PhpStorm, AppCode, PyCharm, CLion, Android Studio
+
+*.iml
+
+## Directory-based project format:
+.idea/
+# if you remove the above rule, at least ignore the following:
+
+# User-specific stuff:
+# .idea/workspace.xml
+# .idea/tasks.xml
+# .idea/dictionaries
+
+# Sensitive or high-churn files:
+# .idea/dataSources.ids
+# .idea/dataSources.xml
+# .idea/sqlDataSources.xml
+# .idea/dynamic.xml
+# .idea/uiDesigner.xml
+
+# Gradle:
+# .idea/gradle.xml
+# .idea/libraries
+
+# Mongo Explorer plugin:
+# .idea/mongoSettings.xml
+
+## File-based project format:
+*.ipr
+*.iws
+
+## Plugin-specific files:
+
+# IntelliJ
+/out/
+
+# mpeltonen/sbt-idea plugin
+.idea_modules/
+
+# JIRA plugin
+atlassian-ide-plugin.xml
+
+# Crashlytics plugin (for Android Studio and IntelliJ)
+com_crashlytics_export_strings.xml
+crashlytics.properties
+crashlytics-build.properties
+### C++ template
+# Compiled Object files
+*.slo
+*.lo
+*.o
+*.obj
+
+# Precompiled Headers
+*.gch
+*.pch
+
+# Compiled Dynamic libraries
+*.so
+*.dylib
+*.dll
+
+# Fortran module files
+*.mod
+
+# Compiled Static libraries
+*.lai
+*.la
+*.a
+*.lib
+
+# Executables
+*.exe
+*.out
+*.app
+### OSX template
+.DS_Store
+.AppleDouble
+.LSOverride
+
+# Icon must end with two \r
+Icon
+
+# Thumbnails
+._*
+
+# Files that might appear in the root of a volume
+.DocumentRevisions-V100
+.fseventsd
+.Spotlight-V100
+.TemporaryItems
+.Trashes
+.VolumeIcon.icns
+
+# Directories potentially created on remote AFP share
+.AppleDB
+.AppleDesktop
+Network Trash Folder
+Temporary Items
+.apdisk
+### Xcode template
+# Xcode
+#
+# gitignore contributors: remember to update Global/Xcode.gitignore, Objective-C.gitignore & Swift.gitignore
+
+## Build generated
+build/
+DerivedData
+
+## Various settings
+*.pbxuser
+!default.pbxuser
+*.mode1v3
+!default.mode1v3
+*.mode2v3
+!default.mode2v3
+*.perspectivev3
+!default.perspectivev3
+xcuserdata
+
+## Other
+*.xccheckout
+*.moved-aside
+*.xcuserstate
+### Scala template
+*.class
+*.log
+
+# sbt specific
+.cache
+.history
+.lib/
+dist/*
+target/
+lib_managed/
+src_managed/
+project/boot/
+project/plugins/project/
+
+# Scala-IDE specific
+.scala_dependencies
+.worksheet
+### Java template
+*.class
+
+# Mobile Tools for Java (J2ME)
+.mtj.tmp/
+
+# Package Files #
+*.jar
+*.war
+*.ear
+
+# virtual machine crash logs, see http://www.java.com/en/download/help/error_hotspot.xml
+hs_err_pid*
+

