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// This file is part of the Noddybox.Emulation C# suite.
//
// Noddybox.Emulation is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// Noddybox.Emulation is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with Noddybox.Emulation. If not, see <http://www.gnu.org/licenses/>.
//
// 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 != 0x34 || r.high != 0x12)
{
throw new Exception("Compile time endian setting incorrect");
}
}
}
}
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