diff --git a/src/crab/gba/apu.cr b/src/crab/gba/apu.cr
index 1c711d3..57e4e2d 100644
--- a/src/crab/gba/apu.cr
+++ b/src/crab/gba/apu.cr
@@ -148,20 +148,19 @@ module GBA
       when 0x82          then @soundcnt_h.value.to_u8!
       when 0x83          then (@soundcnt_h.value >> 8).to_u8!
       when 0x84
-        0x70_u8 |
-          (@sound_enabled ? 0x80 : 0) |
+        (@sound_enabled ? 0x80_u8 : 0_u8) |
           (@channel4.enabled ? 0b1000 : 0) |
           (@channel3.enabled ? 0b0100 : 0) |
           (@channel2.enabled ? 0b0010 : 0) |
           (@channel1.enabled ? 0b0001 : 0)
-      when 0x85 then 0_u8 # unused
-      when 0x88 then @soundbias.value.to_u8!
-      when 0x89 then (@soundbias.value >> 8).to_u8!
-      else           puts "Unmapped APU read ~ addr:#{hex_str io_addr.to_u8}".colorize.fore(:red); 0_u8 # todo: open bus
+      when 0x85, 0x86, 0x87 then 0_u8
+      when 0x88             then @soundbias.value.to_u8!
+      when 0x89             then (@soundbias.value >> 8).to_u8!
+      when 0x8A, 0x8B       then 0_u8
+      else                       @gba.bus.read_open_bus_value(io_addr)
       end
     end
 
-    # write to apu memory
     def []=(io_addr : Int, value : UInt8) : Nil
       return unless @sound_enabled || 0x82 <= io_addr <= 0x89 || Channel3::WAVE_RAM_RANGE.includes?(io_addr)
       case io_addr
@@ -190,7 +189,6 @@ module GBA
       when 0x88 then @soundbias.value = (@soundbias.value & 0xFF00) | value
       when 0x89 then @soundbias.value = (@soundbias.value & 0x00FF) | value.to_u16 << 8
       when 0xA8..0xAF # unused
-      else puts "Unmapped APU write ~ addr:#{hex_str io_addr.to_u8}, val:#{value}".colorize(:yellow)
       end
     end
   end
diff --git a/src/crab/gba/apu/channel1.cr b/src/crab/gba/apu/channel1.cr
index d8097c5..d731d8a 100644
--- a/src/crab/gba/apu/channel1.cr
+++ b/src/crab/gba/apu/channel1.cr
@@ -79,12 +79,11 @@ module GBA
 
     def [](index : Int) : UInt8
       case index
-      when 0x60 then 0x80_u8 | @sweep_period << 4 | (@negate ? 0x08 : 0) | @shift
-      when 0x62 then 0x3F_u8 | @duty << 6
+      when 0x60 then @sweep_period << 4 | (@negate ? 0x08 : 0) | @shift
+      when 0x62 then @duty << 6
       when 0x63 then read_NRx2
-      when 0x64 then 0xFF_u8 # write-only
-      when 0x65 then 0xBF_u8 | (@length_enable ? 0x40 : 0)
-      else           puts "Reading from invalid Channel1 register: #{hex_str index.to_u16}".colorize.fore(:red); 0_u8 # todo: open bus
+      when 0x65 then (@length_enable ? 0x40_u8 : 0_u8)
+      else           0_u8
       end
     end
 
diff --git a/src/crab/gba/apu/channel2.cr b/src/crab/gba/apu/channel2.cr
index 8ca9fa4..508512a 100644
--- a/src/crab/gba/apu/channel2.cr
+++ b/src/crab/gba/apu/channel2.cr
@@ -45,11 +45,10 @@ module GBA
 
     def [](index : Int) : UInt8
       case index
-      when 0x68 then 0x3F_u8 | @duty << 6
+      when 0x68 then @duty << 6
       when 0x69 then read_NRx2
-      when 0x6C then 0xFF_u8 # write-only
-      when 0x6D then 0xBF_u8 | (@length_enable ? 0x40 : 0)
-      else           puts "Reading from invalid Channel2 register: #{hex_str index.to_u16}".colorize.fore(:red); 0_u8 # todo: open bus
+      when 0x6D then (@length_enable ? 0x40_u8 : 0_u8)
+      else           0_u8
       end
     end
 
