diff --git a/rust/ares/src/interpreter.rs b/rust/ares/src/interpreter.rs
index 1572f4e..e651985 100644
--- a/rust/ares/src/interpreter.rs
+++ b/rust/ares/src/interpreter.rs
@@ -21,7 +21,7 @@ use assert_no_alloc::{assert_no_alloc, ensure_alloc_counters, permit_alloc};
 use bitvec::prelude::{BitSlice, Lsb0};
 use either::*;
 use std::convert::TryFrom;
-use std::ffi::c_void;
+use std::ffi::{c_void, c_ulonglong};
 use std::result;
 use std::sync::atomic::Ordering;
 use std::sync::Arc;
@@ -394,20 +394,61 @@ fn debug_assertions(stack: &mut NockStack, noun: Noun) {
 
 use std::marker::PhantomData;
 
-pub struct CCallback<'closure> {
-    pub function: unsafe extern "C" fn(*mut c_void) -> *mut c_void,
+/// See: https://users.rust-lang.org/t/passing-a-closure-to-an-external-c-ffi-library/100271/2
+pub struct BoundsCallback<'closure> {
+    pub function: unsafe extern "C" fn(*mut c_void, *mut c_void) -> *const c_ulonglong,
+    pub bounds_data: *mut c_void,
+
+    // without this it's too easy to accidentally drop the closure too soon
+    _lifetime: PhantomData<&'closure mut c_void>,
+}
+
+impl<'closure> BoundsCallback<'closure> {
+    pub fn new<F>(closure: &'closure mut F) -> Self
+    where
+        F: FnMut(*mut c_void) -> *const c_ulonglong,
+    {
+        let function: unsafe extern "C" fn(*mut c_void, *mut c_void) -> *const c_ulonglong = Self::call_closure::<F>;
+
+        debug_assert_eq!(
+            std::mem::size_of::<&'closure mut F>(),
+            std::mem::size_of::<*const c_void>()
+        );
+        debug_assert_eq!(
+            std::mem::size_of_val(&function),
+            std::mem::size_of::<*const c_void>()
+        );
+
+        Self {
+            function,
+            bounds_data: closure as *mut F as *mut c_void,
+            _lifetime: PhantomData,
+        }
+    }
+
+    unsafe extern "C" fn call_closure<F>(bounds_data: *mut c_void, context_p: *mut c_void) -> *const c_ulonglong
+    where
+        F: FnMut(*mut c_void) -> *const c_ulonglong,
+    {
+        let cb: &mut F = bounds_data.cast::<F>().as_mut().unwrap();
+        (*cb)(context_p)
+    }
+}
+
+pub struct WorkCallback<'closure> {
+    pub function: unsafe extern "C" fn(*mut c_void, *mut c_void) -> *mut c_void,
     pub user_data: *mut c_void,
 
     // without this it's too easy to accidentally drop the closure too soon
     _lifetime: PhantomData<&'closure mut c_void>,
 }
 
-impl<'closure> CCallback<'closure> {
+impl<'closure> WorkCallback<'closure> {
     pub fn new<F>(closure: &'closure mut F) -> Self
     where
-        F: FnMut() -> Result,
+        F: FnMut(*mut c_void) -> Result,
     {
-        let function: unsafe extern "C" fn(*mut c_void) -> *mut c_void = Self::call_closure::<F>;
+        let function: unsafe extern "C" fn(*mut c_void, *mut c_void) -> *mut c_void = Self::call_closure::<F>;
 
         debug_assert_eq!(
             std::mem::size_of::<&'closure mut F>(),
@@ -425,12 +466,12 @@ impl<'closure> CCallback<'closure> {
         }
     }
 
-    unsafe extern "C" fn call_closure<F>(user_data: *mut c_void) -> *mut c_void
+    unsafe extern "C" fn call_closure<F>(user_data: *mut c_void, context_p: *mut c_void) -> *mut c_void
     where
-        F: FnMut() -> Result,
+        F: FnMut(*mut c_void) -> Result,
     {
         let cb: &mut F = user_data.cast::<F>().as_mut().unwrap();
-        let v = (*cb)();
+        let v = (*cb)(context_p);
         permit_alloc(|| {
             let v_box = Box::new(v);
             let v_ptr = Box::into_raw(v_box);
@@ -439,22 +480,29 @@ impl<'closure> CCallback<'closure> {
     }
 }
 