build.sbt

+// See README.md for license details.
+
+def scalacOptionsVersion(scalaVersion: String): Seq[String] = {
+  Seq() ++ {
+    // If we're building with Scala > 2.11, enable the compile option
+    //  switch to support our anonymous Bundle definitions:
+    //  https://github.com/scala/bug/issues/10047
+    CrossVersion.partialVersion(scalaVersion) match {
+      case Some((2, scalaMajor: Long)) if scalaMajor < 12 => Seq()
+      case _ => Seq("-Xsource:2.11")
+    }
+  }
+}
+
+def javacOptionsVersion(scalaVersion: String): Seq[String] = {
+  Seq() ++ {
+    // Scala 2.12 requires Java 8. We continue to generate
+    //  Java 7 compatible code for Scala 2.11
+    //  for compatibility with old clients.
+    CrossVersion.partialVersion(scalaVersion) match {
+      case Some((2, scalaMajor: Long)) if scalaMajor < 12 =>
+        Seq("-source", "1.7", "-target", "1.7")
+      case _ =>
+        Seq("-source", "1.8", "-target", "1.8")
+    }
+  }
+}
+
+name := "noop"
+
+version := "3.1.1"
+
+scalaVersion := "2.11.12"
+
+crossScalaVersions := Seq("2.11.12", "2.12.4")
+
+resolvers ++= Seq(
+  Resolver.sonatypeRepo("snapshots"),
+  Resolver.sonatypeRepo("releases")
+)
+
+// Provide a managed dependency on X if -DXVersion="" is supplied on the command line.
+val defaultVersions = Map(
+  "chisel3" -> "3.1.+",
+  "chisel-iotesters" -> "[1.2.5,1.3-SNAPSHOT["
+  )
+
+libraryDependencies ++= Seq("chisel3","chisel-iotesters").map {
+  dep: String => "edu.berkeley.cs" %% dep % sys.props.getOrElse(dep + "Version", defaultVersions(dep)) }
+
+scalacOptions ++= scalacOptionsVersion(scalaVersion.value)
+
+javacOptions ++= javacOptionsVersion(scalaVersion.value)

project/build.properties

+sbt.version = 1.1.1

project/plugins.sbt

+logLevel := Level.Warn
\ No newline at end of file

scalastyle-config.xml

+<scalastyle>
+ <name>Scalastyle standard configuration</name>
+ <check level="warning" class="org.scalastyle.file.FileTabChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.file.FileLengthChecker" enabled="true">
+  <parameters>
+   <parameter name="maxFileLength"><![CDATA[800]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.file.HeaderMatchesChecker" enabled="true">
+  <parameters>
+   <parameter name="header"><![CDATA[// See README.md for license details.]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.SpacesAfterPlusChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.file.WhitespaceEndOfLineChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.SpacesBeforePlusChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.file.FileLineLengthChecker" enabled="true">
+  <parameters>
+   <parameter name="maxLineLength"><![CDATA[120]]></parameter>
+   <parameter name="tabSize"><![CDATA[4]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.ClassNamesChecker" enabled="true">
+  <parameters>
+   <parameter name="regex"><![CDATA[[A-Z][A-Za-z]*]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.ObjectNamesChecker" enabled="true">
+  <parameters>
+   <parameter name="regex"><![CDATA[[A-Z][A-Za-z]*]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.PackageObjectNamesChecker" enabled="true">
+  <parameters>
+   <parameter name="regex"><![CDATA[^[a-z][A-Za-z]*$]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.EqualsHashCodeChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.IllegalImportsChecker" enabled="true">
+  <parameters>
+   <parameter name="illegalImports"><![CDATA[sun._,java.awt._]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.ParameterNumberChecker" enabled="true">
+  <parameters>
+   <parameter name="maxParameters"><![CDATA[8]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.MagicNumberChecker" enabled="true">
+  <parameters>
+   <parameter name="ignore"><![CDATA[-1,0,1,2,8,10,16]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.NoWhitespaceBeforeLeftBracketChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.NoWhitespaceAfterLeftBracketChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.ReturnChecker" enabled="false"></check>
+ <check level="warning" class="org.scalastyle.scalariform.NullChecker" enabled="false"></check>
+ <check level="warning" class="org.scalastyle.scalariform.NoCloneChecker" enabled="false"></check>
+ <check level="warning" class="org.scalastyle.scalariform.NoFinalizeChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.CovariantEqualsChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.StructuralTypeChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.file.RegexChecker" enabled="true">
+  <parameters>
+   <parameter name="regex"><![CDATA[;(\r|)\n]]></parameter>
+  </parameters>
+  <customMessage>No lines ending with a ;</customMessage>
+ </check>
+ <check level="warning" class="org.scalastyle.file.RegexChecker" enabled="true">
+  <parameters>
+   <parameter name="regex"><![CDATA[println]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.NumberOfTypesChecker" enabled="true">
+  <parameters>
+   <parameter name="maxTypes"><![CDATA[30]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.CyclomaticComplexityChecker" enabled="true">
+  <parameters>
+   <parameter name="maximum"><![CDATA[10]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.UppercaseLChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.SimplifyBooleanExpressionChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.IfBraceChecker" enabled="true">
+  <parameters>
+   <parameter name="singleLineAllowed"><![CDATA[true]]></parameter>
+   <parameter name="doubleLineAllowed"><![CDATA[false]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.MethodLengthChecker" enabled="true">
+  <parameters>
+   <parameter name="maxLength"><![CDATA[50]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.MethodNamesChecker" enabled="true">
+  <parameters>
+   <parameter name="regex"><![CDATA[^[a-z][A-Za-z0-9]*$]]></parameter>
+   <parameter name="ignoreRegex"><![CDATA[^(\+[&%]?|\-[&%]?|\*|/|%|&|\||\^|<|>|\|\||&&|:=|<>|<=|>=|!=|===|<<|>>|##|unary_(~|\-%?|!))$]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.NumberOfMethodsInTypeChecker" enabled="true">
+  <parameters>
+   <parameter name="maxMethods"><![CDATA[30]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.PublicMethodsHaveTypeChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.file.NewLineAtEofChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.file.NoNewLineAtEofChecker" enabled="false"></check>
+</scalastyle>

