diff --git a/src/backplane.rs b/src/backplane.rs
index b23416d..46cfba2 100644
--- a/src/backplane.rs
+++ b/src/backplane.rs
@@ -3,6 +3,7 @@ use std::cell::{Cell, RefCell, RefMut};
 use std::fmt::{Debug, Display};
 use std::marker::PhantomData;
 use std::rc::Rc;
+use std::sync::atomic::{AtomicBool, Ordering};
 
 use anyhow::anyhow;
 use itertools::Itertools;
@@ -67,6 +68,7 @@ pub struct Backplane {
     cards: Vec<Rc<RefCell<dyn Card>>>,
     mmu: Option<usize>,
     in_dma: Cell<bool>,
+    io_at_top_16mb: AtomicBool,
 }
 
 impl Display for Backplane {
@@ -130,6 +132,7 @@ impl Backplane {
             cards,
             mmu,
             in_dma: Cell::new(false),
+            io_at_top_16mb: AtomicBool::new(true),
         })
     }
 
@@ -274,17 +277,35 @@ impl Backplane {
                 .iter()
                 .try_find_map(|card| mem_func(address, card.borrow_mut()))
                 .result(BusError),
-            (0x00ff_0000..=0x00ff_00ff) | (0xffff_0000..=0xffff_00ff) => backplane_io_func(address)
-                .optional_result()
-                .unwrap_or(Ok(T::default())),
-            (0x00ff_0100..=0x00ff_ffff) | (0xffff_0100..=0xffff_ffff) => self
-                .cards
-                .get(((address >> 8) as u8 - 1) as usize)
-                .map_or(Ok(T::default()), |card| {
-                    io_func(address, card.borrow_mut())
+            (0x00ff_0000..=0x00ff_00ff) | (0xffff_0000..=0xffff_00ff) => {
+                if address < 0x0100_0000 && !self.io_at_top_16mb.load(Ordering::Relaxed) {
+                    self.cards
+                        .iter()
+                        .try_find_map(|card| mem_func(address, card.borrow_mut()))
+                        .result(BusError)
+                } else {
+                    backplane_io_func(address)
                         .optional_result()
                         .unwrap_or(Ok(T::default()))
-                }),
+                }
+            }
+            (0x00ff_0100..=0x00ff_ffff) | (0xffff_0100..=0xffff_ffff) => {
+                if address < 0x0100_0000 && !self.io_at_top_16mb.load(Ordering::Relaxed) {
+                    self.cards
+                        .iter()
+                        .try_find_map(|card| mem_func(address, card.borrow_mut()))
+                        .result(BusError)
+                } else {
+                    self.cards.get(((address >> 8) as u8 - 1) as usize).map_or(
+                        Ok(T::default()),
+                        |card| {
+                            io_func(address, card.borrow_mut())
+                                .optional_result()
+                                .unwrap_or(Ok(T::default()))
+                        },
+                    )
+                }
+            }
         };
         let val = res?;
         self.handle_dma();
@@ -303,11 +324,22 @@ impl Backplane {
         }
     }
 
-    fn read_byte_io(&self, _address: u8) -> NullableResult<u8, BusError> {
-        NullableResult::Null
+    fn read_byte_io(&self, address: u8) -> NullableResult<u8, BusError> {
+        #[allow(clippy::single_match)]
+        match address {
+            0x1 => NullableResult::Ok(self.io_at_top_16mb.load(Ordering::Relaxed) as u8),
+            _ => NullableResult::Ok(0),
+        }
     }
 
-    fn write_byte_io(&self, _address: u8, _data: u8) -> NullableResult<(), BusError> {
+    fn write_byte_io(&self, address: u8, data: u8) -> NullableResult<(), BusError> {
+        #[allow(clippy::single_match)]
+        match address {
+            0x1 => self
+                .io_at_top_16mb
+                .store((data & 0x1) > 0, Ordering::Relaxed),
+            _ => (),
+        }
         NullableResult::Ok(())
     }