diff --git a/include/avian/codegen/promise.h b/include/avian/codegen/promise.h
index 6d683533dc..dce86bb9dc 100644
--- a/include/avian/codegen/promise.h
+++ b/include/avian/codegen/promise.h
@@ -118,7 +118,7 @@ class OffsetPromise : public Promise {
 
 class ListenPromise : public Promise {
  public:
-  ListenPromise(vm::System* s, util::Allocator* allocator)
+  ListenPromise(vm::System* s, util::AllocOnly* allocator)
       : s(s), allocator(allocator), listener(0)
   {
   }
@@ -142,7 +142,7 @@ class ListenPromise : public Promise {
   }
 
   vm::System* s;
-  util::Allocator* allocator;
+  util::AllocOnly* allocator;
   Listener* listener;
   Promise* promise;
 };
@@ -150,7 +150,7 @@ class ListenPromise : public Promise {
 class DelayedPromise : public ListenPromise {
  public:
   DelayedPromise(vm::System* s,
-                 util::Allocator* allocator,
+                 util::AllocOnly* allocator,
                  Promise* basis,
                  DelayedPromise* next)
       : ListenPromise(s, allocator), basis(basis), next(next)
diff --git a/include/avian/util/allocator.h b/include/avian/util/allocator.h
index fc4163c775..8b5b1b9232 100644
--- a/include/avian/util/allocator.h
+++ b/include/avian/util/allocator.h
@@ -16,24 +16,21 @@
 namespace avian {
 namespace util {
 
-class Allocator {
+class AllocOnly {
  public:
-
-  // Returns null on failure
-  virtual void* tryAllocate(size_t size) = 0;
-
-  // Aborts on failure
   virtual void* allocate(size_t size) = 0;
+};
 
-  // By contract, size MUST be the original size of the allocated data, and p
-  // MUST point to the original base of the allocated data. No partial frees.
+class Allocator : public AllocOnly {
+ public:
+  virtual void* tryAllocate(size_t size) = 0;
   virtual void free(const void* p, size_t size) = 0;
 };
 
 }  // namespace util
 }  // namespace avian
 
-inline void* operator new(size_t size, avian::util::Allocator* allocator)
+inline void* operator new(size_t size, avian::util::AllocOnly* allocator)
 {
   return allocator->allocate(size);
 }
diff --git a/include/avian/util/slice.h b/include/avian/util/slice.h
index 69667b63c4..13930bc8a6 100644
--- a/include/avian/util/slice.h
+++ b/include/avian/util/slice.h
@@ -69,12 +69,12 @@ class Slice {
     return Slice<T>(this->begin() + begin, count);
   }
 
-  static Slice<T> alloc(Allocator* a, size_t count)
+  static Slice<T> alloc(AllocOnly* a, size_t count)
   {
     return Slice<T>((T*)a->allocate(sizeof(T) * count), count);
   }
 
-  static Slice<T> allocAndSet(Allocator* a, size_t count, const T& item)
+  static Slice<T> allocAndSet(AllocOnly* a, size_t count, const T& item)
   {
     Slice<T> slice(alloc(a, count));
     for (size_t i = 0; i < count; i++) {
@@ -83,14 +83,14 @@ class Slice {
     return slice;
   }
 
-  Slice<T> clone(Allocator* a, size_t newCount)
+  Slice<T> clone(AllocOnly* a, size_t newCount)
   {
     T* newItems = (T*)a->allocate(newCount * sizeof(T));
     memcpy(newItems, items, min(count, newCount) * sizeof(T));
     return Slice<T>(newItems, newCount);
   }
 
-  Slice<T> cloneAndSet(Allocator* a, size_t newCount, const T& item)
+  Slice<T> cloneAndSet(AllocOnly* a, size_t newCount, const T& item)
   {
     Slice<T> slice(clone(a, newCount));
     for (size_t i = count; i < newCount; i++) {
diff --git a/src/avian/zone.h b/src/avian/zone.h
index fa1de3ddeb..e3b0662bd6 100644
--- a/src/avian/zone.h
+++ b/src/avian/zone.h
@@ -17,7 +17,7 @@
 
 namespace vm {
 
-class Zone : public avian::util::Allocator {
+class Zone : public avian::util::AllocOnly {
  public:
   class Segment {
    public:
@@ -31,9 +31,8 @@ class Zone : public avian::util::Allocator {
     uint8_t data[0];
   };
 
-  Zone(System* s, Allocator* allocator, unsigned minimumFootprint)
-      : s(s),
-        allocator(allocator),
+  Zone(avian::util::Allocator* allocator, size_t minimumFootprint)
+      : allocator(allocator),
         segment(0),
         minimumFootprint(minimumFootprint < sizeof(Segment)
                              ? 0
@@ -56,6 +55,46 @@ class Zone : public avian::util::Allocator {
     segment = 0;
   }
 
