make heap compatible with original ws: memory is not initialized to 0

playground.wsa

@@ -1,5 +1,6 @@
 ; hello world in 2 stages
 include memory
+call memory_init
 include io
 
 push 6

src/whitespace/execute.ts

@@ -204,7 +204,13 @@ function stepHeap(state: MachineState, instruction: HeapOp): MachineState {
         `Heap op ${state.pc} failed: Can't access a negative address. addr = ${addr}`
       );
     }
-    state.stack.push(state.heap[Number(addr)] ?? 0n);
+    const value = state.heap[Number(addr)];
+    if (value === undefined) {
+      throw new Error(
+        `Heap op ${state.pc} failed: Access to uninitialized heap address ${addr}`
+      );
+    }
+    state.stack.push(value);
   } else {
     if (state.stack.length < 2) {
       throw new Error(

src/wsa/lib/memory.wsa

@@ -1,6 +1,13 @@
 valueinteger _STACK_HEAD 0
 valueinteger _HEAP_HEAD 4294967294
 
+label memory_init
+push 0
+doub
+store _STACK_HEAD
+store _HEAP_HEAD
+ret
+
 ; arg: size
 label malloc
 retrive _HEAP_HEAD