diff --git a/build.rs b/build.rs
index 551c09a..547030e 100644
--- a/build.rs
+++ b/build.rs
@@ -31,7 +31,7 @@ fn main() -> Result<(), Box<dyn Error>> {
         .define("VB_DIRECT_FRAME", "on_frame")
         .define("VB_DIRECT_READ", "on_read")
         .define("VB_DIRECT_WRITE", "on_write")
-        .define("VBU_REALLOC", "vbu_realloc_shim")
+        .define("VBU_MEMMGR", "VBU_MEMMGR")
         .file(Path::new("shrooms-vb-core/core/vb.c"))
         .file(Path::new("shrooms-vb-core/util/isx.c"))
         .file(Path::new("shrooms-vb-core/util/vbu.c"))
diff --git a/shrooms-vb-core b/shrooms-vb-core
index 31b7403..6e9b727 160000
--- a/shrooms-vb-core
+++ b/shrooms-vb-core
@@ -1 +1 @@
-Subproject commit 31b740325288d9a92debc024a26bf1d689b3ac58
+Subproject commit 6e9b727b319e9ef48fce3f7045afe7f493d093eb
diff --git a/src/emulator/shrooms_vb_util.rs b/src/emulator/shrooms_vb_util.rs
index 4724e30..5675324 100644
--- a/src/emulator/shrooms_vb_util.rs
+++ b/src/emulator/shrooms_vb_util.rs
@@ -1,13 +1,95 @@
-use std::ffi::c_void;
+use std::{ffi::c_void, sync::{LazyLock, Mutex}};
+
+#[derive(Default)]
+struct Reallocator {
+    allocations: Vec<Allocation>,
+}
+
+unsafe impl Send for Reallocator {}
+
+struct Allocation {
+    ptr: *mut u8,
+    size: usize,
+}
+impl Allocation {
+    fn new(size: usize) -> Self {
+        let layout = Self::layout(size);
+        // SAFETY: our layout is not zero, not too big, and properly aligned
+        let ptr = unsafe { std::alloc::alloc_zeroed(layout) };
+        if ptr.is_null() {
+            std::alloc::handle_alloc_error(layout);
+        }
+        Self { ptr, size }
+    }
+
+    fn layout(size: usize) -> std::alloc::Layout {
+        assert!(size != 0);
+        std::alloc::Layout::from_size_align(size, 8).expect("far too large an allocation")
+    }
+
+    fn as_slice(&self) -> &[u8] {
+        // SAFETY: we allocated this ourselves, it's safe
+        unsafe { std::slice::from_raw_parts(self.ptr, self.size) }
+    }
+
+    fn as_mut_slice(&mut self) -> &mut [u8] {
+        // SAFETY: we allocated this ourselves, it's safe
+        unsafe { std::slice::from_raw_parts_mut(self.ptr, self.size) }
+    }
+}
+
+impl Drop for Allocation {
+    fn drop(&mut self) {
+        // SAFETY: we allocated this ourselves with this layout, and it hasn't been freed
+        unsafe { std::alloc::dealloc(self.ptr, Self::layout(self.size)) };
+    }
+}
+
+impl Reallocator {
+    fn realloc(&mut self, ptr: *mut c_void, new_size: usize) -> *mut c_void {
+        if let Some(old_data) = self.load_allocation(ptr) {
+            if new_size == 0 {
+                // free
+                std::ptr::null_mut()
+            } else {
+                // actually reallocate and copy the data over
+                let mut new_data = Allocation::new(new_size);
+                let bytes_to_copy = old_data.size.min(new_size);
+                new_data.as_mut_slice()[..bytes_to_copy].copy_from_slice(&old_data.as_slice()[..bytes_to_copy]);
+                self.store_allocation(new_data)
+            }
+        } else if new_size > 0 {
+            // fresh allocation
+            let allocation = Allocation::new(new_size);
+            self.store_allocation(allocation)
+        } else {
+            std::ptr::null_mut()
+        }
+    }
+
+    fn load_allocation(&mut self, ptr: *mut c_void) -> Option<Allocation> {
+        if ptr.is_null() {
+            return None;
+        }
+        let index = self.allocations.iter().enumerate().find_map(|(i, alloc)| std::ptr::addr_eq(alloc.ptr, ptr).then_some(i))?;
+        let allocation = self.allocations.remove(index);
+        Some(allocation)
+    }
+
+    fn store_allocation(&mut self, allocation: Allocation) -> *mut c_void {
+        let ptr = allocation.ptr;
+        self.allocations.push(allocation);
+        ptr.cast()
+    }
+}
+
+static VBU_ALLOC: LazyLock<Mutex<Reallocator>> = LazyLock::new(|| Mutex::new(Reallocator::default()));
 
 #[unsafe(no_mangle)]
-unsafe fn vbu_realloc_shim(ptr: *mut c_void, new_size: usize) -> *mut c_void {
-    if !ptr.is_null() {
-        // not supporting proper realloc because it needs bookkeeping and it's unnecessary
-        return std::ptr::null_mut();
-    }
-    let allocation = vec![0u8; new_size].into_boxed_slice();
-    Box::into_raw(allocation).cast()
+#[allow(nonstandard_style)]
+unsafe fn VBU_MEMMGR(ptr: *mut c_void, new_size: usize) -> *mut c_void {
+    let mut allocator = VBU_ALLOC.lock().unwrap();
+    allocator.realloc(ptr, new_size)
 }
 
 #[link(name = "vb")]
@@ -25,7 +107,7 @@ pub fn rom_from_isx(bytes: &[u8]) -> Option<Vec<u8>> {
     if raw_rom.is_null() {
         return None;
     }
-    // SAFETY: the rom was allocated by vbu_realloc_shim, which created it from a Vec<u8>.
-    let rom = unsafe { Vec::from_raw_parts(raw_rom.cast(), rom_length, rom_length) };
-    Some(rom)
+    let bytes = VBU_ALLOC.lock().unwrap().load_allocation(raw_rom)?;
+    assert_eq!(bytes.size, rom_length);
+    Some(bytes.as_slice().to_vec())
 }