+  virtual void* allocate(size_t size)
+  {
+    size = pad(size);
+    void* p = tryAllocate(size);
+    if (p) {
+      return p;
+    } else {
+      ensure(size);
+      void* r = segment->data + segment->position;
+      segment->position += size;
+      return r;
+    }
+  }
+
+  void* peek(size_t size)
+  {
+    size = pad(size);
+    Segment* s = segment;
+    while (s->position < size) {
+      size -= s->position;
+      s = s->next;
+    }
+    return s->data + (s->position - size);
+  }
+
+  void pop(size_t size)
+  {
+    size = pad(size);
+    Segment* s = segment;
+    while (s->position < size) {
+      size -= s->position;
+      Segment* next = s->next;
+      allocator->free(s, sizeof(Segment) + s->size);
+      s = next;
+    }
+    s->position -= size;
+    segment = s;
+  }
+
+ private:
   static unsigned padToPage(unsigned size)
   {
     return (size + (LikelyPageSizeInBytes - 1)) & ~(LikelyPageSizeInBytes - 1);
@@ -95,7 +134,7 @@ class Zone : public avian::util::Allocator {
     }
   }
 
-  virtual void* tryAllocate(size_t size)
+  void* tryAllocate(size_t size)
   {
     size = pad(size);
     if (tryEnsure(size)) {
@@ -107,54 +146,7 @@ class Zone : public avian::util::Allocator {
     }
   }
 
-  virtual void* allocate(size_t size)
-  {
-    size = pad(size);
-    void* p = tryAllocate(size);
-    if (p) {
-      return p;
-    } else {
-      ensure(size);
-      void* r = segment->data + segment->position;
-      segment->position += size;
-      return r;
-    }
-  }
-
-  void* peek(unsigned size)
-  {
-    size = pad(size);
-    Segment* s = segment;
-    while (s->position < size) {
-      size -= s->position;
-      s = s->next;
-    }
-    return s->data + (s->position - size);
-  }
-
-  void pop(unsigned size)
-  {
-    size = pad(size);
-    Segment* s = segment;
-    while (s->position < size) {
-      size -= s->position;
-      Segment* next = s->next;
-      allocator->free(s, sizeof(Segment) + s->size);
-      s = next;
-    }
-    s->position -= size;
-    segment = s;
-  }
-
-  virtual void free(const void*, size_t)
-  {
-    // not supported
-    abort(s);
-  }
-
-  System* s;
-  Allocator* allocator;
-  void* context;
+  avian::util::Allocator* allocator;
   Segment* segment;
   unsigned minimumFootprint;
 };
diff --git a/src/compile.cpp b/src/compile.cpp
index 0a0a3775ba..ead6942a1e 100644
--- a/src/compile.cpp
+++ b/src/compile.cpp
@@ -1158,7 +1158,7 @@ class Context {
 
   Context(MyThread* t, BootContext* bootContext, GcMethod* method)
       : thread(t),
-        zone(t->m->system, t->m->heap, InitialZoneCapacityInBytes),
+        zone(t->m->heap, InitialZoneCapacityInBytes),
         assembler(t->arch->makeAssembler(t->m->heap, &zone)),
         client(t),
         compiler(makeCompiler(t->m->system, assembler, &zone, &client)),
@@ -1191,7 +1191,7 @@ class Context {
 
   Context(MyThread* t)
       : thread(t),
-        zone(t->m->system, t->m->heap, InitialZoneCapacityInBytes),
+        zone(t->m->heap, InitialZoneCapacityInBytes),
         assembler(t->arch->makeAssembler(t->m->heap, &zone)),
         client(t),
         compiler(0),
@@ -3743,8 +3743,7 @@ class Stack {
     Stack* s;
   };
 
-  Stack(MyThread* t)
-      : thread(t), zone(t->m->system, t->m->heap, 0), resource(this)
+  Stack(MyThread* t) : thread(t), zone(t->m->heap, 0), resource(this)
   {
   }
 
diff --git a/src/tools/bootimage-generator/main.cpp b/src/tools/bootimage-generator/main.cpp
index b85cc1317c..8860496be3 100644
--- a/src/tools/bootimage-generator/main.cpp
+++ b/src/tools/bootimage-generator/main.cpp
@@ -1431,7 +1431,7 @@ void writeBootImage2(Thread* t,
   // sequence point, for gc (don't recombine statements)
   roots(t)->setOutOfMemoryError(t, throwable);
 
-  Zone zone(t->m->system, t->m->heap, 64 * 1024);
+  Zone zone(t->m->heap, 64 * 1024);
 
   class MyCompilationHandler : public Processor::CompilationHandler {
    public:
diff --git a/unittest/codegen/assembler-test.cpp b/unittest/codegen/assembler-test.cpp
index fa20b9a26c..f463b0c5b4 100644
--- a/unittest/codegen/assembler-test.cpp
+++ b/unittest/codegen/assembler-test.cpp
@@ -52,7 +52,7 @@ class Asm {
   Assembler* a;
 
   Asm(BasicEnv& env)
-      : zone(env.s, env.heap, 8192), a(env.arch->makeAssembler(env.heap, &zone))
+      : zone(env.heap, 8192), a(env.arch->makeAssembler(env.heap, &zone))
   {
   }