diff --git a/spec/gb/cpu_spec.cr b/spec/gb/cpu_spec.cr
new file mode 100644
index 0000000..2633633
--- /dev/null
+++ b/spec/gb/cpu_spec.cr
@@ -0,0 +1,772 @@
+require "./spec_helper"
+
+describe CPU do
+  describe "registers" do
+    it "do computations correctly across registers" do
+      cpu = new_cpu [] of UInt8
+      cpu.b = 0x00
+      cpu.c = 0x00
+      cpu.bc.should eq 0x0000
+      cpu.c += 0x01
+      cpu.b.should eq 0x00
+      cpu.c.should eq 0x01
+      cpu.bc.should eq 0x0001
+      cpu.bc += 0x4320
+      cpu.b.should eq 0x43
+      cpu.c.should eq 0x21
+      cpu.bc.should eq 0x4321
+    end
+  end
+
+  describe "unprefixed opcode" do
+    describe "0x00" do
+      it "does nothing" do
+        cpu = new_cpu [0x00]
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+      end
+    end
+
+    describe "0x01" do
+      it "loads bc with d16" do
+        d16 = 0x1234
+        cpu = new_cpu [0x01, d16 & 0xFF, d16 >> 8]
+        cpu.tick
+
+        cpu.pc.should eq 3
+        cpu.sp.should eq 0xFFFE
+        cpu.bc.should eq d16
+      end
+    end
+
+    describe "0x02" do
+      it "loads (bc) with a" do
+        cpu = new_cpu [0x02]
+        cpu.a = 0x34
+        cpu.bc = 0xA000
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.memory[0xA000].should eq 0x34
+      end
+    end
+
+    describe "0x03" do
+      it "increments bc" do
+        cpu = new_cpu [0x03]
+        cpu.bc = 0x1234
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.bc.should eq 0x1235
+      end
+    end
+
+    describe "0x04" do
+      it "increments b" do
+        cpu = new_cpu [0x04]
+        cpu.b = 0x12
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.b.should eq 0x13
+      end
+    end
+
+    describe "0x05" do
+      it "decrements b" do
+        cpu = new_cpu [0x05]
+        cpu.b = 0x12
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.b.should eq 0x11
+      end
+    end
+
+    describe "0x06" do
+      it "loads b with d8" do
+        d8 = 0x12
+        cpu = new_cpu [0x06, d8]
+        cpu.tick
+
+        cpu.pc.should eq 2
+        cpu.sp.should eq 0xFFFE
+        cpu.b.should eq d8
+      end
+    end
+
+    describe "0x07" do
+      it "rotates accumulator left w/o carry" do
+        cpu = new_cpu [0x07]
+        cpu.a = 0b01011010
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.a.should eq 0b10110100
+        cpu.f_c.should eq false
+      end
+
+      it "rotates accumulator left w/ carry" do
+        cpu = new_cpu [0x07]
+        cpu.a = 0b10100101
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.a.should eq 0b01001011
+        cpu.f_c.should eq true
+      end
+    end
+
+    describe "0x08" do
+      it "loads (d16) with sp" do
+        d16 = 0xA000
+        cpu = new_cpu [0x08, d16 & 0xFF, d16 >> 8]
+        cpu.tick
+
+        cpu.pc.should eq 3
+        cpu.sp.should eq 0xFFFE
+        cpu.memory[0xA000] = 0xFFFE
+      end
+    end
+
+    describe "0x09" do
+      it "adds bc to hl" do
+        cpu = new_cpu [0x09]
+        cpu.hl = 0x1010
+        cpu.bc = 0x1111
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.hl.should eq 0x2121
+        cpu.bc.should eq 0x1111
+      end
+    end
+
+    describe "0x0A" do
+      it "loads a with (bc)" do
+        cpu = new_cpu [0x0A, 0x12]
+        cpu.bc = 0x0001_u8
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.bc.should eq 0x0001
+        cpu.memory[0x01].should eq 0x12
+      end
+    end
+
+    describe "0x0B" do
+      it "decrememnts bc" do
+        cpu = new_cpu [0x0B]
+        cpu.bc = 0x1234
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.bc.should eq 0x1233
+      end
+    end
+
+    describe "0x0C" do
+      it "increments c" do
+        cpu = new_cpu [0x0C]
+        cpu.c = 0x12
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.c.should eq 0x13
+      end
+    end
+
+    describe "0x0D" do
+      it "decrements c" do
+        cpu = new_cpu [0x0D]
+        cpu.c = 0x12
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.c.should eq 0x11
+      end
+    end
+
+    describe "0x0E" do
+      it "loads c with d8" do
+        cpu = new_cpu [0x0E, 0x12]
+        cpu.tick
+
+        cpu.pc.should eq 2
+        cpu.sp.should eq 0xFFFE
+        cpu.c.should eq 0x12
+      end
+    end
+
+    describe "0x0F" do
+      it "rotates accumulator right w/o carry" do
+        cpu = new_cpu [0x0F]
+        cpu.a = 0b01011010
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.a.should eq 0b00101101
+        cpu.f_c.should eq false
+      end
+
+      it "rotates accumulator right w/ carry" do
+        cpu = new_cpu [0x0F]
+        cpu.a = 0b10100101
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.a.should eq 0b11010010
+        cpu.f_c.should eq true
+      end
