diff --git a/include/interp.h b/include/interp.h
index 8b9aad9..7b17daa 100644
--- a/include/interp.h
+++ b/include/interp.h
@@ -32,11 +32,14 @@ typedef struct {
     int option_base;
 
     /* Control flow stacks */
-    for_entry_t for_stack[16];
+#define MAX_FOR_DEPTH   16
+#define MAX_GOSUB_DEPTH 24
+#define MAX_WHILE_DEPTH 16
+    for_entry_t for_stack[MAX_FOR_DEPTH];
     int for_sp;
-    gosub_entry_t gosub_stack[24];
+    gosub_entry_t gosub_stack[MAX_GOSUB_DEPTH];
     int gosub_sp;
-    while_entry_t while_stack[16];
+    while_entry_t while_stack[MAX_WHILE_DEPTH];
     int while_sp;
 
     /* DEF FN */
diff --git a/src/interp.c b/src/interp.c
index 7438e4a..d297de0 100644
--- a/src/interp.c
+++ b/src/interp.c
@@ -1509,7 +1509,7 @@ void gw_exec_stmt(void)
         program_line_t *target = gw_find_line(num);
         if (!target) gw_error(ERR_UL);
 
-        if (gw.gosub_sp >= 24)
+        if (gw.gosub_sp >= MAX_GOSUB_DEPTH)
             gw_error(ERR_OM);
         gw.gosub_stack[gw.gosub_sp].ret_text = gw.text_ptr;
         gw.gosub_stack[gw.gosub_sp].ret_line = gw.cur_line;
@@ -1593,7 +1593,7 @@ void gw_exec_stmt(void)
             }
         }
 
-        if (gw.for_sp >= 16)
+        if (gw.for_sp >= MAX_FOR_DEPTH)
             gw_error(ERR_OM);
 
         for_entry_t *f = &gw.for_stack[gw.for_sp++];
@@ -1759,7 +1759,7 @@ void gw_exec_stmt(void)
                 }
             }
             if (!found) {
-                if (gw.while_sp >= 16)
+                if (gw.while_sp >= MAX_WHILE_DEPTH)
                     gw_error(ERR_OM);
                 gw.while_stack[gw.while_sp].while_text = while_text;
                 gw.while_stack[gw.while_sp].while_line = while_line;
@@ -1887,7 +1887,7 @@ void gw_exec_stmt(void)
         if (!target) gw_error(ERR_UL);
 
         if (is_gosub) {
-            if (gw.gosub_sp >= 24)
+            if (gw.gosub_sp >= MAX_GOSUB_DEPTH)
                 gw_error(ERR_OM);
             gw.gosub_stack[gw.gosub_sp].ret_text = gw.text_ptr;
             gw.gosub_stack[gw.gosub_sp].ret_line = gw.cur_line;