diff --git a/plugins/GSdx/GS.cpp b/plugins/GSdx/GS.cpp
index 20cb695473..06ea0b6356 100644
--- a/plugins/GSdx/GS.cpp
+++ b/plugins/GSdx/GS.cpp
@@ -126,7 +126,7 @@ EXPORT_C GSclose()
 	s_gs->m_wnd.Detach();
 }
 
-static INT32 GSopen(void* dsp, char* title, int mt, int renderer)
+static INT32 _GSopen(void* dsp, char* title, int renderer)
 {
 	GSclose();
 
@@ -153,17 +153,17 @@ static INT32 GSopen(void* dsp, char* title, int mt, int renderer)
 			switch(renderer)
 			{
 			default: 
-			case 0: s_gs = new GSRendererDX9(!!mt, s_irq); break;
-			case 3: s_gs = new GSRendererDX10(!!mt, s_irq); break;
-			case 6: s_gs = new GSRendererDX11(!!mt, s_irq); break;
+			case 0: s_gs = new GSRendererDX9(); break;
+			case 3: s_gs = new GSRendererDX10(); break;
+			case 6: s_gs = new GSRendererDX11(); break;
 	#if 0
-			case 9: s_gs = new GSRendererOGL(!!mt, s_irq); break;
+			case 9: s_gs = new GSRendererOGL(); break;
 	#endif
 			case 2: case 5: case 8: case 11: case 13:
-				s_gs = new GSRendererNull(!!mt, s_irq); break;
+				s_gs = new GSRendererNull(); break;
 
 			case 1: case 4: case 7: case 10: case 12:
-				s_gs = new GSRendererSW(!!mt, s_irq); break;
+				s_gs = new GSRendererSW(); break;
 			}
 		}
 	}
@@ -176,7 +176,8 @@ static INT32 GSopen(void* dsp, char* title, int mt, int renderer)
 		return -1;
 	}
 
-	s_gs->SetRegsMem( s_basemem );
+	s_gs->SetRegsMem(s_basemem);
+	s_gs->SetIrqCallback(s_irq);
 
 	if( *(HWND*)dsp == NULL )
 	{
@@ -196,6 +197,7 @@ static INT32 GSopen(void* dsp, char* title, int mt, int renderer)
 	}
 	else
 	{
+		s_gs->SetMultithreaded( true );
 		s_gs->m_wnd.Attach( *(HWND*)dsp, false );
 	}
 
@@ -221,7 +223,7 @@ EXPORT_C_(INT32) GSopen2( void* dsp, INT32 flags )
 		renderer = 1;
 	}
 
-	return GSopen( dsp, NULL, true, renderer );
+	return _GSopen( dsp, NULL, renderer );
 }
 
 EXPORT_C_(INT32) GSopen(void* dsp, char* title, int mt)
@@ -241,7 +243,15 @@ EXPORT_C_(INT32) GSopen(void* dsp, char* title, int mt)
 	}
 
 	*(HWND*)dsp = NULL;
-	return GSopen(dsp, title, mt, renderer);
+
+	int retval = _GSopen(dsp, title, renderer);
+
+	if( retval == 0 && s_gs )
+	{
+		s_gs->SetMultithreaded( false );
+	}
+	
+	return retval;
 }
 
 EXPORT_C GSreset()
@@ -348,6 +358,10 @@ EXPORT_C GSabout()
 EXPORT_C GSirqCallback(void (*irq)())
 {
 	s_irq = irq;
+	if( s_gs )
+	{
+		s_gs->SetIrqCallback(s_irq);
+	}
 }
 
 EXPORT_C_(int) GSsetupRecording(int start, void* data)
@@ -409,7 +423,7 @@ EXPORT_C GSReplay(HWND hwnd, HINSTANCE hinst, LPSTR lpszCmdLine, int nCmdShow)
 		GSsetBaseMem(regs);
 