+    end
+
+    describe "0x10" do
+      it "stops execution" do
+        # todo: implement and test
+      end
+    end
+
+    describe "0x11" do
+      it "loads de with d16" do
+        d16 = 0x1234
+        cpu = new_cpu [0x11, d16 & 0xFF, d16 >> 8]
+        cpu.tick
+
+        cpu.pc.should eq 3
+        cpu.sp.should eq 0xFFFE
+        cpu.de.should eq d16
+      end
+    end
+
+    describe "0x12" do
+      it "loads (de) with a" do
+        cpu = new_cpu [0x12]
+        cpu.a = 0x34
+        cpu.de = 0xA000
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.memory[0xA000].should eq 0x34
+      end
+    end
+
+    describe "0x13" do
+      it "increments de" do
+        cpu = new_cpu [0x13]
+        cpu.de = 0x1234
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.de.should eq 0x1235
+      end
+    end
+
+    describe "0x14" do
+      it "increments d" do
+        cpu = new_cpu [0x14]
+        cpu.d = 0x12
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.d.should eq 0x13
+      end
+    end
+
+    describe "0x15" do
+      it "decrements d" do
+        cpu = new_cpu [0x15]
+        cpu.d = 0x12
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.d.should eq 0x11
+      end
+    end
+
+    describe "0x16" do
+      it "loads d with d8" do
+        d8 = 0x12
+        cpu = new_cpu [0x16, d8]
+        cpu.tick
+
+        cpu.pc.should eq 2
+        cpu.sp.should eq 0xFFFE
+        cpu.d.should eq d8
+      end
+    end
+
+    describe "0x17" do
+      it "rotates accumulator left through carry w/o carry" do
+        cpu = new_cpu [0x17]
+        cpu.a = 0b01011010
+        cpu.f_c = true
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.a.should eq 0b10110101
+        cpu.f_c.should eq false
+      end
+
+      it "rotates accumulator left through carry w/ carry" do
+        cpu = new_cpu [0x17]
+        cpu.a = 0b10100101
+        cpu.f_c = false
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.a.should eq 0b01001010
+        cpu.f_c.should eq true
+      end
+    end
+
+    # todo codes here
+
+    describe "0x21" do
+      it "loads hl with d16" do
+        d16 = 0x1234
+        cpu = new_cpu [0x21, d16 & 0xFF, d16 >> 8]
+        cpu.tick
+
+        cpu.pc.should eq 3
+        cpu.sp.should eq 0xFFFE
+        cpu.hl.should eq d16
+      end
+    end
+
+    describe "0x22" do
+      it "loads (hl+) with a" do
+        cpu = new_cpu [0x22]
+        cpu.a = 0x34
+        cpu.hl = 0xA000
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.memory[0xA000].should eq 0x34
+        cpu.hl.should eq 0xA001
+      end
+    end
+
+    describe "0x23" do
+      it "increments hl" do
+        cpu = new_cpu [0x23]
+        cpu.hl = 0x1234
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.hl.should eq 0x1235
+      end
+    end
+
+    describe "0x24" do
+      it "increments h" do
+        cpu = new_cpu [0x24]
+        cpu.h = 0x12
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.h.should eq 0x13
+      end
+    end
+
+    describe "0x25" do
+      it "decrements h" do
+        cpu = new_cpu [0x25]
+        cpu.h = 0x12
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.h.should eq 0x11
+      end
+    end
+
+    describe "0x26" do
+      it "loads h with d8" do
+        d8 = 0x12
+        cpu = new_cpu [0x26, d8]
+        cpu.tick
+
+        cpu.pc.should eq 2
+        cpu.sp.should eq 0xFFFE
+        cpu.h.should eq d8
+      end
+    end
+
+    # todo codes here
+
+    describe "0x31" do
+      it "loads sp with d16" do
+        d16 = 0x1234
+        cpu = new_cpu [0x31, d16 & 0xFF, d16 >> 8]
+        cpu.tick
+
+        cpu.pc.should eq 3
+        cpu.sp.should eq d16
+      end
+    end
+
+    describe "0x32" do
+      it "loads (hl-) with a" do
+        cpu = new_cpu [0x32]
+        cpu.a = 0x34
+        cpu.hl = 0xA000
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.memory[0xA000].should eq 0x34
+        cpu.hl.should eq 0x9FFF
+      end
+    end
+
+    describe "0x33" do
+      it "increments sp" do
+        cpu = new_cpu [0x33]
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFF
+      end
+    end
+
+    describe "0x34" do
+      it "increments (hl)" do
+        cpu = new_cpu [0x34]
+        cpu.memory[0xA000] = 0x12_u8
+        cpu.hl = 0xA000
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.memory[0xA000].should eq 0x13
+      end
+    end
+
+    describe "0x35" do
+      it "decrements (hl)" do
+        cpu = new_cpu [0x35]
+        cpu.memory[0xA000] = 0x12_u8
+        cpu.hl = 0xA000
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+        cpu.memory[0xA000].should eq 0x11
+      end
+    end
+
+    describe "0x36" do
+      it "loads (hl) with d8" do
+        d8 = 0x12
+        cpu = new_cpu [0x36, d8]
