From 68f2fa9b09b35586a5f38ef548a3d151fa0fd172 Mon Sep 17 00:00:00 2001
From: crc <>
Date: Tue, 28 Mar 2023 18:30:12 +0000
Subject: [PATCH] nga-c: add stub guard() function (ref #96)

FossilOrigin-Name: 872b8371d1eed28ce2b6405055f5ada8b174c2c55b5dbbbfd35e857dff4665a6
---
 vm/nga-c/retro.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/vm/nga-c/retro.c b/vm/nga-c/retro.c
index c834964..228406d 100644
--- a/vm/nga-c/retro.c
+++ b/vm/nga-c/retro.c
@@ -237,6 +237,25 @@ void inst_iq(NgaState *);  void inst_ii(NgaState *);
 
 /* Global Variables -------------------------------------------------- */
 
+void guard(NgaState *vm, int n, int m, int diff) {
+  if (vm->cpu[vm->active].sp < n) {
+    vm->cpu[vm->active].sp = 0;
+    return;
+  }
+  if (((vm->cpu[vm->active].sp + m) - n) > (STACK_DEPTH - 1)) {
+    vm->cpu[vm->active].sp = 0;
+    return;
+  }
+  if (diff) {
+    if (vm->cpu[vm->active].rp + diff < 0) {
+      return;
+    }
+    if (vm->cpu[vm->active].rp + diff > (ADDRESSES - 1)) {
+      return;
+    }
+  }
+}
+
 /* Dynamic Memory / `malloc` support --------------------------------- */
 #ifdef ENABLE_MALLOC
 #ifdef BIT64