 		HWND hWnd = NULL;
-		GSopen(&hWnd, "", true, renderer);
+		GSopen(&hWnd, "", renderer);
 
 		uint32 crc;
 		fread(&crc, 4, 1, fp);
diff --git a/plugins/GSdx/GSRenderer.cpp b/plugins/GSdx/GSRenderer.cpp
index b6b6c3e53f..f2c484ea3d 100644
--- a/plugins/GSdx/GSRenderer.cpp
+++ b/plugins/GSdx/GSRenderer.cpp
@@ -22,8 +22,8 @@
 #include "StdAfx.h"
 #include "GSRenderer.h"
 
-GSRenderer::GSRenderer(bool mt, void (*irq)())
-	: GSState(mt, irq)
+GSRenderer::GSRenderer()
+	: GSState()
 	, m_dev(NULL)
 	, m_shader(0)
 	, m_vt(this)
diff --git a/plugins/GSdx/GSRenderer.h b/plugins/GSdx/GSRenderer.h
index 420e46c7f0..6b441368d9 100644
--- a/plugins/GSdx/GSRenderer.h
+++ b/plugins/GSdx/GSRenderer.h
@@ -69,7 +69,7 @@ public:
 	int s_saven;
 
 public:
-	GSRenderer(bool mt, void (*irq)());
+	GSRenderer();
 	virtual ~GSRenderer();
 
 	virtual bool CreateWnd(const string& title, int w, int h);
@@ -217,8 +217,8 @@ protected:
 	virtual void Draw() = 0;
 
 public:
-	GSRendererT(bool mt, void (*irq)())
-		: GSRenderer(mt, irq)
+	GSRendererT()
+		: GSRenderer()
 		, m_vertices(NULL)
 		, m_count(0)
 		, m_maxcount(0)
diff --git a/plugins/GSdx/GSRendererDX.h b/plugins/GSdx/GSRendererDX.h
index ef67661cc2..2034d8282f 100644
--- a/plugins/GSdx/GSRendererDX.h
+++ b/plugins/GSdx/GSRendererDX.h
@@ -40,8 +40,8 @@ protected:
 	virtual void UpdateFBA(GSTexture* rt) {}
 
 public:
-	GSRendererDX(bool mt, void (*irq)(), GSTextureCache* tc, const GSVector2& pixelcenter = GSVector2(0, 0))
-		: GSRendererHW<Vertex>(mt, irq, tc)
+	GSRendererDX(GSTextureCache* tc, const GSVector2& pixelcenter = GSVector2(0, 0))
+		: GSRendererHW<Vertex>(tc)
 		, m_pixelcenter(pixelcenter)
 		, m_topology(-1)
 	{
diff --git a/plugins/GSdx/GSRendererDX10.cpp b/plugins/GSdx/GSRendererDX10.cpp
index 61f471d2eb..32bebe2cca 100644
--- a/plugins/GSdx/GSRendererDX10.cpp
+++ b/plugins/GSdx/GSRendererDX10.cpp
@@ -24,8 +24,8 @@
 #include "GSCrc.h"
 #include "resource.h"
 
-GSRendererDX10::GSRendererDX10(bool mt, void (*irq)())
-	: GSRendererDX<GSVertexHW10>(mt, irq, new GSTextureCache10(this), GSVector2(-0.5f, -0.5f))
+GSRendererDX10::GSRendererDX10()
+	: GSRendererDX<GSVertexHW10>(new GSTextureCache10(this), GSVector2(-0.5f, -0.5f))
 {
 	InitVertexKick<GSRendererDX10>();
 }
diff --git a/plugins/GSdx/GSRendererDX10.h b/plugins/GSdx/GSRendererDX10.h
index eca99b27c6..17b51589b2 100644
--- a/plugins/GSdx/GSRendererDX10.h
+++ b/plugins/GSdx/GSRendererDX10.h
@@ -38,7 +38,7 @@ protected:
 	void SetupDATE(GSTexture* rt, GSTexture* ds);
 