+        cpu.hl = 0xA000
+        cpu.tick
+
+        cpu.pc.should eq 2
+        cpu.sp.should eq 0xFFFE
+        cpu.memory[0xA000].should eq d8
+      end
+    end
+
+    # todo codes here
+
+    describe "0xC0" do
+      it "returns if nz" do
+        cpu = new_cpu [0xC0]
+        cpu.sp = 0xFFF0_u16
+        cpu.memory[0xFFF0] = 0x1234_u16
+        cpu.f_z = false
+        cpu.tick
+
+        cpu.pc.should eq 0x1234
+        cpu.sp.should eq 0xFFF2
+      end
+
+      it "doesn't return if not nz" do
+        cpu = new_cpu [0xC0]
+        cpu.f_z = true
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+      end
+    end
+
+    describe "0xC1" do
+      it "pops bc" do
+        cpu = new_cpu [0xC1]
+        cpu.sp = 0xFFF0_u16
+        cpu.memory[0xFFF0_u16] = 0x1234
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFF2
+        cpu.bc.should eq 0x1234
+      end
+    end
+
+    describe "0xC2" do
+      it "jumps to a16 if nz" do
+        a16 = 0xA000
+        cpu = new_cpu [0xC2, a16 & 0xFF, a16 >> 8]
+        cpu.f_z = false
+        cpu.tick
+
+        cpu.pc.should eq a16
+        cpu.sp.should eq 0xFFFE
+      end
+
+      it "doesn't jump to a16 if not nz" do
+        a16 = 0xA000
+        cpu = new_cpu [0xC2, a16 & 0xFF, a16 >> 8]
+        cpu.f_z = true
+        cpu.tick
+
+        cpu.pc.should eq 3
+        cpu.sp.should eq 0xFFFE
+      end
+    end
+
+    describe "0xC3" do
+      it "jumps to a16" do
+        a16 = 0xA000
+        cpu = new_cpu [0xC3, a16 & 0xFF, a16 >> 8]
+        cpu.tick
+
+        cpu.pc.should eq 0xA000
+        cpu.sp.should eq 0xFFFE
+      end
+
+      it "jumps to a16 regardless of nz" do
+        a16 = 0xAC00
+        cpu = new_cpu [0xC3, a16 & 0xFF, a16 >> 8]
+        cpu.f_z = false
+        cpu.tick
+        cpu.pc.should eq a16
+        cpu = new_cpu [0xC3, a16 & 0xFF, a16 >> 8]
+        cpu.f_z = true
+        cpu.tick
+        cpu.pc.should eq a16
+      end
+    end
+
+    describe "0xC4" do
+      it "calls a16 if nz" do
+        a16 = 0xAC00
+        cpu = new_cpu [0xC4, a16 & 0xFF, a16 >> 8]
+        cpu.f_z = false
+        cpu.tick
+
+        cpu.pc.should eq 0xAC00
+        cpu.sp.should eq 0xFFFC
+        cpu.memory[0xFFFD].should eq 0x00
+        cpu.memory[0xFFFC].should eq 0x03
+      end
+
+      it "doesn't call a16 if not nz" do
+        a16 = 0xAC00
+        cpu = new_cpu [0xC4, a16 & 0xFF, a16 >> 8]
+        cpu.f_z = true
+        cpu.tick
+
+        cpu.pc.should eq 3
+        cpu.sp.should eq 0xFFFE
+      end
+    end
+
+    describe "0xC5" do
+      it "pushes bc" do
+        cpu = new_cpu [0xC5]
+        cpu.bc = 0x1234_u16
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFC
+        cpu.memory[0xFFFD].should eq 0x12
+        cpu.memory[0xFFFC].should eq 0x34
+      end
+    end
+
+    describe "0xC6" do
+      it "adds d8 to a" do
+        d8 = 0x01
+        cpu = new_cpu [0xC6, d8]
+        cpu.a = 0xFF
+        cpu.tick
+
+        cpu.pc.should eq 2
+        cpu.sp.should eq 0xFFFE
+        cpu.a.should eq 0x00
+        cpu.f_z.should eq true
+        cpu.f_n.should eq false
+        cpu.f_h.should eq true
+        cpu.f_c.should eq true
+      end
+    end
+
+    # todo codes here
+
+    describe "0xD0" do
+      it "returns if nc" do
+        cpu = new_cpu [0xD0]
+        cpu.sp = 0xFFF0_u16
+        cpu.memory[0xFFF0] = 0x1234_u16
+        cpu.f_c = false
+        cpu.tick
+
+        cpu.pc.should eq 0x1234
+        cpu.sp.should eq 0xFFF2
+      end
+
+      it "doesn't return if not nc" do
+        cpu = new_cpu [0xD0]
+        cpu.f_c = true
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFE
+      end
+    end
+
+    describe "0xD1" do
+      it "pops de" do
+        cpu = new_cpu [0xD1]
+        cpu.sp = 0xFFF0_u16
+        cpu.memory[0xFFF0_u16] = 0x1234
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFF2
+        cpu.de.should eq 0x1234
+      end
+    end
+
+    describe "0xD2" do
+      it "jumps to a16 if nc" do
+        a16 = 0xA000
+        cpu = new_cpu [0xD2, a16 & 0xFF, a16 >> 8]
+        cpu.f_c = false
+        cpu.tick
+
+        cpu.pc.should eq a16
+        cpu.sp.should eq 0xFFFE
+      end
+
+      it "doesn't jump to a16 if not nc" do
+        a16 = 0xA000
+        cpu = new_cpu [0xD2, a16 & 0xFF, a16 >> 8]
+        cpu.f_c = true
+        cpu.tick
+
+        cpu.pc.should eq 3
+        cpu.sp.should eq 0xFFFE
+      end
+    end
+
+    describe "0xD3" do
+      # unused opcode
+    end
+
+    describe "0xD4" do
+      it "calls a16 if nc" do
+        a16 = 0xAC00
+        cpu = new_cpu [0xD4, a16 & 0xFF, a16 >> 8]
+        cpu.f_c = false
+        cpu.tick
+
+        cpu.pc.should eq 0xAC00
+        cpu.sp.should eq 0xFFFC
