diff --git a/nix/pkgs/urbit/default.nix b/nix/pkgs/urbit/default.nix
index e9da7b305..c9023c077 100644
--- a/nix/pkgs/urbit/default.nix
+++ b/nix/pkgs/urbit/default.nix
@@ -68,6 +68,7 @@ in stdenv.mkDerivation {
 
   CFLAGS = oFlags ++ lib.optionals (!enableDebug) [ "-Werror" ];
 
+  GUARD_PAGE = true;
   MEMORY_DEBUG = enableDebug;
   CPU_DEBUG = enableDebug;
   EVENT_TIME_DEBUG = false;
diff --git a/pkg/urbit/configure b/pkg/urbit/configure
index 22958e3ed..54be912e5 100755
--- a/pkg/urbit/configure
+++ b/pkg/urbit/configure
@@ -64,6 +64,7 @@ do
   shift
 done
 
+[ -n "${GUARD_PAGE-}" ]       && defmacro U3_GUARD_PAGE 1
 [ -n "${MEMORY_DEBUG-}" ]     && defmacro U3_MEMORY_DEBUG 1
 [ -n "${MEMORY_LOG-}" ]       && defmacro U3_MEMORY_LOG 1
 [ -n "${CPU_DEBUG-}" ]        && defmacro U3_CPU_DEBUG 1
diff --git a/pkg/urbit/include/noun/allocate.h b/pkg/urbit/include/noun/allocate.h
index 13f4a545f..529ad6fb6 100644
--- a/pkg/urbit/include/noun/allocate.h
+++ b/pkg/urbit/include/noun/allocate.h
@@ -398,28 +398,26 @@
           {
             u3R->cap_p += pil_u->mov_ws;
 
-            //  XX define symbol to control guard page
-            //
-#if 0
+#ifndef U3_GUARD_PAGE
             //  !off means we're on a north road
             //
             if ( !pil_u->off_ws ) {
               if( !(u3R->cap_p > u3R->hat_p) ) {
                 u3m_bail(c3__meme);
               }
-  #ifdef U3_MEMORY_DEBUG
+# ifdef U3_MEMORY_DEBUG
               c3_assert( pil_u->top_p >= u3R->cap_p );
-  #endif
+# endif
             }
             else {
               if( !(u3R->cap_p < u3R->hat_p) ) {
                 u3m_bail(c3__meme);
               }
-  #ifdef U3_MEMORY_DEBUG
+# ifdef U3_MEMORY_DEBUG
               c3_assert( pil_u->top_p <= u3R->cap_p );
-  #endif
+# endif
             }
-#endif
+#endif /* ifndef U3_GUARD_PAGE */
 
 #ifdef U3_MEMORY_DEBUG
             c3_assert( pil_u->rod_u == u3R );
diff --git a/pkg/urbit/noun/events.c b/pkg/urbit/noun/events.c
index 455137efc..3d4d47e10 100644
--- a/pkg/urbit/noun/events.c
+++ b/pkg/urbit/noun/events.c
@@ -186,6 +186,7 @@ _ce_mapfree(void* map_v)
 }
 #endif
 
+#ifdef U3_GUARD_PAGE
 //! Place a guard page at the (approximate) middle of the free space between
 //! the heap and stack of the current road, bailing if memory has been
 //! exhausted.
@@ -237,6 +238,7 @@ _ce_center_guard_page(void)
 fail:
   u3m_signal(c3__meme);
 }
+#endif /* ifdef U3_GUARD_PAGE */
 
 /* u3e_fault(): handle a memory event with libsigsegv protocol.
 */
@@ -265,11 +267,14 @@ u3e_fault(void* adr_v, c3_i ser_i)
   c3_w      blk_w  = (pag_w >> 5);
   c3_w      bit_w  = (pag_w & 31);
 
+#ifdef U3_GUARD_PAGE
   // The fault happened in the guard page.
   if ( gar_pag_p <= adr_p && adr_p < gar_pag_p + pag_wiz_i ) {
     _ce_center_guard_page();
   }
-  else if ( 0 != (u3P.dit_w[blk_w] & (1 << bit_w)) ) {
+  else
+#endif /* ifdef U3_GUARD_PAGE */
+  if ( 0 != (u3P.dit_w[blk_w] & (1 << bit_w)) ) {
     fprintf(stderr, "strange page: %d, at %p, off %x\r\n", pag_w, adr_w, adr_p);
     c3_assert(0);
     return 0;
@@ -1093,5 +1098,7 @@ u3e_foul(void)
 void
 u3e_init(void)
 {
+#ifdef U3_GUARD_PAGE
   _ce_center_guard_page();
+#endif
 }