diff --git a/src/crab/gba/apu/channel3.cr b/src/crab/gba/apu/channel3.cr
index 379cc7a..3ce88e9 100644
--- a/src/crab/gba/apu/channel3.cr
+++ b/src/crab/gba/apu/channel3.cr
@@ -51,18 +51,16 @@ module GBA
 
     def [](index : Int) : UInt8
       case index
-      when 0x70 then 0x7F_u8 | (@dac_enabled ? 0x80 : 0)
-      when 0x72 then 0xFF_u8
-      when 0x73 then 0x9F_u8 | @volume_code << 5
-      when 0x74 then 0xFF_u8
-      when 0x75 then 0xBF_u8 | (@length_enable ? 0x40 : 0)
+      when 0x70 then (@dac_enabled ? 0x80_u8 : 0_u8) | @wave_ram_bank << 6 | (@wave_ram_dimension ? 0x20 : 0)
+      when 0x73 then (@volume_force ? 0x80_u8 : 0_u8) | @volume_code << 5
+      when 0x75 then (@length_enable ? 0x40_u8 : 0_u8)
       when WAVE_RAM_RANGE
         if @enabled
           @wave_ram[@wave_ram_bank][@wave_ram_position // 2]
         else
           @wave_ram[@wave_ram_bank][index - WAVE_RAM_RANGE.begin]
         end
-      else puts "Reading from invalid Channel3 register: #{hex_str index.to_u16}".colorize.fore(:red); 0_u8 # todo: open bus
+      else 0_u8
       end
     end
 
diff --git a/src/crab/gba/apu/channel4.cr b/src/crab/gba/apu/channel4.cr
index 89ebe07..fde63f5 100644
--- a/src/crab/gba/apu/channel4.cr
+++ b/src/crab/gba/apu/channel4.cr
@@ -45,11 +45,10 @@ module GBA
 
     def [](index : Int) : UInt8
       case index
-      when 0x78 then 0xFF_u8
       when 0x79 then read_NRx2
       when 0x7C then @clock_shift << 4 | @width_mode << 3 | @divisor_code
-      when 0x7D then 0xBF_u8 | (@length_enable ? 0x40 : 0)
-      else           puts "Reading from invalid Channel4 register: #{hex_str index.to_u16}".colorize.fore(:red); 0_u8 # todo: open bus
+      when 0x7D then (@length_enable ? 0x40_u8 : 0_u8)
+      else           0_u8
       end
     end
 
diff --git a/src/crab/gba/apu/dma_channels.cr b/src/crab/gba/apu/dma_channels.cr
index 46434aa..ff444bf 100644
--- a/src/crab/gba/apu/dma_channels.cr
+++ b/src/crab/gba/apu/dma_channels.cr
@@ -20,7 +20,7 @@ module GBA
     end
 
     def [](index : Int) : UInt8
-      0_u8
+      @gba.bus.read_open_bus_value(index)
     end
 
     def []=(index : Int, value : Byte) : Nil
diff --git a/src/crab/gba/bus.cr b/src/crab/gba/bus.cr
index 6cfbdeb..5f8fba5 100644
--- a/src/crab/gba/bus.cr
+++ b/src/crab/gba/bus.cr
@@ -266,5 +266,19 @@ module GBA
       write_byte_internal(index + 2, (value >> 16).to_u8!)
       write_byte_internal(index + 3, (value >> 24).to_u8!)
     end
+
+    def read_open_bus_value(index : Int, _file = __FILE__) : Byte
+      log "Reading open bus at #{hex_str index.to_u32} from #{_file}"
+      shift = (index & 3) * 8
+      if @gba.cpu.cpsr.thumb
+        # todo: special handling for 16-bit vs 32-bit regions
+        # todo: does this need to have both of the previous opcodes?
+        opcode = read_half_internal(@gba.cpu.r[15] & ~1).to_u32!
+        word = opcode << 16 | opcode
+      else
+        word = read_word_internal(@gba.cpu.r[15] & ~3)
+      end
+      (word >> shift).to_u8!
+    end
   end
 end
diff --git a/src/crab/gba/cpu.cr b/src/crab/gba/cpu.cr
index d24ffeb..d53ea10 100644
--- a/src/crab/gba/cpu.cr
+++ b/src/crab/gba/cpu.cr
@@ -43,7 +43,7 @@ module GBA
     end
 