+        cpu.memory[0xFFFD].should eq 0x00
+        cpu.memory[0xFFFC].should eq 0x03
+      end
+
+      it "doesn't call a16 if not nc" do
+        a16 = 0xAC00
+        cpu = new_cpu [0xD4, a16 & 0xFF, a16 >> 8]
+        cpu.f_c = true
+        cpu.tick
+
+        cpu.pc.should eq 3
+        cpu.sp.should eq 0xFFFE
+      end
+    end
+
+    describe "0xD5" do
+      it "pushes de" do
+        cpu = new_cpu [0xD5]
+        cpu.de = 0x1234_u16
+        cpu.tick
+
+        cpu.pc.should eq 1
+        cpu.sp.should eq 0xFFFC
+        cpu.memory[0xFFFD].should eq 0x12
+        cpu.memory[0xFFFC].should eq 0x34
+      end
+    end
+
+    describe "0xD6" do
+      it "subs d8 from a" do
+        d8 = 0x01
+        cpu = new_cpu [0xD6, d8]
+        cpu.a = 0x10
+        cpu.tick
+
+        cpu.pc.should eq 2
+        cpu.sp.should eq 0xFFFE
+        cpu.a.should eq 0x0F
+        cpu.f_z.should eq false
+        cpu.f_n.should eq true
+        cpu.f_h.should eq true
+        cpu.f_c.should eq false
+      end
+    end
+
+    # todo codes here
+  end
+
+  describe "prefixed opcode" do
+    # todo codes here
+
+    describe "0x40" do
+      it "tests bit 0 of b" do
+        cpu = new_cpu [0xCB, 0x40]
+        cpu.b = 0b01010101
+        cpu.tick
+
+        cpu.pc.should eq 2
+        cpu.sp.should eq 0xFFFE
+        cpu.b.should eq 0b01010101
+        cpu.f_z.should eq false
+        cpu.f_c.should eq false
+        cpu.f_h.should eq true
+      end
+    end
+
+    # todo codes here
+  end
+end
diff --git a/spec/gb/cryboy_spec.cr b/spec/gb/cryboy_spec.cr
new file mode 100644
index 0000000..db95d13
--- /dev/null
+++ b/spec/gb/cryboy_spec.cr
@@ -0,0 +1,4 @@
+require "./spec_helper"
+
+describe Crab do
+end
diff --git a/spec/gb/joypad_spec.cr b/spec/gb/joypad_spec.cr
new file mode 100644
index 0000000..2abc4c9
--- /dev/null
+++ b/spec/gb/joypad_spec.cr
@@ -0,0 +1,54 @@
+describe Joypad do
+  it "defaults to buttons not pressed" do
+    joypad = Joypad.new
+    joypad.read.should eq 0b00111111
+  end
+
+  it "toggles button select" do
+    joypad = Joypad.new
+    joypad.write 0b00011111
+    joypad.read.should eq 0b00011111
+    joypad.write 0b00111111
+    joypad.read.should eq 0b00111111
+  end
+
+  it "toggles direction select" do
+    joypad = Joypad.new
+    joypad.write 0b00101111
+    joypad.read.should eq 0b00101111
+    joypad.write 0b00111111
+    joypad.read.should eq 0b00111111
+  end
+
+  it "only allows writing to selection keys" do
+    joypad = Joypad.new
+    joypad.write 0b00000000
+    joypad.read.should eq 0b00001111
+    joypad.write 0b11111111
+    joypad.read.should eq 0b00111111
+  end
+
+  it "sets correct bits for one key down" do
+    joypad = Joypad.new
+    joypad.down = true
+    joypad.write 0b00100000
+    joypad.read.should eq 0b00100111
+  end
+
+  it "sets correct bits for two different keys down" do
+    joypad = Joypad.new
+    joypad.down = true
+    joypad.b = true
+    joypad.write 0b00000000
+    joypad.read.should eq 0b00000101
+  end
+
+  it "sets correct bits for two parallel keys down" do
+    joypad = Joypad.new
+    joypad.up = true
+    joypad.select = true
+    joypad.button_keys = true
+    joypad.direction_keys = true
+    joypad.read.should eq 0b00001011
+  end
+end
diff --git a/spec/gb/memory_spec.cr b/spec/gb/memory_spec.cr
new file mode 100644
index 0000000..404a40a
--- /dev/null
+++ b/spec/gb/memory_spec.cr
@@ -0,0 +1,31 @@
+require "./spec_helper"
+
+describe GB::Memory do
+  it "can't write over rom" do
+    bytes = Array.new 0x8000, 0
+    bytes[0] = 0x01
+    bytes[1] = 0x02
+    bytes[2] = 0x03
+    memory = new_memory bytes
+    memory[0x0000] = 0x05_u8
+    memory[0x0001] = 0x06_u8
+    memory[0x3FFF] = 0x07_u8
+    memory[0x4000] = 0x07_u8
+    memory[0x7FFF] = 0x08_u8
+
+    memory[0x0000].should eq 0x01
+    memory[0x0001].should eq 0x02
+    memory[0x0002].should eq 0x03
+    memory[0x3FFF].should eq 0x00
+    memory[0x4000].should eq 0x00
+    memory[0x7FFF].should eq 0x00
+  end
+
+  it "writes to external ram simple" do
+    memory = new_memory [0x00]
+    memory[0xA000] = 0x12.to_u8
+    memory[0xBFFF] = 0x34.to_u8
+    memory[0xA000].should eq 0x12
+    memory[0xBFFF].should eq 0x34
+  end
+end
diff --git a/spec/gb/run_test_roms.cr b/spec/gb/run_test_roms.cr
new file mode 100644
index 0000000..6d76026
--- /dev/null
+++ b/spec/gb/run_test_roms.cr
@@ -0,0 +1,109 @@
+require "option_parser"
+
+TEST_RESULTS_DIR = "spec/gb/test_results"
+SCREENSHOT_DIR = "#{TEST_RESULTS_DIR}/screenshots"
+README_FILE = "#{TEST_RESULTS_DIR}/README.md"
+
+# kill process after given number of seconds
+def kill(process : Process, after : Number = 5) : Nil
+  spawn do
+    sleep after
+    process.signal Signal::KILL if process.exists?