scalastyle-test-config.xml

+<scalastyle>
+ <name>Scalastyle configuration for Chisel3 unit tests</name>
+ <check level="warning" class="org.scalastyle.file.FileTabChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.file.FileLengthChecker" enabled="true">
+  <parameters>
+   <parameter name="maxFileLength"><![CDATA[800]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.file.HeaderMatchesChecker" enabled="true">
+  <parameters>
+   <parameter name="header"><![CDATA[// See README.md for license details.]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.SpacesAfterPlusChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.file.WhitespaceEndOfLineChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.SpacesBeforePlusChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.file.FileLineLengthChecker" enabled="true">
+  <parameters>
+   <parameter name="maxLineLength"><![CDATA[120]]></parameter>
+   <parameter name="tabSize"><![CDATA[4]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.ClassNamesChecker" enabled="true">
+  <parameters>
+   <parameter name="regex"><![CDATA[[A-Z][A-Za-z]*]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.ObjectNamesChecker" enabled="true">
+  <parameters>
+   <parameter name="regex"><![CDATA[[A-Z][A-Za-z]*]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.PackageObjectNamesChecker" enabled="true">
+  <parameters>
+   <parameter name="regex"><![CDATA[^[a-z][A-Za-z]*$]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.EqualsHashCodeChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.IllegalImportsChecker" enabled="true">
+  <parameters>
+   <parameter name="illegalImports"><![CDATA[sun._,java.awt._]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.ParameterNumberChecker" enabled="true">
+  <parameters>
+   <parameter name="maxParameters"><![CDATA[8]]></parameter>
+  </parameters>
+ </check>
+ <!-- Numerical constants are used a lot in test setups, it would be burdensome to require each one be its own val
+ declaration. -->
+ <check level="warning" class="org.scalastyle.scalariform.MagicNumberChecker" enabled="false"></check>
+ <!-- Scalatest's exception checking syntax looks like "a [ChiselException] should be thrownBy". -->
+ <check level="warning" class="org.scalastyle.scalariform.NoWhitespaceBeforeLeftBracketChecker" enabled="false"></check>
+ <check level="warning" class="org.scalastyle.scalariform.NoWhitespaceAfterLeftBracketChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.ReturnChecker" enabled="false"></check>
+ <check level="warning" class="org.scalastyle.scalariform.NullChecker" enabled="false"></check>
+ <check level="warning" class="org.scalastyle.scalariform.NoCloneChecker" enabled="false"></check>
+ <check level="warning" class="org.scalastyle.scalariform.NoFinalizeChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.CovariantEqualsChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.StructuralTypeChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.file.RegexChecker" enabled="true">
+  <parameters>
+   <parameter name="regex"><![CDATA[^.*;(\r|)\n]]></parameter>
+  </parameters>
+  <customMessage>No lines ending with a ;</customMessage>
+ </check>
+ <check level="warning" class="org.scalastyle.file.RegexChecker" enabled="true">
+  <parameters>
+   <parameter name="regex"><![CDATA[println]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.NumberOfTypesChecker" enabled="true">
+  <parameters>
+   <parameter name="maxTypes"><![CDATA[30]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.CyclomaticComplexityChecker" enabled="true">
+  <parameters>
+   <parameter name="maximum"><![CDATA[10]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.UppercaseLChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.SimplifyBooleanExpressionChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.scalariform.IfBraceChecker" enabled="true">
+  <parameters>
+   <parameter name="singleLineAllowed"><![CDATA[true]]></parameter>
+   <parameter name="doubleLineAllowed"><![CDATA[false]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.MethodLengthChecker" enabled="true">
+  <parameters>
+   <parameter name="maxLength"><![CDATA[50]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.MethodNamesChecker" enabled="true">
+  <parameters>
+   <parameter name="regex"><![CDATA[^[a-z][A-Za-z0-9]*$]]></parameter>
+   <parameter name="ignoreRegex"><![CDATA[^(\+[&%]?|\-[&%]?|\*|/|%|&|\||\^|<|>|\|\||&&|:=|<>|<=|>=|!=|===|<<|>>|##|unary_(~|\-%?|!))$]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.NumberOfMethodsInTypeChecker" enabled="true">
+  <parameters>
+   <parameter name="maxMethods"><![CDATA[30]]></parameter>
+  </parameters>
+ </check>
+ <check level="warning" class="org.scalastyle.scalariform.PublicMethodsHaveTypeChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.file.NewLineAtEofChecker" enabled="true"></check>
+ <check level="warning" class="org.scalastyle.file.NoNewLineAtEofChecker" enabled="false"></check>
+</scalastyle>

