From e8102339d718d0b5cd50010837d81f7b2b35cf50 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Juhani=20Krekel=C3=A4?= <juhani@krekelä.fi>
Date: Thu, 18 Aug 2022 14:51:47 +0300
Subject: [PATCH] Add stack underflow detection to popByte in calculator.asm

---
 calculator.asm | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/calculator.asm b/calculator.asm
index 11ce5c2..30bdf8a 100644
--- a/calculator.asm
+++ b/calculator.asm
@@ -481,6 +481,13 @@ pushByte:
 popByte:
 	load r0, SP+0
 	load r1, SP+1
+
+	load r2, SPStart+0
+	load r3, SPStart+1
+	brneq r0, r2, popByteNoUnderflow
+	breq r1, r3, error
+
+popByteNoUnderflow:
 	cleq r0, r0, decWord
 	store SP+0, r0
 	store SP+1, r1