+  end
+end
+
+def get_test_name(dir : String, test : String) : String
+  test.rpartition('.')[0][dir.size + 1, test.size]
+end
+
+alias TestResult = NamedTuple(test: String, pass: Bool)
+alias TestSuite = NamedTuple(suite: String, results: Array(TestResult))
+
+test_results : Array(TestSuite) = [] of TestSuite
+
+acid_dir = ""
+blargg_dir = ""
+mealybug_dir = ""
+mooneye_dir = ""
+
+OptionParser.parse do |parser|
+  parser.on("--acid PATH", "Path to directory with acid tests") { |path| acid_dir = path }
+  parser.on("--blargg PATH", "Path to directory with blargg tests") { |path| blargg = path }
+  parser.on("--mealybug PATH", "Path to directory with mealybug tests") { |path| mealybug_dir = path }
+  parser.on("--mooneye PATH", "Path to directory with mooneye tests") { |path| mooneye_dir = path }
+  parser.invalid_option { abort parser }
+end
+
+system "shards build -Dgraphics_test > /dev/null"
+
+unless acid_dir == ""
+  [true, false].each do |fifo|
+    test_results << {suite: "Acid#{" Fifo" if fifo}", results: [] of TestResult}
+    puts "Acid #{"Fifo " if fifo}Tests"
+    Dir.glob("#{acid_dir}/*acid2.gb*").sort.each do |path|
+      test_name = get_test_name acid_dir, path
+      Process.run "bin/crab", [path, "--headless"] + (fifo ? ["--fifo"] : [] of String) do |process|
+        kill process, after: 1
+      end
+      system %[touch out.png] # touch image in case something went wrong
+      system %[mv out.png #{SCREENSHOT_DIR}/#{test_name}#{"_fifo" if fifo}.png]
+      system %[compare -metric AE #{SCREENSHOT_DIR}/#{test_name}#{"_fifo" if fifo}.png #{SCREENSHOT_DIR}/expected/#{test_name}.png /tmp/crab_diff 2>/dev/null]
+      passed = $?.exit_status == 0
+      test_results[test_results.size - 1][:results] << {test: test_name, pass: passed}
+      print passed ? "." : "F"
+    end
+    print "\n"
+  end
+end
+
+unless mealybug_dir == ""
+  test_results << {suite: "Mealybug Fifo", results: [] of TestResult}
+  puts "Mealybug Fifo Tests"
+  Dir.glob("#{mealybug_dir}/**/*.gb").sort.each do |path|
+    test_name = get_test_name mealybug_dir, path
+    Process.run "bin/crab", [path, "--headless", "--fifo"] do |process|
+      kill process, after: 1
+    end
+    system %[touch out.png] # touch image in case something went wrong
+    system %[mv out.png #{SCREENSHOT_DIR}/#{test_name}.png]
+    system %[compare -metric AE #{SCREENSHOT_DIR}/#{test_name}.png #{SCREENSHOT_DIR}/expected/#{test_name}.png /tmp/crab_diff 2>/dev/null]
+    passed = $?.exit_status == 0
+    test_results[test_results.size - 1][:results] << {test: test_name, pass: passed}
+    print passed ? "." : "F"
+  end
+  print "\n"
+end
+
+system "shards build --release -Dprint_serial > /dev/null"
+
+unless mooneye_dir == ""
+  test_results << {suite: "Mooneye", results: [] of TestResult}
+  puts "Mooneye Tests"
+  fib_string = "358132134"
+  Dir.glob("#{mooneye_dir}/**/*.gb").sort.each do |path|
+    next if path.includes?("util") || path.includes?("manual-only") || path.includes?("dmg") || path.includes?("mgb") || path.includes?("sgb")
+    test_name = get_test_name mooneye_dir, path
+    passed = false
+    Process.run("bin/crab", [path, "--headless"]) do |process|
+      kill process, after: 10 # seconds
+      result = process.output.gets 9
+      process.terminate if process.exists?
+      passed = result == fib_string
+    end
+    test_results[test_results.size - 1][:results] << {test: test_name, pass: passed}
+    print passed ? "." : "F"
+  end
+  print "\n"
+end
+
+File.open README_FILE, "w" do |file|
+  file.puts "# Test Results"
+  test_results.each do |test_suite|
+    file.puts "## #{test_suite[:suite]} Tests"
+    file.puts "| Result | Test Name |"
+    file.puts "|--------|-----------|"
+    test_suite[:results].each do |test_result|
+      file.puts "| #{test_result[:pass] ? "👌" : "👀"} | #{test_result[:test]} |"
+    end
+  end
+end
diff --git a/spec/gb/spec_helper.cr b/spec/gb/spec_helper.cr
new file mode 100644
index 0000000..eacae48
--- /dev/null
+++ b/spec/gb/spec_helper.cr
@@ -0,0 +1,28 @@
+# define a new CPU with the given bytes as rom
+def new_cpu(bytes : Array(Int), cgb_enabled = true, boot = false)
+  interrupts = Interrupts.new
+  display = Display.new
+  ppu = PPU.new display, interrupts, pointerof(cgb_enabled)
+  apu = APU.new
+  timer = Timer.new interrupts
+
+  cpu = CPU.new new_memory(bytes), interrupts, ppu, apu, timer, boot
+  cpu.sp = 0xFFFE_u16
+  cpu
+end
+
+# define a new Memory with the given bytes as rom
+def new_memory(bytes : Array(Int), cgb_enabled = true, bootrom = nil)
+  rom = Bytes.new 0x8000
+  bytes.each_with_index do |byte, i|
+    rom[i] = byte.to_u8!
