diff --git a/src/main.rs b/src/main.rs
index 1527512..92ddeb3 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -66,21 +66,25 @@ fn main() {
     let mouse = Arc::new(Mutex::new(Mouse::new()));
 
     // 32 MiB of shared memory
-    let shared_memory = Arc::new(Mutex::new(vec![0u8; 0x02000000]));
+    // plus 3 bytes at the end to allow for reading the last byte of memory as an immediate pointer
+    // Operand::ImmediatePtr always reads 4 bytes
+    let shared_memory = Arc::new(Mutex::new(vec![0u8; 0x02000003]));
 
     let mut cpu = {
         // 32 MiB of fast memory
-        let cpu_fast_memory = vec![0; 0x02000000];
+        // plus 3 extra bytes at the end to allow for reading the last byte of memory as an immediate pointer
+        // Operand::ImmediatePtr always reads 4 bytes
+        let cpu_fast_memory = vec![0; 0x02000003];
         let cpu_shared_memory = Arc::clone(&shared_memory);
         let cpu_overlays = Arc::clone(&display.overlays);
         let cpu_read_only_memory = read_rom();
 
-        let fast_size = cpu_fast_memory.len();
+        let fast_size = cpu_fast_memory.len() - 3;
         let fast_bottom_address = 0x00000000;
         let fast_top_address = fast_bottom_address + fast_size - 1;
         println!("Fast:   {:.2}MB mapped at {:#010X}-{:#010X}", fast_size / 1048576, fast_bottom_address, fast_top_address);
 
-        let shared_size = { cpu_shared_memory.lock().unwrap().len() };
+        let shared_size = { cpu_shared_memory.lock().unwrap().len() - 3 };
         let shared_bottom_address = 0x80000000;
         let shared_top_address = shared_bottom_address + shared_size - 1;
         println!("Shared: {:.2}MB mapped at {:#010X}-{:#010X}", shared_size / 1048576, shared_bottom_address, shared_top_address);
diff --git a/src/memory.rs b/src/memory.rs
index df21937..a9ce675 100644
--- a/src/memory.rs
+++ b/src/memory.rs
@@ -18,9 +18,10 @@ pub struct Memory {
 
 impl Memory {
     pub fn new(fast_memory: Vec<u8>, shared_memory: Arc<Mutex<Vec<u8>>>, overlays: Arc<Mutex<Vec<Overlay>>>, rom: Vec<u8>) -> Self {
-        let shared_memory_size = { shared_memory.lock().unwrap().len() };
+        // 3 extra bytes at the end to allow for reading the last byte of memory as an immediate pointer
+        let shared_memory_size = { shared_memory.lock().unwrap().len() - 3 };
         Memory {
-            fast_memory_size: fast_memory.len(),
+            fast_memory_size: fast_memory.len() - 3,
             fast_memory,
             shared_memory_size,
             shared_memory,