passing some breaking specs

spec/mmu_spec.rb

@@ -41,12 +41,12 @@ describe Waterfoul::MMU do
 
   describe '#read_word' do
     before do
-      subject.write_byte 0xFFFF, 0x1
-      subject.write_byte 0xFFFE, 0x1
+      subject.write_byte 0xC001, 0x1
+      subject.write_byte 0xC000, 0x1
     end
 
     it 'reads two bytes from memory into a word' do
-      expect(subject.read_word(0xFFFE)).to eq 0x2
+      expect(subject.read_word(0xC000)).to eq 0x101
     end
   end
 
@@ -57,8 +57,4 @@ describe Waterfoul::MMU do
       expect(subject[0xFF41]).to eq 0xFF
     end
   end
-
-  context 'DMA transfer' do
-
-  end
 end

spec/registers_spec.rb

@@ -3,7 +3,27 @@ require 'spec_helper'
 describe Waterfoul::CPU do
   subject { Waterfoul::CPU.new }
 
-  [[:a, :f], [:b, :c], [:d, :e], [:h, :l]].each do |i|
+  describe '#af=' do
+    it 'doesnt overwrite the F register' do
+      subject.af = 0x101
+      expect(subject.f).to eq 0
+    end
+
+    it 'writes values into A register' do
+      subject.af = 0x101
+      expect(subject.a).to eq 0x1
+    end
+  end
+
+  describe '#af' do
+    it 'pairs the A, F register to form a 16 bit register' do
+      subject.set_register :a, 0x5
+      subject.set_register :f, 0x3
+      expect(subject.af).to eq 0x503
+    end
+  end
+
+  [[:b, :c], [:d, :e], [:h, :l]].each do |i|
     read_register_method = i.join
     write_register_method = i.join + '='