From 4a14a990665f766e349b9027dbf980bc39f395f6 Mon Sep 17 00:00:00 2001
From: Ian C <ianc@noddybox.co.uk>
Date: Sat, 31 Dec 2011 00:10:45 +0000
Subject: Development check-in.

---
 Noddybox.Emulation.EightBit/Register16.cs | 88 +++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)
 create mode 100644 Noddybox.Emulation.EightBit/Register16.cs

(limited to 'Noddybox.Emulation.EightBit/Register16.cs')

diff --git a/Noddybox.Emulation.EightBit/Register16.cs b/Noddybox.Emulation.EightBit/Register16.cs
new file mode 100644
index 0000000..06c9c01
--- /dev/null
+++ b/Noddybox.Emulation.EightBit/Register16.cs
@@ -0,0 +1,88 @@
+//
+// Copyright (c) 2012 Ian Cowburn
+//
+using System;
+using System.Runtime.InteropServices;
+
+namespace Noddybox.Emulation.EightBit
+{
+    /// <summary>
+    /// Provides a common 8-bit register pattern, which is two 8-bit registers rolled into one.
+    /// While this could be done at runtime using <see cref="BitConverter.IsLittleEndian"/> this
+    /// method was chosen for speed reasons; interfaces cannot define fields, which is sensible
+    /// enoough, but accessing fields directly would allow ref variables to be used with this type's
+    /// fields.
+    /// </summary>
+    [StructLayout(LayoutKind.Explicit)]
+    public struct Register16
+    {
+#if TARGET_IS_BIGENDIAN
+
+        /// <summary>
+        /// The 16-bit register.
+        /// </summary>
+        [FieldOffset(0)]
+        public ushort reg;
+
+        /// <summary>
+        /// The high 8-bits of the register.
+        /// </summary>
+        [FieldOffset(0)]
+        public byte high;
+
+        /// <summary>
+        /// The low 8-bits of the register.
+        /// </summary>
+        [FieldOffset(1)]
+        public byte low;
+
+#else
+
+        /// <summary>
+        /// The 16-bit register.
+        /// </summary>
+        [FieldOffset(0)]
+        public ushort reg;
+
+        /// <summary>
+        /// The low 8-bits of the register.
+        /// </summary>
+        [FieldOffset(0)]
+        public byte low;
+
+        /// <summary>
+        /// The high 8-bits of the register.
+        /// </summary>
+        [FieldOffset(1)]
+        public byte high;
+
+#endif
+
+        /// <summary>
+        /// Public constructor.  This is provided to stop the compiler complaining when you use the struct as it
+        /// doesn't realise that setting <see cref="reg"/> will also set <see cref="low"/> and <see cref="high"/>.
+        /// </summary>
+        /// <param name="val">The 16-bit value to assign.</param>
+        public Register16(ushort val)
+        {
+            high = 0;
+            low = 0;
+            reg = val;
+        }
+
+        /// <summary>
+        /// Used to verify the compile-time setting at runtime.
+        /// </summary>
+        public static void Verify()
+        {
+            Register16 r = new Register16(0);
+
+            r.reg = 0x1234;
+
+            if (r.low != 0x24 || r.high != 0x12)
+            {
+                throw new Exception("Compile time endian setting incorrect");
+            }
+        }
+    }
+}
-- 
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