From 7ce55e660176c08f1472ab8800f5373296cb90fc Mon Sep 17 00:00:00 2001
From: Asnivor <coding@asnitech.co.uk>
Date: Wed, 13 Jun 2018 09:24:43 +0100
Subject: [PATCH] ZXHawk: ReadPort method was incorrectly snagging occational
 floating bus requests and processing them as kempston joystick input

---
 .../SinclairSpectrum/Machine/ZXSpectrum128K/ZX128.Port.cs | 6 ++++--
 .../Machine/ZXSpectrum128KPlus2a/ZX128Plus2a.Port.cs      | 6 ++++--
 .../Machine/ZXSpectrum128KPlus3/ZX128Plus3.Port.cs        | 6 ++++--
 .../SinclairSpectrum/Machine/ZXSpectrum48K/ZX48.Port.cs   | 8 +++++---
 4 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128K/ZX128.Port.cs b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128K/ZX128.Port.cs
index 0596dce779..a1dcc1427e 100644
--- a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128K/ZX128.Port.cs
+++ b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128K/ZX128.Port.cs
@@ -43,9 +43,11 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
             if (AYDevice.ReadPort(port, ref result))
                 return (byte)result;
 
-            // Kempston joystick input takes priority over all other input
+            byte lowByte = (byte)(port & 0xff);
+
+            // Kempston joystick input takes priority over keyboard input
             // if this is detected just return the kempston byte
-            if ((port & 0xe0) == 0 || (port & 0x20) == 0)
+            if (lowByte == 0x1f)
             {
                 if (LocateUniqueJoystick(JoystickType.Kempston) != null)
                     return (byte)((KempstonJoystick)LocateUniqueJoystick(JoystickType.Kempston) as KempstonJoystick).JoyLine;
diff --git a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128KPlus2a/ZX128Plus2a.Port.cs b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128KPlus2a/ZX128Plus2a.Port.cs
index a2cd41da7d..214442e49f 100644
--- a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128KPlus2a/ZX128Plus2a.Port.cs
+++ b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128KPlus2a/ZX128Plus2a.Port.cs
@@ -24,9 +24,11 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
             if (AYDevice.ReadPort(port, ref result))
                 return (byte)result;
 
-            // Kempston joystick input takes priority over all other input
+            byte lowByte = (byte)(port & 0xff);
+
+            // Kempston joystick input takes priority over all keyboard input
             // if this is detected just return the kempston byte
-            if ((port & 0xe0) == 0 || (port & 0x20) == 0)
+            if (lowByte == 0x1f)
             {
                 if (LocateUniqueJoystick(JoystickType.Kempston) != null)
                     return (byte)((KempstonJoystick)LocateUniqueJoystick(JoystickType.Kempston) as KempstonJoystick).JoyLine;
diff --git a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128KPlus3/ZX128Plus3.Port.cs b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128KPlus3/ZX128Plus3.Port.cs
index 31c2e08e44..f71ab0af90 100644
--- a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128KPlus3/ZX128Plus3.Port.cs
+++ b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128KPlus3/ZX128Plus3.Port.cs
@@ -24,9 +24,11 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
             if (AYDevice.ReadPort(port, ref result))
                 return (byte)result;
 
-            // Kempston joystick input takes priority over all other input
+            byte lowByte = (byte)(port & 0xff);
+
+            // Kempston joystick input takes priority over all keyboard input
             // if this is detected just return the kempston byte
-            if ((port & 0xe0) == 0 || (port & 0x20) == 0)
+            if (lowByte == 0x1f)
             {
                 if (LocateUniqueJoystick(JoystickType.Kempston) != null)
                     return (byte)((KempstonJoystick)LocateUniqueJoystick(JoystickType.Kempston) as KempstonJoystick).JoyLine;
diff --git a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum48K/ZX48.Port.cs b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum48K/ZX48.Port.cs
index 31fd2ac00b..632caceac2 100644
--- a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum48K/ZX48.Port.cs
+++ b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum48K/ZX48.Port.cs
@@ -20,10 +20,11 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
             // Check whether the low bit is reset
             // Technically the ULA should respond to every even I/O address
             bool lowBitReset = (port & 0x0001) == 0;
-
-            // Kempston joystick input takes priority over all other input
+            byte lowByte = (byte)(port & 0xff);
+            
+            // Kempston joystick input takes priority over keyboard input
             // if this is detected just return the kempston byte
-            if ((port & 0xe0) == 0 || (port & 0x20) == 0)
+            if (lowByte == 0x1f)
             {
                 if (LocateUniqueJoystick(JoystickType.Kempston) != null)
                     return (byte)((KempstonJoystick)LocateUniqueJoystick(JoystickType.Kempston) as KempstonJoystick).JoyLine;
@@ -31,6 +32,7 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
                 // not a lag frame
                 InputRead = true;
             }
+            // Even ports always address the ULA
             else if (lowBitReset)
             {
                 // Even I/O address so get input from keyboard