Lots more code written, and I think most of it's OK; but it doesn't compile yet.

src/c/memory/stack.c

@@ -0,0 +1,38 @@
+/**
+ *  memory/stack.c
+ *
+ *  The execution stack.
+ *
+ *  (c) 2026 Simon Brooke <simon@journeyman.cc>
+ *  Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#include "memory/stack.h"
+
+/** 
+ * @brief The maximum depth of stack before we throw an exception.
+ *
+ * `0` is interpeted as `unlimited`.
+ */
+uint32_t stack_limit = 0;
+
+/**
+ * Fetch a pointer to the value of the local variable at this index.
+ */
+struct cons_pointer fetch_arg( struct stack_frame *frame, unsigned int index ) {
+    struct cons_pointer result = NIL;
+
+    if ( index < args_in_frame ) {
+        result = frame->arg[index];
+    } else {
+        struct cons_pointer p = frame->more;
+
+        for ( int i = args_in_frame; i < index; i++ ) {
+            p = pointer2cell( p ).payload.cons.cdr;
+        }
+
+        result = pointer2cell( p ).payload.cons.car;
+    }
+
+    return result;
+}