diff --git a/src/crab/gba/mmio.cr b/src/crab/gba/mmio.cr
index ef43f2c..2512273 100644
--- a/src/crab/gba/mmio.cr
+++ b/src/crab/gba/mmio.cr
@@ -12,7 +12,7 @@ module GBA
       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
+      when 0x120..0x12B, 0x134..0x15B # todo: serial
         if io_addr == 0x135
           0x80_u8
         else
@@ -22,7 +22,10 @@ module GBA
       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)
+      when 0x206..0x207,
+           0x20A..0x20B,
+           0x302..0x303 then 0_u8 # do not read open bus
+      else @gba.bus.read_open_bus_value(io_addr)
       end
     end
 
@@ -33,7 +36,7 @@ module GBA
       when 0x060..0x0A7 then @gba.apu[io_addr] = value
       when 0x0B0..0x0DF then @gba.dma[io_addr] = value
       when 0x100..0x10F then @gba.timer[io_addr] = value
-      when 0x120..0x12B, 0x134..0x159 # todo: serial
+      when 0x120..0x12B, 0x134..0x15B # todo: serial
       when 0x130..0x133 then @gba.keypad[io_addr] = value
       when 0x200..0x203,
            0x208..0x209 then @gba.interrupts[io_addr] = value