+  end
+  cartridge = GB::Cartridge.new rom
+  interrupts = GB::Interrupts.new
+  display = GB::Display.new
+  ppu = GB::PPU.new display, interrupts, pointerof(cgb_enabled)
+  apu = GB::APU.new
+  timer = GB::Timer.new interrupts
+  joypad = GB::Joypad.new
+  GB::Memory.new cartridge, interrupts, ppu, apu, timer, joypad, pointerof(cgb_enabled), bootrom
+end
diff --git a/spec/gb/test_results/README.md b/spec/gb/test_results/README.md
new file mode 100644
index 0000000..15d50ef
--- /dev/null
+++ b/spec/gb/test_results/README.md
@@ -0,0 +1,149 @@
+# Test Results
+## Acid Fifo Tests
+| Result | Test Name |
+|--------|-----------|
+| 👌 | cgb-acid2 |
+| 👌 | dmg-acid2 |
+## Acid Tests
+| Result | Test Name |
+|--------|-----------|
+| 👌 | cgb-acid2 |
+| 👌 | dmg-acid2 |
+## Mealybug Fifo Tests
+| Result | Test Name |
+|--------|-----------|
+| 👀 | m2_win_en_toggle |
+| 👀 | m3_bgp_change |
+| 👀 | m3_bgp_change_sprites |
+| 👀 | m3_lcdc_bg_en_change |
+| 👀 | m3_lcdc_bg_en_change2 |
+| 👀 | m3_lcdc_bg_map_change |
+| 👀 | m3_lcdc_bg_map_change2 |
+| 👀 | m3_lcdc_obj_en_change |
+| 👀 | m3_lcdc_obj_en_change_variant |
+| 👀 | m3_lcdc_obj_size_change |
+| 👀 | m3_lcdc_obj_size_change_scx |
+| 👀 | m3_lcdc_tile_sel_change |
+| 👀 | m3_lcdc_tile_sel_change2 |
+| 👀 | m3_lcdc_tile_sel_win_change |
+| 👀 | m3_lcdc_tile_sel_win_change2 |
+| 👀 | m3_lcdc_win_en_change_multiple |
+| 👀 | m3_lcdc_win_en_change_multiple_wx |
+| 👀 | m3_lcdc_win_map_change |
+| 👀 | m3_lcdc_win_map_change2 |
+| 👀 | m3_obp0_change |
+| 👀 | m3_scx_high_5_bits |
+| 👀 | m3_scx_high_5_bits_change2 |
+| 👌 | m3_scx_low_3_bits |
+| 👀 | m3_scy_change |
+| 👀 | m3_scy_change2 |
+| 👀 | m3_window_timing |
+| 👀 | m3_window_timing_wx_0 |
+| 👀 | m3_wx_4_change |
+| 👀 | m3_wx_4_change_sprites |
+| 👀 | m3_wx_5_change |
+| 👀 | m3_wx_6_change |
+## Mooneye Tests
+| Result | Test Name |
+|--------|-----------|
+| 👌 | acceptance/add_sp_e_timing |
+| 👌 | acceptance/bits/mem_oam |
+| 👌 | acceptance/bits/reg_f |
+| 👀 | acceptance/bits/unused_hwio-GS |
+| 👀 | acceptance/boot_div-S |
+| 👀 | acceptance/boot_div2-S |
+| 👀 | acceptance/boot_hwio-S |
+| 👌 | acceptance/call_cc_timing |
+| 👌 | acceptance/call_cc_timing2 |
+| 👌 | acceptance/call_timing |
+| 👌 | acceptance/call_timing2 |
+| 👌 | acceptance/di_timing-GS |
+| 👌 | acceptance/div_timing |
+| 👌 | acceptance/ei_sequence |
+| 👌 | acceptance/ei_timing |
+| 👌 | acceptance/halt_ime0_ei |
+| 👌 | acceptance/halt_ime0_nointr_timing |
+| 👌 | acceptance/halt_ime1_timing |
+| 👌 | acceptance/halt_ime1_timing2-GS |
+| 👌 | acceptance/if_ie_registers |
+| 👌 | acceptance/instr/daa |
+| 👌 | acceptance/interrupts/ie_push |
+| 👌 | acceptance/intr_timing |
+| 👌 | acceptance/jp_cc_timing |
+| 👌 | acceptance/jp_timing |
+| 👌 | acceptance/ld_hl_sp_e_timing |
+| 👌 | acceptance/oam_dma/basic |
+| 👌 | acceptance/oam_dma/reg_read |
+| 👀 | acceptance/oam_dma/sources-GS |
+| 👌 | acceptance/oam_dma_restart |
+| 👌 | acceptance/oam_dma_start |
+| 👌 | acceptance/oam_dma_timing |
+| 👌 | acceptance/pop_timing |
+| 👀 | acceptance/ppu/hblank_ly_scx_timing-GS |
+| 👌 | acceptance/ppu/intr_1_2_timing-GS |
+| 👌 | acceptance/ppu/intr_2_0_timing |
+| 👀 | acceptance/ppu/intr_2_mode0_timing |
+| 👀 | acceptance/ppu/intr_2_mode0_timing_sprites |
+| 👀 | acceptance/ppu/intr_2_mode3_timing |
+| 👀 | acceptance/ppu/intr_2_oam_ok_timing |
+| 👀 | acceptance/ppu/lcdon_timing-GS |
+| 👀 | acceptance/ppu/lcdon_write_timing-GS |
+| 👌 | acceptance/ppu/stat_irq_blocking |
+| 👀 | acceptance/ppu/stat_lyc_onoff |
+| 👀 | acceptance/ppu/vblank_stat_intr-GS |
+| 👌 | acceptance/push_timing |
+| 👌 | acceptance/rapid_di_ei |
+| 👌 | acceptance/ret_cc_timing |
+| 👌 | acceptance/ret_timing |
+| 👌 | acceptance/reti_intr_timing |
+| 👌 | acceptance/reti_timing |
+| 👌 | acceptance/rst_timing |
+| 👌 | acceptance/timer/div_write |