 public:
-	GSRendererDX10(bool mt, void (*irq)());
+	GSRendererDX10();
 
 	bool CreateDevice(GSDevice* dev);
 
diff --git a/plugins/GSdx/GSRendererDX11.cpp b/plugins/GSdx/GSRendererDX11.cpp
index 53912dfce3..d7eeb0a46c 100644
--- a/plugins/GSdx/GSRendererDX11.cpp
+++ b/plugins/GSdx/GSRendererDX11.cpp
@@ -24,8 +24,8 @@
 #include "GSCrc.h"
 #include "resource.h"
 
-GSRendererDX11::GSRendererDX11(bool mt, void (*irq)())
-	: GSRendererDX<GSVertexHW11>(mt, irq, new GSTextureCache11(this), GSVector2(-0.5f, -0.5f))
+GSRendererDX11::GSRendererDX11()
+	: GSRendererDX<GSVertexHW11>(new GSTextureCache11(this), GSVector2(-0.5f, -0.5f))
 {
 	InitVertexKick<GSRendererDX11>();
 }
diff --git a/plugins/GSdx/GSRendererDX11.h b/plugins/GSdx/GSRendererDX11.h
index a389de7012..35411f90b8 100644
--- a/plugins/GSdx/GSRendererDX11.h
+++ b/plugins/GSdx/GSRendererDX11.h
@@ -39,7 +39,7 @@ protected:
 	void SetupDATE(GSTexture* rt, GSTexture* ds);
 
 public:
-	GSRendererDX11(bool mt, void (*irq)());
+	GSRendererDX11();
 
 	bool CreateDevice(GSDevice* dev);
 
diff --git a/plugins/GSdx/GSRendererDX9.cpp b/plugins/GSdx/GSRendererDX9.cpp
index 9bc79a3849..78135c2742 100644
--- a/plugins/GSdx/GSRendererDX9.cpp
+++ b/plugins/GSdx/GSRendererDX9.cpp
@@ -24,8 +24,8 @@
 #include "GSCrc.h"
 #include "resource.h"
 
-GSRendererDX9::GSRendererDX9(bool mt, void (*irq)())
-	: GSRendererDX<GSVertexHW9>(mt, irq, new GSTextureCache9(this))
+GSRendererDX9::GSRendererDX9()
+	: GSRendererDX<GSVertexHW9>(new GSTextureCache9(this))
 {
 	InitVertexKick<GSRendererDX9>();
 }
diff --git a/plugins/GSdx/GSRendererDX9.h b/plugins/GSdx/GSRendererDX9.h
index 26884ae89d..1772830694 100644
--- a/plugins/GSdx/GSRendererDX9.h
+++ b/plugins/GSdx/GSRendererDX9.h
@@ -45,7 +45,7 @@ protected:
 	void UpdateFBA(GSTexture* rt);
 
 public:
-	GSRendererDX9(bool mt, void (*irq)());
+	GSRendererDX9();
 
 	bool CreateDevice(GSDevice* dev);
 
diff --git a/plugins/GSdx/GSRendererHW.h b/plugins/GSdx/GSRendererHW.h
index 597392f858..985b447089 100644
--- a/plugins/GSdx/GSRendererHW.h
+++ b/plugins/GSdx/GSRendererHW.h
@@ -704,8 +704,8 @@ protected:
 	}
 
 public:
-	GSRendererHW(bool mt, void (*irq)(), GSTextureCache* tc)
-		: GSRendererT<Vertex>(mt, irq)
+	GSRendererHW(GSTextureCache* tc)
+		: GSRendererT<Vertex>()
 		, m_tc(tc)
 		, m_width(1024)
 		, m_height(1024)
diff --git a/plugins/GSdx/GSRendererNull.h b/plugins/GSdx/GSRendererNull.h
index 2fbb488a12..b1c5b60b14 100644
--- a/plugins/GSdx/GSRendererNull.h
+++ b/plugins/GSdx/GSRendererNull.h
@@ -37,8 +37,8 @@ protected:
 	}
 