-pub fn call_with_guard<F: FnMut() -> Result>(
+pub fn call_with_guard<F: FnMut(*mut c_void) -> Result, G: FnMut(*mut c_void) -> *const c_ulonglong, H: FnMut(*mut c_void) -> *const c_ulonglong>(
     f: &mut F,
-    stack: *const *mut c_void,
-    alloc: *const *mut c_void,
+    low_f: &mut G,
+    high_f: &mut H,
+    context: &mut Context,
 ) -> Result {
-    let c = CCallback::new(f);
+    let work = WorkCallback::new(f);
+    let low = BoundsCallback::new(low_f);
+    let high = BoundsCallback::new(high_f);
+    let context_p = context as *mut Context as *mut c_void;
+
     let mut ret: Result = Ok(D(0));
     let ret_p = &mut ret as *mut _ as *mut c_void;
     let ret_pp = &ret_p as *const *mut c_void;
 
     unsafe {
         let err = guard(
-            Some(c.function as unsafe extern "C" fn(*mut c_void) -> *mut c_void),
-            c.user_data as *mut c_void,
-            stack,
-            alloc,
+            Some(work.function as unsafe extern "C" fn(*mut c_void, *mut c_void) -> *mut c_void),
+            work.user_data as *mut c_void,
+            Some(low.function as unsafe extern "C" fn(*mut c_void, *mut c_void) -> *const c_ulonglong),
+            Some(high.function as unsafe extern "C" fn(*mut c_void, *mut c_void) -> *const c_ulonglong),
+            high.bounds_data as *mut c_void,
+            context_p,
             ret_pp,
         );
 
@@ -488,15 +536,12 @@ pub fn call_with_guard<F: FnMut() -> Result>(
 
 /** Interpret nock */
 pub fn interpret(context: &mut Context, mut subject: Noun, formula: Noun) -> Result {
+    // print the addresses of the context.stack.stack_pointer and context.stack.alloc_pointer
     let terminator = Arc::clone(&TERMINATOR);
     let orig_subject = subject; // for debugging
     let snapshot = context.save();
     let virtual_frame: *const u64 = context.stack.get_frame_pointer();
     let mut res: Noun = D(0);
-    let stack_p = context.stack.get_stack_pointer() as *mut c_void;
-    let alloc_p = context.stack.get_alloc_pointer() as *mut c_void;
-    let stack_pp = &stack_p as *const *mut c_void;
-    let alloc_pp = &alloc_p as *const *mut c_void;
 
     // Setup stack for Nock computation
     unsafe {
@@ -523,7 +568,24 @@ pub fn interpret(context: &mut Context, mut subject: Noun, formula: Noun) -> Res
     // (See https://docs.rs/assert_no_alloc/latest/assert_no_alloc/#advanced-use)
     let nock = assert_no_alloc(|| {
         ensure_alloc_counters(|| {
-            let work_closure = &mut || unsafe {
+            let low_f = &mut |context_p: *mut c_void| {
+                let context = unsafe { &mut *(context_p as *mut Context) };
+                if context.stack.is_west() {
+                    context.stack.get_stack_pointer() as *const c_ulonglong
+                } else {
+                    context.stack.get_alloc_pointer() as *const c_ulonglong
+                }
+            };
+            let high_f = &mut |context_p: *mut c_void| {
+                let context = unsafe { &mut *(context_p as *mut Context) };
+                if context.stack.is_west() {
+                    context.stack.get_alloc_pointer() as *const c_ulonglong
+                } else {
+                    context.stack.get_stack_pointer() as *const c_ulonglong
+                }
+            };
+            let work_closure = &mut |context_p: *mut c_void| unsafe {
+                let context = &mut *(context_p as *mut Context);
                 push_formula(&mut context.stack, formula, true)?;
 
                 loop {
@@ -1072,7 +1134,7 @@ pub fn interpret(context: &mut Context, mut subject: Noun, formula: Noun) -> Res
                     };
                 }
             };
-            call_with_guard(work_closure, stack_pp, alloc_pp)
+            call_with_guard(work_closure, low_f, high_f, context)
         })
     });
 
diff --git a/rust/ares/src/mem.rs b/rust/ares/src/mem.rs
index 31f81c5..58838b1 100644
--- a/rust/ares/src/mem.rs
+++ b/rust/ares/src/mem.rs
@@ -147,11 +147,21 @@ impl NockStack {
         self.stack_pointer
     }
 