     getter r = Slice(Word).new 16
-    @cpsr : PSR = PSR.new CPU::Mode::SYS.value
+    getter cpsr : PSR = PSR.new CPU::Mode::SYS.value
     @spsr : PSR = PSR.new CPU::Mode::SYS.value
     getter pipeline = Pipeline.new
     getter lut : Slice(Proc(Word, Nil)) { fill_lut }
@@ -100,12 +100,12 @@ module GBA
     def fill_pipeline : Nil
       if @cpsr.thumb
         pc = @r[15] & ~1
-        @pipeline.push @gba.bus.read_half(@r[15] &- 2).to_u32! if @pipeline.size == 0
-        @pipeline.push @gba.bus.read_half(@r[15]).to_u32! if @pipeline.size == 1
+        @pipeline.push @gba.bus.read_half(pc &- 2).to_u32! if @pipeline.size == 0
+        @pipeline.push @gba.bus.read_half(pc).to_u32! if @pipeline.size == 1
       else
         pc = @r[15] & ~3
-        @pipeline.push @gba.bus.read_word(@r[15] &- 4) if @pipeline.size == 0
-        @pipeline.push @gba.bus.read_word(@r[15]) if @pipeline.size == 1
+        @pipeline.push @gba.bus.read_word(pc &- 4) if @pipeline.size == 0
+        @pipeline.push @gba.bus.read_word(pc) if @pipeline.size == 1
       end
     end
 
diff --git a/src/crab/gba/dma.cr b/src/crab/gba/dma.cr
index 62d0332..dfe81b7 100644
--- a/src/crab/gba/dma.cr
+++ b/src/crab/gba/dma.cr
@@ -46,17 +46,12 @@ module GBA
       channel = (io_addr - 0xB0) // 12
       reg = (io_addr - 0xB0) % 12
       case reg
-      when 0, 1, 2, 3 # dmasad
-        0_u8          # todo: OOB read
-      when 4, 5, 6, 7 # dmadad
-        0_u8          # todo: OOB read
-      when 8, 9       # dmacnt_l
-        0_u8          # write-only
-      when 10, 11     # dmacnt_h
+      when 8, 9 then 0_u8 # dmacnt_l write-only
+      when 10, 11
         val = @dmacnt_h[channel].read_byte(io_addr & 1)
         val |= 0b1000 if io_addr == 0xDF && @dmacnt_h[3].game_pak # DMA3 only
         val
-      else abort "Unmapped DMA read ~ addr:#{hex_str io_addr.to_u8}"
+      else @gba.bus.read_open_bus_value(io_addr)
       end
     end
 
diff --git a/src/crab/gba/mmio.cr b/src/crab/gba/mmio.cr
index 22490c0..bca5b15 100644
--- a/src/crab/gba/mmio.cr
+++ b/src/crab/gba/mmio.cr
@@ -7,29 +7,23 @@ module GBA
 
     def [](index : Int) : Byte
       io_addr = 0xFFFFFF_u32 & index
-      if io_addr <= 0x05F
-        @gba.ppu[io_addr]
-      elsif io_addr <= 0xAF
-        @gba.apu[io_addr]
-      elsif io_addr <= 0xFF
-        @gba.dma[io_addr]
-      elsif 0x100 <= io_addr <= 0x10F
-        @gba.timer[io_addr]
-      elsif 0x130 <= io_addr <= 0x133
-        @gba.keypad[io_addr]
-      elsif 0x120 <= io_addr <= 0x12F || 0x134 <= io_addr <= 0x1FF
+      case io_addr
+      when 0x000..0x055 then @gba.ppu[io_addr]
+      when 0x060..0x0A7 then @gba.apu[io_addr]
+      when 0x0B0..0x0DF then @gba.dma[io_addr]
+      when 0x100..0x10F then @gba.timer[io_addr]
+      when 0x120..0x12B, 0x134..0x159
         # todo: serial
         if io_addr == 0x135
           0x80_u8
         else
           0_u8
         end
-      elsif 0x200 <= io_addr <= 0x203 || 0x208 <= io_addr <= 0x209
-        @gba.interrupts[io_addr]
-      elsif 0x204 <= io_addr <= 0x205
-        @waitcnt.read_byte(io_addr & 1)
-      else
-        0_u8 # todo: oob reads
+      when 0x130..0x133 then @gba.keypad[io_addr]
+      when 0x200..0x203,
+           0x208..0x209 then @gba.interrupts[io_addr]
+      when 0x204..0x205 then @waitcnt.read_byte(io_addr & 1)
+      else                   @gba.bus.read_open_bus_value(io_addr)
       end
     end
 
