From fc1ad39a8dbe3d0c5ae8395cf3fda37d2b13b523 Mon Sep 17 00:00:00 2001
From: Adrian Conlon <adrian.conlon@gmail.com>
Date: Mon, 11 May 2026 22:27:30 +0100
Subject: [PATCH] Add half-carry tests

---
 MC6809/MC6809.UnitTest/AddTests.cs | 26 ++++++++++++++++++++++++++
 MC6809/MC6809.UnitTest/DecTests.cs | 24 ++++++++++++++++++++++++
 MC6809/MC6809.UnitTest/IncTests.cs | 24 ++++++++++++++++++++++++
 3 files changed, 74 insertions(+)

diff --git a/MC6809/MC6809.UnitTest/AddTests.cs b/MC6809/MC6809.UnitTest/AddTests.cs
index 47f5596..692eb83 100644
--- a/MC6809/MC6809.UnitTest/AddTests.cs
+++ b/MC6809/MC6809.UnitTest/AddTests.cs
@@ -59,6 +59,32 @@ namespace MC6809.UnitTest
             Assert.AreEqual(2, this.cpu.Cycles);
         }
 
+        // 0x08 + 0x08 = 0x10: carry out of low nibble sets H
+        [TestMethod]
+        public void TestImmediateADDA_half_carry_set()
+        {
+            this.board.Poke(0, 0x8b);
+            this.board.Poke(1, 0x08);
+            this.cpu.CC = 0;
+            this.cpu.A = 0x08;
+            this.cpu.Step();
+            Assert.AreEqual(0x10, this.cpu.A);
+            Assert.IsTrue(this.cpu.HalfCarry);
+        }
+
+        // 0x01 + 0x01 = 0x02: no carry out of low nibble, H clear
+        [TestMethod]
+        public void TestImmediateADDA_half_carry_clear()
+        {
+            this.board.Poke(0, 0x8b);
+            this.board.Poke(1, 0x01);
+            this.cpu.CC = 0;
+            this.cpu.A = 0x01;
+            this.cpu.Step();
+            Assert.AreEqual(0x02, this.cpu.A);
+            Assert.IsFalse(this.cpu.HalfCarry);
+        }
+
         // The overflow (V) bit indicates signed two’s complement overflow, which occurs when the
         // sign bit differs from the carry bit after an arithmetic operation.
         // A=0x03 + 0xFF becomes 0x02
diff --git a/MC6809/MC6809.UnitTest/DecTests.cs b/MC6809/MC6809.UnitTest/DecTests.cs
index b0ece2d..0ff5aae 100644
--- a/MC6809/MC6809.UnitTest/DecTests.cs
+++ b/MC6809/MC6809.UnitTest/DecTests.cs
@@ -69,5 +69,29 @@ namespace MC6809.UnitTest
             Assert.IsTrue(this.cpu.Negative);
             Assert.AreEqual(2, this.cpu.Cycles);
         }
+
+        // DEC does not affect H: H=1 before must be H=1 after
+        [TestMethod]
+        public void TestInherentDECA_preserves_half_carry_set()
+        {
+            this.board.Poke(0, 0x4a);
+            this.cpu.CC = (byte)StatusBits.HF;
+            this.cpu.A = 0x10;
+            this.cpu.Step();
+            Assert.AreEqual(0x0f, this.cpu.A);
+            Assert.IsTrue(this.cpu.HalfCarry);
+        }
+
+        // DEC does not affect H: H=0 before must be H=0 after
+        [TestMethod]
+        public void TestInherentDECA_preserves_half_carry_clear()
+        {
+            this.board.Poke(0, 0x4a);
+            this.cpu.CC = 0;
+            this.cpu.A = 0x10;
+            this.cpu.Step();
+            Assert.AreEqual(0x0f, this.cpu.A);
+            Assert.IsFalse(this.cpu.HalfCarry);
+        }
     }
 }
diff --git a/MC6809/MC6809.UnitTest/IncTests.cs b/MC6809/MC6809.UnitTest/IncTests.cs
index da1d69a..d323444 100644
--- a/MC6809/MC6809.UnitTest/IncTests.cs
+++ b/MC6809/MC6809.UnitTest/IncTests.cs
@@ -67,5 +67,29 @@ namespace MC6809.UnitTest
             Assert.IsFalse(this.cpu.Carry);
             Assert.AreEqual(2, this.cpu.Cycles);
         }
+
+        // INC does not affect H: H=1 before must be H=1 after
+        [TestMethod]
+        public void TestInherentINCA_preserves_half_carry_set()
+        {
+            this.board.Poke(0, 0x4c);
+            this.cpu.CC = (byte)StatusBits.HF;
+            this.cpu.A = 0x01;
+            this.cpu.Step();
+            Assert.AreEqual(0x02, this.cpu.A);
+            Assert.IsTrue(this.cpu.HalfCarry);
+        }
+
+        // INC does not affect H: a nibble-crossing increment must not set H=1
+        [TestMethod]
+        public void TestInherentINCA_preserves_half_carry_clear()
+        {
+            this.board.Poke(0, 0x4c);
+            this.cpu.CC = 0;
+            this.cpu.A = 0x0f;
+            this.cpu.Step();
+            Assert.AreEqual(0x10, this.cpu.A);
+            Assert.IsFalse(this.cpu.HalfCarry);
+        }
     }
 }