From a036bc1a487c4330adabf2509f6cd34e862fba8a Mon Sep 17 00:00:00 2001
From: ParantezTech <12572227+par274@users.noreply.github.com>
Date: Wed, 23 Sep 2026 14:46:17 +0300
Subject: [PATCH] [CPU] Host RBX is restored from the saved host frame below

---
 .../Cpu/Native/DirectExecutionBackend.cs      |  2 ++
 .../Cpu/Gen5NativeReturnSmokeTests.cs         | 34 ++++++++++++++++++-
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/src/SharpEmu.Core/Cpu/Native/DirectExecutionBackend.cs b/src/SharpEmu.Core/Cpu/Native/DirectExecutionBackend.cs
index 1de1af5..5fdce4d 100644
--- a/src/SharpEmu.Core/Cpu/Native/DirectExecutionBackend.cs
+++ b/src/SharpEmu.Core/Cpu/Native/DirectExecutionBackend.cs
@@ -2610,6 +2610,7 @@ public sealed unsafe partial class DirectExecutionBackend : INativeCpuBackend, I
 
 		byte* code = (byte*)ptr;
 		int offset = 0;
+		EmitByte(code, ref offset, 0x48); EmitByte(code, ref offset, 0x89); EmitByte(code, ref offset, 0xC3); // mov rbx,rax
 		EmitByte(code, ref offset, 0x48); // sub rsp, 0x20
 		EmitByte(code, ref offset, 0x83);
 		EmitByte(code, ref offset, 0xEC);
@@ -2639,6 +2640,7 @@ public sealed unsafe partial class DirectExecutionBackend : INativeCpuBackend, I
 		EmitByte(code, ref offset, 0x5E);
 		EmitByte(code, ref offset, 0x5F);
 		EmitByte(code, ref offset, 0x5D);
+		EmitByte(code, ref offset, 0x48); EmitByte(code, ref offset, 0x89); EmitByte(code, ref offset, 0xD8); // mov rax,rbx
 		EmitByte(code, ref offset, 0x5B);
 		EmitByte(code, ref offset, 0xC3);
 
diff --git a/tests/SharpEmu.Libs.Tests/Cpu/Gen5NativeReturnSmokeTests.cs b/tests/SharpEmu.Libs.Tests/Cpu/Gen5NativeReturnSmokeTests.cs
index 9426786..6a85028 100644
--- a/tests/SharpEmu.Libs.Tests/Cpu/Gen5NativeReturnSmokeTests.cs
+++ b/tests/SharpEmu.Libs.Tests/Cpu/Gen5NativeReturnSmokeTests.cs
@@ -82,7 +82,24 @@ public sealed class Gen5NativeReturnSmokeTests
         ]);
     }
 
-    private static void ExecuteSyntheticGuest(byte[]? callbackInstructions = null)
+    [Fact]
+    public async Task SyntheticGen5Continuation_PreservesFullReturnValue()
+    {
+        if (!IsSupportedHost)
+            return;
+
+        if (Environment.GetEnvironmentVariable(WorkerEnvironmentVariable) != "1")
+        {
+            var result = await RunIsolatedWorker(nameof(SyntheticGen5Continuation_PreservesFullReturnValue));
+            Assert.True(result.Completed, result.Output);
+            Assert.True(result.ExitCode == 0, result.Output);
+            return;
+        }
+
+        ExecuteSyntheticGuest(continuation: true);
+    }
+
+    private static void ExecuteSyntheticGuest(byte[]? callbackInstructions = null, bool continuation = false)
     {
         using var memory = new PhysicalVirtualMemory();
         var image = new SelfLoader().Load(BuildSyntheticElf(callbackInstructions), memory);
@@ -119,6 +136,21 @@ public sealed class Gen5NativeReturnSmokeTests
         Assert.Null(dispatcher.LastNotImplementedInfo);
 
         var callerContext = new CpuContext(new TrackedCpuMemory(memory), Generation.Gen5);
+        if (continuation)
+        {
+            Assert.True(memory.TryAllocateAtOrAbove(0x1_0000_0000, 0x4000, false, 0x4000, out var stack));
+            callerContext[CpuRegister.Rsp] = stack + 0x4000 - sizeof(ulong);
+            // Resume directly into mov rax, imm64; ret through the shared guest return stub.
+            var execute = typeof(DirectExecutionBackend).GetMethod(
+                "ExecuteGuestContinuationEntry", BindingFlags.Instance | BindingFlags.NonPublic)!;
+            object?[] arguments = [callerContext, image.EntryPoint + 3, callerContext[CpuRegister.Rsp],
+                "synthetic-native-continuation-return", null];
+            var exitReason = execute.Invoke(backend, arguments);
+            Assert.True(exitReason?.ToString() == "Returned", arguments[4]?.ToString());
+            Assert.Equal(CallbackReturnValue, callerContext[CpuRegister.Rax]);
+            return;
+        }
+
         ulong dataAddress = 0;
         var recoveryCounter = typeof(DirectExecutionBackend).GetField(
             "_sse4aInstructionsEmulated", BindingFlags.Static | BindingFlags.NonPublic)!;