+    /** Current stack pointer's address */
+    pub fn get_stack_pointer_pointer(&self) -> *const *mut u64 {
+        &self.stack_pointer as *const *mut u64
+    }
+
     /** Current alloc pointer of this NockStack */
     pub fn get_alloc_pointer(&self) -> *const u64 {
         self.alloc_pointer
     }
 
+    /** Current alloc pointer's address */
+    pub fn get_alloc_pointer_pointer(&self) -> *const *mut u64 {
+        &self.alloc_pointer as *const *mut u64
+    }
+
     /** Start of the memory range for this NockStack */
     pub fn get_start(&self) -> *const u64 {
         self.start
@@ -255,7 +265,7 @@ impl NockStack {
         }
     }
 
-    /** Pointer to where the previous stack pointer is saved in a frame */
+    /** Pointer to where the previous alloc pointer is saved in a frame */
     unsafe fn prev_alloc_pointer_pointer(&self) -> *mut *mut u64 {
         if !self.pc {
             self.slot_pointer(ALLOC) as *mut *mut u64
diff --git a/rust/ares_guard/c-src/guard.c b/rust/ares_guard/c-src/guard.c
index 15f0c31..c10d1d0 100644
--- a/rust/ares_guard/c-src/guard.c
+++ b/rust/ares_guard/c-src/guard.c
@@ -11,23 +11,30 @@
 #define GD_PAGESIZE (1ULL << GD_PAGEBITS) /* 16K */
 
 static uint64_t *guard_p = 0;
-static uint64_t **stack = 0;
-static uint64_t **alloc = 0;
 static jmp_buf env_buffer;
 volatile sig_atomic_t err = guard_sound;
 static void (*prev_sigsegv_handler)(int, siginfo_t *, void *);
 
+static const uint64_t *(*low)(void *, void *) = 0;
+static const uint64_t *(*high)(void *, void *) = 0;
+static void *bounds = 0;
+static void *context = 0;
 
 // Center the guard page.
 static guard_err
 _focus_guard()
 {
-  uint64_t *stack_p = *stack;
-  uint64_t *alloc_p = *alloc;
+  const uint64_t *low_p = low(bounds, context);
+  const uint64_t *high_p = high(bounds, context);
+
+  // Check if we're spent already.
+  if (low_p == high_p || low_p > high_p) {
+    return guard_spent;
+  }
 
   // Check for strange situations.
-  if (stack_p == 0 || alloc_p == 0) {
-    fprintf(stderr, "guard: stack or alloc pointer is null\r\n");
+  if (low_p == 0 || high_p == 0) {
+    fprintf(stderr, "guard: low or high bound pointer is null\r\n");
     return guard_weird;
   }
 
@@ -38,18 +45,9 @@ _focus_guard()
     return guard_armor;
   }
 
-  // Place the new guard page in the center.
-  if (stack_p > alloc_p) {
-    guard_p = stack_p - ((stack_p - alloc_p) / 2);
-  }
-  else if (stack_p < alloc_p) {
-    guard_p = stack_p + ((alloc_p - stack_p) / 2);
-  }
-  else {
-    fprintf(stderr, "guard: weird; stack and alloc pointers are equal\r\n");
-    return guard_weird;
-  }
-  guard_p = (void *)((uintptr_t)guard_p & ~(GD_PAGESIZE - 1));
+  // Place the new guard page in the low-aligned center.
+  guard_p = (uint64_t *)low_p + ((high_p - low_p) / 2);
+  guard_p = (uint64_t *)((uintptr_t)guard_p & ~(GD_PAGESIZE - 1));
 
   // Mark the new guard page.
   if (guard_p != old_guard_p) {
@@ -60,8 +58,6 @@ _focus_guard()
     return guard_spent;
   }
 
-  fprintf(stderr, "guard: installed guard page at %p\r\n", (void *) guard_p);
-
   return guard_sound;
 }
 