src/main/scala/gcd/GCD.scala

+// See README.md for license details.
+
+package gcd
+
+import chisel3._
+
+/**
+  * Compute GCD using subtraction method.
+  * Subtracts the smaller from the larger until register y is zero.
+  * value in register x is then the GCD
+  */
+class GCD extends Module {
+  val io = IO(new Bundle {
+    val value1        = Input(UInt(16.W))
+    val value2        = Input(UInt(16.W))
+    val loadingValues = Input(Bool())
+    val outputGCD     = Output(UInt(16.W))
+    val outputValid   = Output(Bool())
+  })
+
+  val x  = Reg(UInt())
+  val y  = Reg(UInt())
+
+  when(x > y) { x := x - y }
+    .otherwise { y := y - x }
+
+  when(io.loadingValues) {
+    x := io.value1
+    y := io.value2
+  }
+
+  io.outputGCD := x
+  io.outputValid := y === 0.U
+}

src/test/scala/gcd/GCDMain.scala

+// See README.md for license details.
+
+package gcd
+
+import chisel3._
+
+/**
+  * This provides an alternate way to run tests, by executing then as a main
+  * From sbt (Note: the test: prefix is because this main is under the test package hierarchy):
+  * {{{
+  * test:runMain gcd.GCDMain
+  * }}}
+  * To see all command line options use:
+  * {{{
+  * test:runMain gcd.GCDMain --help
+  * }}}
+  * To run with verilator:
+  * {{{
+  * test:runMain gcd.GCDMain --backend-name verilator
+  * }}}
+  * To run with verilator from your terminal shell use:
+  * {{{
+  * sbt 'test:runMain gcd.GCDMain --backend-name verilator'
+  * }}}
+  */
+object GCDMain extends App {
+  iotesters.Driver.execute(args, () => new GCD) {
+    c => new GCDUnitTester(c)
+  }
+}
+
+/**
+  * This provides a way to run the firrtl-interpreter REPL (or shell)
+  * on the lowered firrtl generated by your circuit. You will be placed
+  * in an interactive shell. This can be very helpful as a debugging
+  * technique. Type help to see a list of commands.
+  *
+  * To run from sbt
+  * {{{
+  * test:runMain gcd.GCDRepl
+  * }}}
+  * To run from sbt and see the half a zillion options try
+  * {{{
+  * test:runMain gcd.GCDRepl --help
+  * }}}
+  */
+object GCDRepl extends App {
+  iotesters.Driver.executeFirrtlRepl(args, () => new GCD)
+}
\ No newline at end of file

