From 998b64d14c9d055562d8c1611813d40af4cb030b Mon Sep 17 00:00:00 2001
From: Ian C <ianc@noddybox.co.uk>
Date: Fri, 9 Mar 2012 23:01:13 +0000
Subject: Further bug fixes to Z80.  Now starts the Spectrum ROM OK.

---
 src/Noddybox.Emulation.EightBit.Z80/Z80Cpu.cs | 40 +++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

(limited to 'src/Noddybox.Emulation.EightBit.Z80/Z80Cpu.cs')

diff --git a/src/Noddybox.Emulation.EightBit.Z80/Z80Cpu.cs b/src/Noddybox.Emulation.EightBit.Z80/Z80Cpu.cs
index 2da155e..74a8df1 100644
--- a/src/Noddybox.Emulation.EightBit.Z80/Z80Cpu.cs
+++ b/src/Noddybox.Emulation.EightBit.Z80/Z80Cpu.cs
@@ -15,6 +15,12 @@
 //
 // Copyright (c) 2012 Ian Cowburn
 //
+// Some of the concepts and code in here are taken from FUSE, the open source UNIX
+// Spectrum emulator Copyright (c) 1999-2003 Philip Kendall.
+// See <http://fuse-emulator.sourceforge.net/>
+//
+// Any introduced bugs in that code are mine, and not the original authors.
+//
 using System;
 
 namespace Noddybox.Emulation.EightBit.Z80
@@ -45,6 +51,40 @@ namespace Noddybox.Emulation.EightBit.Z80
         private Z80Flags[] Ztable = new Z80Flags[512];
         private Z80Flags[] H35table = new Z80Flags[512];
 
+        // Whether a half carry occurred or not can be determined by looking at
+        // the 3rd bit of the two arguments and the result; these are hashed
+        // into this table in the form r12, where r is the 3rd bit of the
+        // result, 1 is the 3rd bit of the 1st argument and 2 is the
+        // third bit of the 2nd argument; the tables differ for add and subtract
+        // operations.
+        //
+        private readonly ReadOnlyArray<Z80Flags> halfcarry_add_table =
+                            new ReadOnlyArray<Z80Flags>
+                                    (new Z80Flags[8]
+                                        {Z80Flags.None, Z80Flags.HalfCarry, Z80Flags.HalfCarry, Z80Flags.HalfCarry,
+                                         Z80Flags.None, Z80Flags.None, Z80Flags.None, Z80Flags.HalfCarry});
+
+        private readonly ReadOnlyArray<Z80Flags> halfcarry_sub_table =
+                            new ReadOnlyArray<Z80Flags>
+                                    (new Z80Flags[8]
+                                        {Z80Flags.None, Z80Flags.None, Z80Flags.HalfCarry, Z80Flags.None,
+                                         Z80Flags.HalfCarry, Z80Flags.None, Z80Flags.HalfCarry, Z80Flags.HalfCarry});
+
+        // Similarly, overflow can be determined by looking at the 7th bits; again
+        // the hash into this table is r12
+        //
+        private readonly ReadOnlyArray<Z80Flags> overflow_add_table =
+                            new ReadOnlyArray<Z80Flags>
+                                    (new Z80Flags[8]
+                                        {Z80Flags.None, Z80Flags.None, Z80Flags.None, Z80Flags.PV,
+                                         Z80Flags.PV, Z80Flags.None, Z80Flags.None, Z80Flags.None});
+
+        private readonly ReadOnlyArray<Z80Flags> overflow_sub_table =
+                            new ReadOnlyArray<Z80Flags>
+                                    (new Z80Flags[8]
+                                        {Z80Flags.None, Z80Flags.PV, Z80Flags.None, Z80Flags.None,
+                                         Z80Flags.None, Z80Flags.None, Z80Flags.PV, Z80Flags.None});
+
         // Machine accessors
         //
         private IMemory memory;
-- 
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