From 291cb3d1a22812ef2cfe99bd7ef9cee34e187622 Mon Sep 17 00:00:00 2001
From: Andrew Alderwick <andrew@alderwick.co.uk>
Date: Wed, 5 Jan 2022 14:02:06 +0000
Subject: [PATCH] Simplify stack determination logic.

---
 src/uxn.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/uxn.c b/src/uxn.c
index ba5c911..be9d942 100644
--- a/src/uxn.c
+++ b/src/uxn.c
@@ -100,8 +100,8 @@ uxn_eval(Uxn *u, Uint16 pc)
 
 err:
 	/* set 1 in errcode if it involved the return stack instead of the working stack */
-	/*        (stack overflow & (       instruction opcode was STH or JSR        )) ^ Return Mode */
-	errcode |= ((errcode >> 1 & instr >> 1 & instr >> 2 & instr >> 3 & ~instr >> 4) ^ instr >> 6) & 1;
+	/*        (stack overflow & (opcode was STH / JSR)) ^ Return Mode */
+	errcode |= ((errcode >> 1 & (instr & 0x1e) == 0x0e) ^ instr >> 6) & 1;
 	return uxn_halt(u, errcode, pc - 1);
 }