diff --git a/gate.rb b/gate.rb
index bc0d23d..5eeb3c0 100644
--- a/gate.rb
+++ b/gate.rb
@@ -1,6 +1,47 @@
 require_relative "port.rb"
 
-# @abstract Subclass and override {#input_changed} to implement a gate
+# @abstract Subclass and override {#inputs_changed} to implement a gate
 ## Base class for gates
 class Gate
+
+  # @!attribute [r] out
+    # @return [Port] the output port of the gate
+  attr_reader :out
+
+  # @param args [Array<Port>] These are the ports to be added to the gate.
+  # @note The first argument determines the width of the gate.
+  def initialize(*args)
+    @inputs=[]
+    @width=args[0].width
+    @out=Port.new(@width)
+    args.each do |input|
+      add_input(input)
+    end
+  end
+
+  # Add a port to the gate.
+  # @param port [Port] The port to be added. It must match the width of the gate.
+  # @return [void]
+  def add_input(port)
+    if port.width != @width then
+      raise ArgumentError, "Incorrect width #{port.width}, expected #{@width}"
+    end
+    @inputs.push(port)
+    port.add_callback do |value|
+      vals=[]
+      @inputs.each do |port|
+        vals.push port.val
+      end
+      inputs_changed(vals)
+     end
+    vals=[]
+    @inputs.each do |port|
+      vals.push port.val
+    end
+    inputs_changed(vals)
+  end
+
+  # @abstract Override this to implement a gate.
+  # @param vals [Array<Integer>] List of values for connected ports.
+  def inputs_changed(vals); raise NotImplementedError; end
 end
diff --git a/port.rb b/port.rb
index 5581300..238dc8d 100644
--- a/port.rb
+++ b/port.rb
@@ -19,6 +19,7 @@ class Port
   #   @param name [String] The name of the port
   def initialize(width, name="")
     @connected=[]
+    @callbacks=[]
     @propagating=false
     @val=0
     @width=width
@@ -26,8 +27,9 @@ class Port
     @strname=name
   end
 
-  # Sets the port's value
-  # @param val [Integer] The new value for the port
+  # Sets the port's value and calls all registered callbacks
+  # @param val [Integer] The new value for the port.
+  #   It must not exceed (width^2)-1,or an ArgumentError will be raised.
   # @return [void]
   def setval(val)
     # Prevent infinite loops when the connected port calls back when propagating.
@@ -37,6 +39,9 @@ class Port
       else
         raise ArgumentError,"#{val} is over maximum of #{@maxval}"
       end
+      @callbacks.each do |callback|
+        callback.call(@val)
+      end
       @propagating=true
       propagate()
       @propagating=false
@@ -51,6 +56,14 @@ class Port
     port.connect_back(self)
   end
 
+  # Adds a callback.
+  # @yield When callback called, gives the value of the port.
+  # @yieldparam value [Integer] The value of the port.
+  def add_callback(&callback)
+    #add block to the list, put at head
+    @callbacks.insert(0, callback)
+  end
+
   # Returns a string representation for debugging
   # @return [String] String name of the port, or super if none
   def to_s()
diff --git a/spec/gate_spec.rb b/spec/gate_spec.rb
new file mode 100644
index 0000000..1f11e22
--- /dev/null
+++ b/spec/gate_spec.rb
@@ -0,0 +1,34 @@
+require_relative "../gate.rb"
+require_relative "../port.rb"
+
+describe Gate do
+  let(:klass) do
+    Class.new(Gate) do
+      def inputs_changed(vals)
+        tot=0
+        vals.each do |val|
+          tot+=val
+        end
+        out.setval(tot)
+      end
+    end
+  end
+  it "calls #inputs_changed when input values change or an input is added" do
+    a=Port.new(8)
+    b=Port.new(8)
+    gate=klass.new(a,b)
+    expect(gate).to receive(:inputs_changed).with([8,0])
+    a.setval(8)
+    expect(gate).to receive(:inputs_changed).with([8,10])
+    b.setval(10)
+  end
+
+  it "can perform computations in #inputs_changed" do
+    a=Port.new(8)
+    b=Port.new(8)
+    gate=klass.new(a,b)
+    a.setval(8)
+    b.setval(10)
+    expect(gate.out.val).to eq(18)
+  end
+end
diff --git a/spec/port_spec.rb b/spec/port_spec.rb
index 35789ea..9c6b977 100644
--- a/spec/port_spec.rb
+++ b/spec/port_spec.rb
@@ -1,13 +1,13 @@
 require_relative "../port.rb"
 
 describe Port do
-  it "should set the port's value when we call setval" do
+  it "should set the port's value when we call #setval" do
     a=Port.new(4)
     a.setval(1)
     expect(a).to eq 1
   end
 
-  it "should propagate values when we call setval" do
+  it "should propagate values when we call #setval" do
     a=Port.new(4)
     b=Port.new(4)
     c=Port.new(4)
@@ -42,4 +42,10 @@ describe Port do
     a=Port.new(4)
     expect {a.setval(16)}.to raise_error ArgumentError
   end
+
+  it "should call registered callbacks" do
+    a=Port.new(4)
+    expect { |b|  a.add_callback(&b); a.setval(10) }.to yield_with_args(10)
+    expect { |b| a.add_callback(&b); a.setval(8) }.to yield_with_args(8)
+  end
 end