From b3e617f1e5698362ce8a04687ededd19c418c993 Mon Sep 17 00:00:00 2001
From: Andrea <mariofutire@gmail.com>
Date: Fri, 17 Jun 2022 22:27:52 +0200
Subject: [PATCH] Uthernet II: the real hardware only decodes A0 and A1, so we
 introduce a 0x03 mask. (PR #1115)

Fixes #1113
---
 source/Uthernet2.cpp |  2 +-
 source/W5100.h       | 11 +++++++----
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/source/Uthernet2.cpp b/source/Uthernet2.cpp
index 7399beb8..dba2c61b 100644
--- a/source/Uthernet2.cpp
+++ b/source/Uthernet2.cpp
@@ -1407,7 +1407,7 @@ BYTE Uthernet2::IO_C0(WORD programcounter, WORD address, BYTE write, BYTE value,
     const uint16_t oldAddress = myDataAddress;
 #endif
 
-    const uint8_t loc = address & 0x0F;
+    const uint8_t loc = address & U2_C0X_MASK;
 
     if (write)
     {
diff --git a/source/W5100.h b/source/W5100.h
index 169fec93..7d8cec8d 100644
--- a/source/W5100.h
+++ b/source/W5100.h
@@ -2,10 +2,13 @@
 
 // Uthernet II registers
 
-#define U2_C0X_MODE_REGISTER      0x04
-#define U2_C0X_ADDRESS_HIGH       0x05
-#define U2_C0X_ADDRESS_LOW        0x06
-#define U2_C0X_DATA_PORT          0x07
+// only A0 and A1 are decoded
+#define U2_C0X_MASK               0x03
+
+#define U2_C0X_MODE_REGISTER      (0x04 & U2_C0X_MASK)
+#define U2_C0X_ADDRESS_HIGH       (0x05 & U2_C0X_MASK)
+#define U2_C0X_ADDRESS_LOW        (0x06 & U2_C0X_MASK)
+#define U2_C0X_DATA_PORT          (0x07 & U2_C0X_MASK)
 
 // W5100 registers and values