@@ -73,12 +69,12 @@ _signal_handler(int sig, siginfo_t *si, void *unused)
       if (si->si_addr >= (void *)guard_p && 
           si->si_addr < (void *)guard_p + GD_PAGESIZE)
       {
-        fprintf(stderr, "guard: fault in guard\r\n");
+        fprintf(stderr, "guard: fault in guard: %p\r\n", si->si_addr);
         err = _focus_guard();
         break;
       }
       else {
-        fprintf(stderr, "guard: fault outside guard\r\n");
+        fprintf(stderr, "guard: fault outside guard %p\r\n", si->si_addr);
         if (NULL != prev_sigsegv_handler) {
           prev_sigsegv_handler(sig, si, unused);
           break;
@@ -124,26 +120,34 @@ _register_handler()
 
 guard_err
 guard(
-  void *(*f)(void *),
-  void *user_data,
-  void *const *stack_pp,
-  void *const *alloc_pp,
+  void *(*work_f)(void *, void *),
+  void *work_data,
+  const uint64_t *(*low_f)(void *, void *),
+  const uint64_t *(*high_f)(void *, void *),
+  void *bounds_data,
+  void *context_p,
   void *const *ret
 )
 {
-  stack = (uint64_t**) stack_pp;
-  alloc = (uint64_t**) alloc_pp;
+  // Set globals.
+  low = low_f;
+  high = high_f;
+  bounds= bounds_data;
+  context = context_p;
 
-  // fprintf(stderr, "guard: stack pointer at %p\r\n", (void *) *stack);
-  // fprintf(stderr, "guard: alloc pointer at %p\r\n", (void *) *alloc);
+  const uint64_t *low_p = low_f(bounds_data, context_p);
+  const uint64_t *high_p = high_f(bounds_data, context_p);
 
-  if (guard_p == 0) {
-    guard_err install_err = _focus_guard();
-    if (install_err != guard_sound) {
-      fprintf(stderr, "guard: failed to install guard page\r\n");
-      err = install_err;
-      goto fail;
-    }
+  // fprintf(stderr, "guard: low: %p high: %p\r\n", (void *)low_p, (void *)high_p);
+
+  const unsigned long free_mb = (unsigned long)(high_p - low_p) / 1024 / 1024;
+  // fprintf(stderr, "guard: free space: %lu MB\r\n", free_mb);
+
+  guard_err focus_err = _focus_guard();
+  if (focus_err != guard_sound && focus_err != guard_spent) {
+    fprintf(stderr, "guard: failed to install guard page\r\n");
+    err = focus_err;
+    goto fail;
   }
 
   if (_register_handler() != guard_sound) {
@@ -153,7 +157,7 @@ guard(
 
   void *result;
   if (sigsetjmp(env_buffer, 1) == 0) {
-    result = f(user_data);
+    result = work_f(work_data, context_p);
   }
   else {
     if (err != guard_sound) {
@@ -167,15 +171,15 @@ guard(
     err = guard_armor;
     goto fail;
   }
-  // fprintf(stderr, "guard: sound; uninstalled guard page\r\n");
 
   return guard_sound;
 
 fail:
-  if (mprotect(guard_p, GD_PAGESIZE, PROT_READ | PROT_WRITE) == -1) {
+  if (guard_p != NULL && 
+      mprotect(guard_p, GD_PAGESIZE, PROT_READ | PROT_WRITE) == -1)
+  {
     fprintf(stderr, "guard: failed to uninstall guard page\r\n");
   }
-  fprintf(stderr, "guard: fail; uninstalled guard page\r\n");
   switch (err) {
     case guard_armor:
       fprintf(stderr, "guard: armor error\r\n");
diff --git a/rust/ares_guard/c-src/guard.h b/rust/ares_guard/c-src/guard.h
index f91b188..7354701 100644
--- a/rust/ares_guard/c-src/guard.h
+++ b/rust/ares_guard/c-src/guard.h
@@ -42,10 +42,12 @@ typedef enum {
  */
 guard_err
 guard(
-  void *(*f)(void *),
-  void *user_data,
-  void *const *stack_pp,
-  void *const *alloc_pp,
+  void *(*work_f)(void *, void *),
+  void *work_data,
+  const uint64_t *(*low_f)(void *, void *),
+  const uint64_t *(*high_f)(void *, void *),
+  void *bounds_data,
+  void *context_p,
   void *const *ret
 );