diff --git a/src/crab/gba/ppu.cr b/src/crab/gba/ppu.cr
index 1c8d4da..565d4fd 100644
--- a/src/crab/gba/ppu.cr
+++ b/src/crab/gba/ppu.cr
@@ -402,34 +402,16 @@ module GBA
       when 0x002..0x003 then 0_u8 # todo green swap
       when 0x004..0x005 then @dispstat.read_byte(io_addr & 1)
       when 0x006..0x007 then (@vcount >> (8 * (io_addr & 1))).to_u8!
-      when 0x008..0x00F then @bgcnt[(io_addr - 0x008) >> 1].read_byte(io_addr & 1)
-      when 0x010..0x01F
-        bg_num = (io_addr - 0x010) >> 2
-        if bit?(io_addr, 1)
-          @bgvofs[bg_num].read_byte(io_addr & 1)
-        else
-          @bghofs[bg_num].read_byte(io_addr & 1)
-        end
-      when 0x020..0x03F
-        bg_num = (io_addr & 0x10) >> 4 # (bg 0/1 represents bg 2/3, since those are the only aff bgs)
-        offs = io_addr & 0xF
-        if offs >= 8
-          offs -= 8
-          @bgref[bg_num][offs >> 2].read_byte(offs & 3)
-        else
-          @bgaff[bg_num][offs >> 1].read_byte(offs & 1)
-        end
-      when 0x040..0x041 then @win0h.read_byte(io_addr & 1)
-      when 0x042..0x043 then @win1h.read_byte(io_addr & 1)
-      when 0x044..0x045 then @win0v.read_byte(io_addr & 1)
-      when 0x046..0x047 then @win1v.read_byte(io_addr & 1)
+      when 0x008..0x00F
+        bg_num = (io_addr - 0x008) >> 1
+        val = @bgcnt[bg_num].read_byte(io_addr & 1)
+        val |= 0x20 if (io_addr == 0xD || io_addr == 0xF) && @bgcnt[bg_num].affine_wrap
+        val
       when 0x048..0x049 then @winin.read_byte(io_addr & 1)
       when 0x04A..0x04B then @winout.read_byte(io_addr & 1)
-      when 0x04C..0x04D then @mosaic.read_byte(io_addr & 1)
       when 0x050..0x051 then @bldcnt.read_byte(io_addr & 1)
       when 0x052..0x053 then @bldalpha.read_byte(io_addr & 1)
-      when 0x054..0x055 then @bldy.read_byte(io_addr & 1)
-      else                   log "Unmapped PPU read ~ addr:#{hex_str io_addr.to_u8}"; 0_u8 # todo: open bus
+      else                   @gba.bus.read_open_bus_value(io_addr)
       end
     end
 
diff --git a/src/crab/gba/reg.cr b/src/crab/gba/reg.cr
index 390ddb1..ee3cd8f 100644
--- a/src/crab/gba/reg.cr
+++ b/src/crab/gba/reg.cr
@@ -169,11 +169,11 @@ module GBA
     class BGCNT < BitField(UInt16)
       include Base16
       num screen_size, 2
-      bool affine_wrap
+      bool affine_wrap, write_only: true # used only in bg2 and bg3
       num screen_base_block, 5
       bool color_mode_8bpp
       bool mosaic
-      num not_used, 2, read_only: true
+      num not_used, 2
       num character_base_block, 2
       num priority, 2
     end