 public:
-	GSRendererNull(bool mt, void (*irq)())
-		: GSRendererT<GSVertexNull>(mt, irq)
+	GSRendererNull()
+		: GSRendererT<GSVertexNull>()
 	{
 		InitVertexKick<GSRendererNull>();
 	}
diff --git a/plugins/GSdx/GSRendererOGL.cpp b/plugins/GSdx/GSRendererOGL.cpp
index dba86b4b15..cbbe87f381 100644
--- a/plugins/GSdx/GSRendererOGL.cpp
+++ b/plugins/GSdx/GSRendererOGL.cpp
@@ -24,8 +24,8 @@
 #include "GSCrc.h"
 #include "resource.h"
 
-GSRendererOGL::GSRendererOGL(bool mt, void (*irq)())
-	: GSRendererHW<GSVertexOGL>(mt, irq, new GSTextureCacheOGL(this))
+GSRendererOGL::GSRendererOGL()
+	: GSRendererHW<GSVertexOGL>(new GSTextureCacheOGL(this))
 {
 	InitVertexKick<GSRendererOGL>();
 }
diff --git a/plugins/GSdx/GSRendererOGL.h b/plugins/GSdx/GSRendererOGL.h
index a769898c44..e9accf1a87 100644
--- a/plugins/GSdx/GSRendererOGL.h
+++ b/plugins/GSdx/GSRendererOGL.h
@@ -31,7 +31,7 @@ protected:
 	void Draw(GSTexture* rt, GSTexture* ds, GSTextureCache::Source* tex);
 
 public:
-	GSRendererOGL(bool mt, void (*irq)());
+	GSRendererOGL();
 
 	bool CreateDevice(GSDevice* dev);
 
diff --git a/plugins/GSdx/GSRendererSW.cpp b/plugins/GSdx/GSRendererSW.cpp
index 00171e2e33..10cc08b782 100644
--- a/plugins/GSdx/GSRendererSW.cpp
+++ b/plugins/GSdx/GSRendererSW.cpp
@@ -24,8 +24,8 @@
 
 const GSVector4 g_pos_scale(1.0f / 16, 1.0f / 16, 1.0f, 128.0f);
 
-GSRendererSW::GSRendererSW(bool mt, void (*irq)())
-	: GSRendererT(mt, irq)
+GSRendererSW::GSRendererSW()
+	: GSRendererT()
 {
 	m_tc = new GSTextureCacheSW(this);
 
diff --git a/plugins/GSdx/GSRendererSW.h b/plugins/GSdx/GSRendererSW.h
index b14462f004..0aac07b450 100644
--- a/plugins/GSdx/GSRendererSW.h
+++ b/plugins/GSdx/GSRendererSW.h
@@ -44,7 +44,7 @@ protected:
 	void GetScanlineParam(GSScanlineParam& p, GS_PRIM_CLASS primclass);
 
 public:
-	GSRendererSW(bool mt, void (*irq)());
+	GSRendererSW();
 	virtual ~GSRendererSW();
 
 	template<uint32 prim, uint32 tme, uint32 fst> 
diff --git a/plugins/GSdx/GSState.cpp b/plugins/GSdx/GSState.cpp
index d41a92c5c2..ef01439854 100644
--- a/plugins/GSdx/GSState.cpp
+++ b/plugins/GSdx/GSState.cpp
@@ -22,9 +22,10 @@
 #include "stdafx.h"
 #include "GSState.h"
 
-GSState::GSState(bool mt, void (*irq)())
-	: m_mt(mt)
-	, m_irq(irq)
+GSState::GSState()
+	: m_mt(false)
+	, m_irq(NULL)
+	, m_regs(NULL)
 	, m_crc(0)
 	, m_options(0)
 	, m_path3hack(0)
@@ -97,12 +98,38 @@ GSState::~GSState()
 {
 }
 