src/test/scala/gcd/GCDUnitTest.scala

+// See README.md for license details.
+
+package gcd
+
+import java.io.File
+
+import chisel3.iotesters
+import chisel3.iotesters.{ChiselFlatSpec, Driver, PeekPokeTester}
+
+class GCDUnitTester(c: GCD) extends PeekPokeTester(c) {
+  /**
+    * compute the gcd and the number of steps it should take to do it
+    *
+    * @param a positive integer
+    * @param b positive integer
+    * @return the GCD of a and b
+    */
+  def computeGcd(a: Int, b: Int): (Int, Int) = {
+    var x = a
+    var y = b
+    var depth = 1
+    while(y > 0 ) {
+      if (x > y) {
+        x -= y
+      }
+      else {
+        y -= x
+      }
+      depth += 1
+    }
+    (x, depth)
+  }
+
+  private val gcd = c
+
+  for(i <- 1 to 40 by 3) {
+    for (j <- 1 to 40 by 7) {
+      poke(gcd.io.value1, i)
+      poke(gcd.io.value2, j)
+      poke(gcd.io.loadingValues, 1)
+      step(1)
+      poke(gcd.io.loadingValues, 0)
+
+      val (expected_gcd, steps) = computeGcd(i, j)
+
+      step(steps - 1) // -1 is because we step(1) already to toggle the enable
+      expect(gcd.io.outputGCD, expected_gcd)
+      expect(gcd.io.outputValid, 1)
+    }
+  }
+}
+
+/**
+  * This is a trivial example of how to run this Specification
+  * From within sbt use:
+  * {{{
+  * testOnly gcd.GCDTester
+  * }}}
+  * From a terminal shell use:
+  * {{{
+  * sbt 'testOnly gcd.GCDTester'
+  * }}}
+  */
+class GCDTester extends ChiselFlatSpec {
+  // Disable this until we fix isCommandAvailable to swallow stderr along with stdout
+  private val backendNames = if(false && firrtl.FileUtils.isCommandAvailable(Seq("verilator", "--version"))) {
+    Array("firrtl", "verilator")
+  }
+  else {
+    Array("firrtl")
+  }
+  for ( backendName <- backendNames ) {
+    "GCD" should s"calculate proper greatest common denominator (with $backendName)" in {
+      Driver(() => new GCD, backendName) {
+        c => new GCDUnitTester(c)
+      } should be (true)
+    }
+  }
+
+  "Basic test using Driver.execute" should "be used as an alternative way to run specification" in {
+    iotesters.Driver.execute(Array(), () => new GCD) {
+      c => new GCDUnitTester(c)
+    } should be (true)
+  }
+
+  "using --backend-name verilator" should "be an alternative way to run using verilator" in {
+    if(backendNames.contains("verilator")) {
+      iotesters.Driver.execute(Array("--backend-name", "verilator"), () => new GCD) {
+        c => new GCDUnitTester(c)
+      } should be(true)
+    }
+  }
+
+  "running with --is-verbose" should "show more about what's going on in your tester" in {
+    iotesters.Driver.execute(Array("--is-verbose"), () => new GCD) {
+      c => new GCDUnitTester(c)
+    } should be(true)
+  }
+
+  /**
+    * By default verilator backend produces vcd file, and firrtl and treadle backends do not.
+    * Following examples show you how to turn on vcd for firrtl and treadle and how to turn it off for verilator
+    */
+
+  "running with --generate-vcd-output on" should "create a vcd file from your test" in {
+    iotesters.Driver.execute(
+      Array("--generate-vcd-output", "on", "--target-dir", "test_run_dir/make_a_vcd", "--top-name", "make_a_vcd"),
+      () => new GCD
+    ) {
+
+      c => new GCDUnitTester(c)
+    } should be(true)
+
+    new File("test_run_dir/make_a_vcd/make_a_vcd.vcd").exists should be (true)
+  }
+
+  "running with --generate-vcd-output off" should "not create a vcd file from your test" in {
+    iotesters.Driver.execute(
+      Array("--generate-vcd-output", "off", "--target-dir", "test_run_dir/make_no_vcd", "--top-name", "make_no_vcd",
+      "--backend-name", "verilator"),
+      () => new GCD
+    ) {
+
+      c => new GCDUnitTester(c)
+    } should be(true)
+
+    new File("test_run_dir/make_no_vcd/make_a_vcd.vcd").exists should be (false)
+
+  }
+
+}