+| 👌 | acceptance/timer/rapid_toggle |
+| 👌 | acceptance/timer/tim00 |
+| 👌 | acceptance/timer/tim00_div_trigger |
+| 👌 | acceptance/timer/tim01 |
+| 👌 | acceptance/timer/tim01_div_trigger |
+| 👌 | acceptance/timer/tim10 |
+| 👌 | acceptance/timer/tim10_div_trigger |
+| 👌 | acceptance/timer/tim11 |
+| 👌 | acceptance/timer/tim11_div_trigger |
+| 👌 | acceptance/timer/tima_reload |
+| 👌 | acceptance/timer/tima_write_reloading |
+| 👌 | acceptance/timer/tma_write_reloading |
+| 👌 | emulator-only/mbc1/bits_bank1 |
+| 👌 | emulator-only/mbc1/bits_bank2 |
+| 👌 | emulator-only/mbc1/bits_mode |
+| 👌 | emulator-only/mbc1/bits_ramg |
+| 👀 | emulator-only/mbc1/multicart_rom_8Mb |
+| 👌 | emulator-only/mbc1/ram_256kb |
+| 👌 | emulator-only/mbc1/ram_64kb |
+| 👌 | emulator-only/mbc1/rom_16Mb |
+| 👌 | emulator-only/mbc1/rom_1Mb |
+| 👌 | emulator-only/mbc1/rom_2Mb |
+| 👌 | emulator-only/mbc1/rom_4Mb |
+| 👌 | emulator-only/mbc1/rom_512kb |
+| 👌 | emulator-only/mbc1/rom_8Mb |
+| 👌 | emulator-only/mbc2/bits_ramg |
+| 👌 | emulator-only/mbc2/bits_romb |
+| 👌 | emulator-only/mbc2/bits_unused |
+| 👌 | emulator-only/mbc2/ram |
+| 👌 | emulator-only/mbc2/rom_1Mb |
+| 👌 | emulator-only/mbc2/rom_2Mb |
+| 👌 | emulator-only/mbc2/rom_512kb |
+| 👌 | emulator-only/mbc5/rom_16Mb |
+| 👌 | emulator-only/mbc5/rom_1Mb |
+| 👌 | emulator-only/mbc5/rom_2Mb |
+| 👌 | emulator-only/mbc5/rom_32Mb |
+| 👌 | emulator-only/mbc5/rom_4Mb |
+| 👌 | emulator-only/mbc5/rom_512kb |
+| 👌 | emulator-only/mbc5/rom_64Mb |
+| 👌 | emulator-only/mbc5/rom_8Mb |
+| 👌 | misc/bits/unused_hwio-C |
+| 👀 | misc/boot_div-A |
+| 👀 | misc/boot_div-cgb0 |
+| 👌 | misc/boot_div-cgbABCDE |
+| 👀 | misc/boot_hwio-C |
+| 👀 | misc/boot_regs-A |
+| 👌 | misc/boot_regs-cgb |
+| 👀 | misc/ppu/vblank_stat_intr-C |
diff --git a/spec/gb/test_results/readme.md b/spec/gb/test_results/readme.md
new file mode 100644
index 0000000..15d50ef
--- /dev/null
+++ b/spec/gb/test_results/readme.md
@@ -0,0 +1,149 @@
+# Test Results
+## Acid Fifo Tests
+| Result | Test Name |
+|--------|-----------|
+| 👌 | cgb-acid2 |
+| 👌 | dmg-acid2 |
+## Acid Tests
+| Result | Test Name |
+|--------|-----------|
+| 👌 | cgb-acid2 |
+| 👌 | dmg-acid2 |
+## Mealybug Fifo Tests
+| Result | Test Name |
+|--------|-----------|
+| 👀 | m2_win_en_toggle |
+| 👀 | m3_bgp_change |
+| 👀 | m3_bgp_change_sprites |
+| 👀 | m3_lcdc_bg_en_change |
+| 👀 | m3_lcdc_bg_en_change2 |
+| 👀 | m3_lcdc_bg_map_change |
+| 👀 | m3_lcdc_bg_map_change2 |
+| 👀 | m3_lcdc_obj_en_change |
+| 👀 | m3_lcdc_obj_en_change_variant |
+| 👀 | m3_lcdc_obj_size_change |
+| 👀 | m3_lcdc_obj_size_change_scx |
+| 👀 | m3_lcdc_tile_sel_change |
+| 👀 | m3_lcdc_tile_sel_change2 |
+| 👀 | m3_lcdc_tile_sel_win_change |
+| 👀 | m3_lcdc_tile_sel_win_change2 |
+| 👀 | m3_lcdc_win_en_change_multiple |
+| 👀 | m3_lcdc_win_en_change_multiple_wx |
+| 👀 | m3_lcdc_win_map_change |
+| 👀 | m3_lcdc_win_map_change2 |
+| 👀 | m3_obp0_change |
+| 👀 | m3_scx_high_5_bits |
+| 👀 | m3_scx_high_5_bits_change2 |
+| 👌 | m3_scx_low_3_bits |
+| 👀 | m3_scy_change |
+| 👀 | m3_scy_change2 |
+| 👀 | m3_window_timing |
+| 👀 | m3_window_timing_wx_0 |
+| 👀 | m3_wx_4_change |
+| 👀 | m3_wx_4_change_sprites |
+| 👀 | m3_wx_5_change |
+| 👀 | m3_wx_6_change |
+## Mooneye Tests
+| Result | Test Name |
+|--------|-----------|
+| 👌 | acceptance/add_sp_e_timing |
+| 👌 | acceptance/bits/mem_oam |
+| 👌 | acceptance/bits/reg_f |
+| 👀 | acceptance/bits/unused_hwio-GS |
+| 👀 | acceptance/boot_div-S |
+| 👀 | acceptance/boot_div2-S |
+| 👀 | acceptance/boot_hwio-S |
+| 👌 | acceptance/call_cc_timing |
+| 👌 | acceptance/call_cc_timing2 |
+| 👌 | acceptance/call_timing |
+| 👌 | acceptance/call_timing2 |
+| 👌 | acceptance/di_timing-GS |
+| 👌 | acceptance/div_timing |
+| 👌 | acceptance/ei_sequence |
+| 👌 | acceptance/ei_timing |
+| 👌 | acceptance/halt_ime0_ei |
+| 👌 | acceptance/halt_ime0_nointr_timing |
