From 3534d54b61a8ce858d9fb447374ee495df694049 Mon Sep 17 00:00:00 2001
From: Jon Herron <jon.herron@yahoo.com>
Date: Sun, 7 Jul 2024 20:38:40 -0400
Subject: [PATCH] Check for underflow

---
 data_stack.inc | 7 +++++++
 defs.inc       | 1 +
 2 files changed, 8 insertions(+)

diff --git a/data_stack.inc b/data_stack.inc
index 8c8844c..45e3019 100644
--- a/data_stack.inc
+++ b/data_stack.inc
@@ -28,12 +28,19 @@ data_stack_push:
 
 data_stack_pop:
 	mov	rsi, [_data_stack.here]
+	cmp	rsi, [_data_stack.base]
+	jle	.underflow
+	
 	std
 	lodsq
 	mov	[_data_stack.here], rsi
 	lodsq
 	cld
+	
 	ret
+	
+	.underflow:
+	die	ERR_UNDERFLOW
 
 data_stack_depth:
 	mov	rax, [_data_stack.here]
diff --git a/defs.inc b/defs.inc
index a715aa3..94efb71 100644
--- a/defs.inc
+++ b/defs.inc
@@ -11,6 +11,7 @@ ERR_BAD_BASE	= 167
 ERR_BAD_PAD	= 168
 ERR_NO_WORD	= 169
 ERR_BAD_SE	= 170
+ERR_UNDERFLOW	= 171
 
 TYPE_OFFSET	= 0x00