diff --git a/src/arena.c b/src/arena.c
index ea5a11c..b21e351 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -1,40 +1,43 @@
 internal Arena *
-arena_initialize(u64 size)
+arena_initialize(u64 size, b8 growable)
 {
   Arena *result = 0;
-
-  void *backing_buffer = malloc(size);
+  void *backing_buffer = platform_memory_reserve(size);
 
   if (backing_buffer != NULL)
   {
-    result = (Arena*) backing_buffer;
+    platform_memory_commit(backing_buffer, ARENA_INITIAL_COMMIT_SIZE);
 
-    result->buffer = backing_buffer;
-    result->offset = sizeof(Arena);
-    result->size   = sizeof(Arena) + size;
+    result = (Arena*) backing_buffer;
+    result->backing_buffer = backing_buffer;
+
+    // rhjr: immutable arena header
+    result->base_pos = sizeof(Arena);
+    result->commit_pos = sizeof(Arena);
+
+    result->growable = growable;
+    result->size = size;
   }
 
+  // rhjr: arenas should be instantiated early on in the programs lifetime, that
+  //       is why this assertion will also be used in production.
+  ASSERT(result != 0);
+  return result;
+}
+
+internal Arena *
+arena_initialize_default()
+{
+  Arena *result = arena_initialize(ARENA_DEFAULT_RESERVE_SIZE, 1);
   return result;
 }
 
 internal void *
 arena_allocate(Arena *arena, u64 size)
 {
-  void *result = 0;
-
-  if (arena->offset + size <= arena->size)
-  {
-    result = (void*)((u64) arena->buffer + (u64) arena->offset);
-    arena->offset += size;
-
-    memset(result, 0, size);
-  }
-
-  return result;
 }
 
 internal void
 arena_release(Arena *arena)
 {
-  arena->offset = 0;
 }
diff --git a/src/arena.h b/src/arena.h
index b07866f..103a367 100644
--- a/src/arena.h
+++ b/src/arena.h
@@ -1,19 +1,32 @@
 #ifndef ARENA_H
 #define ARENA_H
 
+#define ARENA_INITIAL_COMMIT_SIZE sizeof(struct Arena)
+
+#ifndef ARENA_DEFAULT_RESERVE_SIZE
+#  define ARENA_DEFAULT_RESERVE_SIZE KB(4)
+#endif // ARENA_DEFAULT_RESERVE_SIZE
+
 typedef struct Arena Arena;
 struct Arena {
-  u64 *buffer;
-  u64 offset;
+  struct Arena *current;
+  struct Arena *prev;
+  u64 *backing_buffer;
+  u64 base_pos;
+  u64 commit_pos;
   u64 size;
+  b8 growable;
 };
 
-internal Arena * arena_initialize(u64 size);
+STATIC_ASSERT(ARENA_INITIAL_COMMIT_SIZE <= ARENA_DEFAULT_RESERVE_SIZE,
+	      arena_default_allocation_size);
 
-internal void * arena_allocate(Arena *arena, u64 size);
-internal void   arena_release(Arena *arena);
+//= rhjr: arenas
 
-//= rhjr: arena helpers
+internal Arena * arena_initialize(u64 size, b8 growable);
+internal Arena * arena_initialize_default();
+
+internal void *  arena_allocate(Arena *arena, u64 size);
 
 #define arena_push_array(arena, type, count) \
   (type*) arena_allocate((arena), sizeof(type) * (count))
@@ -21,11 +34,6 @@ internal void   arena_release(Arena *arena);
 #define arena_push(arena, type) \
   (type*) arena_allocate((arena), sizeof(type))
 
-//= rhjr: memory helpers
-
-#define memory_zero(s,z)       memset((s), 0, (z))
-#define memory_zero_struct(s)  memory_zero((s), sizeof(*(s)))
-
-#define memory_copy(dst, src, length) memmove((dst), (src), (length))
+internal void arena_release(Arena *arena);
 
 #endif // ARENA_H
diff --git a/src/base.h b/src/base.h
index 6b578c1..e7d93f9 100644
--- a/src/base.h
+++ b/src/base.h
@@ -18,10 +18,26 @@ typedef int16_t i16;
 typedef int32_t i32;
 typedef int64_t i64;
 
+typedef uint8_t  b8;
+typedef uint8_t  b16;
+typedef uint8_t  b32;
+typedef uint8_t  b64;
+
 #define KB(b) ((b) << 10)
 #define MB(b) ((b) << 20)
 
+#define Glue_(A,B) A##B
+#define Glue(A,B) Glue_(A,B)
+
 #define STATEMENT(S) do{ S } while(0)
 #define ASSERT(c) STATEMENT( if (!(c)){ (*(volatile int*)0 = 0); } )
+#define STATIC_ASSERT(c,l) typedef u8 Glue(l,__LINE__) [(c)?1:-1]
+
+//= rhjr: memory helpers
+
+#define memory_zero(s,z)       memset((s), 0, (z))
+#define memory_zero_struct(s)  memory_zero((s), sizeof(*(s)))
+
+#define memory_copy(dst, src, length) memmove((dst), (src), (length))
 
 #endif // BASE_H
diff --git a/src/main.c b/src/main.c
index 3731b84..0d8fb62 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1,10 +1,13 @@
 #include "base.h"
 #include "arena.h"
+#include "win32_platform.h"
 
 #include "arena.c"
+#include "win32_platform.c"
 
 int
 main(void)
 {
+  Arena *arena = arena_initialize_default();
   return 0;
 }