+| 👌 | acceptance/halt_ime1_timing |
+| 👌 | acceptance/halt_ime1_timing2-GS |
+| 👌 | acceptance/if_ie_registers |
+| 👌 | acceptance/instr/daa |
+| 👌 | acceptance/interrupts/ie_push |
+| 👌 | acceptance/intr_timing |
+| 👌 | acceptance/jp_cc_timing |
+| 👌 | acceptance/jp_timing |
+| 👌 | acceptance/ld_hl_sp_e_timing |
+| 👌 | acceptance/oam_dma/basic |
+| 👌 | acceptance/oam_dma/reg_read |
+| 👀 | acceptance/oam_dma/sources-GS |
+| 👌 | acceptance/oam_dma_restart |
+| 👌 | acceptance/oam_dma_start |
+| 👌 | acceptance/oam_dma_timing |
+| 👌 | acceptance/pop_timing |
+| 👀 | acceptance/ppu/hblank_ly_scx_timing-GS |
+| 👌 | acceptance/ppu/intr_1_2_timing-GS |
+| 👌 | acceptance/ppu/intr_2_0_timing |
+| 👀 | acceptance/ppu/intr_2_mode0_timing |
+| 👀 | acceptance/ppu/intr_2_mode0_timing_sprites |
+| 👀 | acceptance/ppu/intr_2_mode3_timing |
+| 👀 | acceptance/ppu/intr_2_oam_ok_timing |
+| 👀 | acceptance/ppu/lcdon_timing-GS |
+| 👀 | acceptance/ppu/lcdon_write_timing-GS |
+| 👌 | acceptance/ppu/stat_irq_blocking |
+| 👀 | acceptance/ppu/stat_lyc_onoff |
+| 👀 | acceptance/ppu/vblank_stat_intr-GS |
+| 👌 | acceptance/push_timing |
+| 👌 | acceptance/rapid_di_ei |
+| 👌 | acceptance/ret_cc_timing |
+| 👌 | acceptance/ret_timing |
+| 👌 | acceptance/reti_intr_timing |
+| 👌 | acceptance/reti_timing |
+| 👌 | acceptance/rst_timing |
+| 👌 | acceptance/timer/div_write |
+| 👌 | acceptance/timer/rapid_toggle |
+| 👌 | acceptance/timer/tim00 |
+| 👌 | acceptance/timer/tim00_div_trigger |
+| 👌 | acceptance/timer/tim01 |
+| 👌 | acceptance/timer/tim01_div_trigger |
+| 👌 | acceptance/timer/tim10 |
+| 👌 | acceptance/timer/tim10_div_trigger |
+| 👌 | acceptance/timer/tim11 |
+| 👌 | acceptance/timer/tim11_div_trigger |
+| 👌 | acceptance/timer/tima_reload |
+| 👌 | acceptance/timer/tima_write_reloading |
+| 👌 | acceptance/timer/tma_write_reloading |
+| 👌 | emulator-only/mbc1/bits_bank1 |
+| 👌 | emulator-only/mbc1/bits_bank2 |
+| 👌 | emulator-only/mbc1/bits_mode |
+| 👌 | emulator-only/mbc1/bits_ramg |
+| 👀 | emulator-only/mbc1/multicart_rom_8Mb |
+| 👌 | emulator-only/mbc1/ram_256kb |
+| 👌 | emulator-only/mbc1/ram_64kb |
+| 👌 | emulator-only/mbc1/rom_16Mb |
+| 👌 | emulator-only/mbc1/rom_1Mb |
+| 👌 | emulator-only/mbc1/rom_2Mb |
+| 👌 | emulator-only/mbc1/rom_4Mb |
+| 👌 | emulator-only/mbc1/rom_512kb |
+| 👌 | emulator-only/mbc1/rom_8Mb |
+| 👌 | emulator-only/mbc2/bits_ramg |
+| 👌 | emulator-only/mbc2/bits_romb |
+| 👌 | emulator-only/mbc2/bits_unused |
+| 👌 | emulator-only/mbc2/ram |
+| 👌 | emulator-only/mbc2/rom_1Mb |
+| 👌 | emulator-only/mbc2/rom_2Mb |
+| 👌 | emulator-only/mbc2/rom_512kb |
+| 👌 | emulator-only/mbc5/rom_16Mb |
+| 👌 | emulator-only/mbc5/rom_1Mb |
+| 👌 | emulator-only/mbc5/rom_2Mb |
+| 👌 | emulator-only/mbc5/rom_32Mb |
+| 👌 | emulator-only/mbc5/rom_4Mb |
+| 👌 | emulator-only/mbc5/rom_512kb |
+| 👌 | emulator-only/mbc5/rom_64Mb |
+| 👌 | emulator-only/mbc5/rom_8Mb |
+| 👌 | misc/bits/unused_hwio-C |
+| 👀 | misc/boot_div-A |
+| 👀 | misc/boot_div-cgb0 |
+| 👌 | misc/boot_div-cgbABCDE |
+| 👀 | misc/boot_hwio-C |
+| 👀 | misc/boot_regs-A |
+| 👌 | misc/boot_regs-cgb |
+| 👀 | misc/ppu/vblank_stat_intr-C |
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diff --git a/spec/gb/util_spec.cr b/spec/gb/util_spec.cr
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--- /dev/null
+++ b/spec/gb/util_spec.cr
@@ -0,0 +1,17 @@
+require "./spec_helper"
+
+describe "util" do
+  describe "array_to_uint8" do
+    it "converts array to uint8" do
+      res = array_to_uint8 [false, true, 0, 1, 0_u8, 1_u8, -0, -1]
+      res.should eq 0b01010101_u8
+    end
+  end
+
+  describe "array_to_uint16" do
+    it "converts array to uint16" do
+      res = array_to_uint16 [false, true, 0, 1, 0_u8, 1_u8, -0, -1, false, true, 0, 1, 0_u8, 1_u8, -0, -1]
+      res.should eq 0b0101010101010101_u16
+    end
+  end
+end