From 98169fd6b88e6efeb73478aecf7a001470e9ecc1 Mon Sep 17 00:00:00 2001
From: Adrian Conlon <adrian.conlon@gmail.com>
Date: Mon, 18 May 2026 20:06:57 +0100
Subject: [PATCH] Fix Z80 halt implementation

---
 Z80/Z80.UnitTests/HaltTests.cs | 14 ++++++++++++++
 Z80/Z80.cs                     |  4 ++--
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/Z80/Z80.UnitTests/HaltTests.cs b/Z80/Z80.UnitTests/HaltTests.cs
index d8ad88b..c1aa15e 100644
--- a/Z80/Z80.UnitTests/HaltTests.cs
+++ b/Z80/Z80.UnitTests/HaltTests.cs
@@ -44,6 +44,20 @@ namespace Z80.UnitTests
             Assert.AreEqual(pcAfterHalt, this.cpu.PC.Joined, "PC must not advance while CPU is halted");
         }
 
+        [TestMethod]
+        public void Halt_State_Forces_Nop_Regardless_Of_Memory_Contents()
+        {
+            // The Z80 manual states: "the data received from the memory is ignored
+            // and an NOP instruction is forced internally to the CPU."
+            // INC A (0x3C) at the looping address must not execute — A must be unchanged.
+            this.board.Poke(0x0000, 0x76); // HALT
+            this.board.Poke(0x0001, 0x3C); // INC A — must be suppressed while halted
+            this.cpu.A = 0x42;
+            this.cpu.Step(); // execute HALT — enters halt state, PC moves to 0x0001
+            this.cpu.Step(); // looping — memory byte (INC A) must be ignored, NOP forced
+            Assert.AreEqual(0x42, this.cpu.A, "Memory contents must be ignored during HALT; NOP must be forced");
+        }
+
         [TestMethod]
         public void Halt_Nmi_Raises_Halt_Pin()
         {
diff --git a/Z80/Z80.cs b/Z80/Z80.cs
index fe49e78..7b56350 100644
--- a/Z80/Z80.cs
+++ b/Z80/Z80.cs
@@ -1676,9 +1676,9 @@ namespace Z80
         protected override byte FetchInstruction()
         {
             this.LowerM1();
-                base.FetchInstruction();
+                var returned = base.FetchInstruction();
             this.RaiseM1();
-            return this.Bus.Data;
+            return returned;
         }
 
         private byte Subtract(byte operand, byte value, int carry = 0)