-void GSState::SetRegsMem( uint8* basemem )
+void GSState::SetRegsMem(uint8* basemem)
 {
 	ASSERT(basemem);
 	m_regs = (GSPrivRegSet*)basemem;
 }
 
+void GSState::SetIrqCallback(void (*irq)())
+{
+	m_irq = irq;
+}
+
+void GSState::SetMultithreaded( bool isMT )
+{
+	// Some older versions of PCSX2 didn't properly set the irq callback to NULL
+	// in multithreaded mode (possibly because ZeroGS itself would assert in such
+	// cases), and didn't bind them to a dummy callback either.  PCSX2 handles all
+	// IRQs internally when multithreaded anyway -- so let's ignore them here:
+	
+	m_mt = isMT;
+	if( isMT )
+	{
+		m_fpGIFRegHandlers[GIF_A_D_REG_SIGNAL] = &GSState::GIFRegHandlerNull;
+		m_fpGIFRegHandlers[GIF_A_D_REG_FINISH] = &GSState::GIFRegHandlerNull;
+		m_fpGIFRegHandlers[GIF_A_D_REG_LABEL] = &GSState::GIFRegHandlerNull;
+	}
+	else
+	{
+		m_fpGIFRegHandlers[GIF_A_D_REG_SIGNAL] = &GSState::GIFRegHandlerSIGNAL;
+		m_fpGIFRegHandlers[GIF_A_D_REG_FINISH] = &GSState::GIFRegHandlerFINISH;
+		m_fpGIFRegHandlers[GIF_A_D_REG_LABEL] = &GSState::GIFRegHandlerLABEL;
+	}
+}
 
 void GSState::Reset()
 {
@@ -915,8 +942,6 @@ void GSState::GIFRegHandlerHWREG(GIFReg* r)
 
 void GSState::GIFRegHandlerSIGNAL(GIFReg* r)
 {
-	if(m_mt) return;
-
 	m_regs->SIGLBLID.SIGID = (m_regs->SIGLBLID.SIGID & ~r->SIGNAL.IDMSK) | (r->SIGNAL.ID & r->SIGNAL.IDMSK);
 
 	if(m_regs->CSR.wSIGNAL) m_regs->CSR.rSIGNAL = 1;
@@ -925,16 +950,12 @@ void GSState::GIFRegHandlerSIGNAL(GIFReg* r)
 
 void GSState::GIFRegHandlerFINISH(GIFReg* r)
 {
-	if(m_mt) return;
-
 	if(m_regs->CSR.wFINISH) m_regs->CSR.rFINISH = 1;
 	if(!m_regs->IMR.FINISHMSK && m_irq) m_irq();
 }
 
 void GSState::GIFRegHandlerLABEL(GIFReg* r)
 {
-	if(m_mt) return;
-
 	m_regs->SIGLBLID.LBLID = (m_regs->SIGLBLID.LBLID & ~r->LABEL.IDMSK) | (r->LABEL.ID & r->LABEL.IDMSK);
 }
 
diff --git a/plugins/GSdx/GSState.h b/plugins/GSdx/GSState.h
index 01ca087d8e..b752b752c6 100644
--- a/plugins/GSdx/GSState.h
+++ b/plugins/GSdx/GSState.h
@@ -215,7 +215,7 @@ public:
 	GSDump m_dump;
 
 public:
-	GSState(bool mt, void (*irq)());
+	GSState();
 	virtual ~GSState();
 
 	void ResetHandlers();
@@ -251,5 +251,7 @@ public:
 	void SetFrameSkip(int skip);
 	void SetFrameLimit(bool limit);
 	void SetRegsMem( uint8* basemem );
+	void SetIrqCallback(void (*irq)());
+	void SetMultithreaded( bool isMT=true );
 };