From 301a92655f284141687df4538f4ca40d99597c1d Mon Sep 17 00:00:00 2001
From: Edward Li <vk.edward.li@gmail.com>
Date: Thu, 7 Nov 2024 13:25:12 +0800
Subject: [PATCH] Mimic 8-bit overflow manually in SCBRR2_write

---
 core/hw/sh4/modules/serial.cpp | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/core/hw/sh4/modules/serial.cpp b/core/hw/sh4/modules/serial.cpp
index 3c1da16f2c..d99fb0c744 100644
--- a/core/hw/sh4/modules/serial.cpp
+++ b/core/hw/sh4/modules/serial.cpp
@@ -339,7 +339,12 @@ void SCIFSerialPort::SCFCR2_write(u16 data)
 // SCBRR2 - Bit Rate Register
 void SCIFSerialPort::SCBRR2_write(u32 addr, u8 data)
 {
-	SCIF_SCBRR2 = data;
+	// Mimic 8-bit overflow manually: -94 could become 4294967202 with 32-bit overflow in macOS arm64
+	if (static_cast<signed char>(data) < 0)
+		SCIF_SCBRR2 = static_cast<signed char>(data) + 256;
+	else
+		SCIF_SCBRR2 = data;
+	
 	Instance().updateBaudRate();
 }