diff --git a/src/NDS.cpp b/src/NDS.cpp
index c750bc6e..98c29248 100644
--- a/src/NDS.cpp
+++ b/src/NDS.cpp
@@ -764,6 +764,7 @@ u32 RunFrame()
     FrameSysClockCycles = 0;
 
     if (!Running) return 263; // dorp
+    if (CPUStop & 0x40000000) return 263;
 
     GPU::StartFrame();
 
@@ -852,6 +853,14 @@ u32 RunFrame()
         SysClockCycles += ndscyclestorun;
         LastSysClockCycles += ndscyclestorun;
         FrameSysClockCycles += ndscyclestorun;
+
+        if (CPUStop & 0x40000000)
+        {
+            // checkme: when is sleep mode effective?
+            //CancelEvent(Event_LCD);
+            //GPU::TotalScanlines = 263;
+            break;
+        }
     }
 
 #ifdef DEBUG_CHECK_DESYNC
@@ -953,6 +962,20 @@ void SetKeyMask(u32 mask)
     KeyInput |= key_lo | (key_hi << 16);
 }
 
+void SetLidClosed(bool closed)
+{
+    if (closed)
+    {
+        KeyInput |= (1<<23);
+    }
+    else
+    {
+        KeyInput &= ~(1<<23);
+        SetIRQ(1, IRQ_LidOpen);
+        CPUStop &= ~0x40000000;
+    }
+}
+
 void MicInputFrame(s16* data, int samples)
 {
     return SPI_TSC::MicInputFrame(data, samples);
@@ -1111,6 +1134,14 @@ void GXFIFOUnstall()
     CPUStop &= ~0x80000000;
 }
 
+void EnterSleepMode()
+{
+    if (CPUStop & 0x40000000) return;
+
+    CPUStop |= 0x40000000;
+    ARM7->Halt(2);
+}
+
 u32 GetPC(u32 cpu)
 {
     return cpu ? ARM7->R[15] : ARM9->R[15];
@@ -3224,7 +3255,10 @@ void ARM7IOWrite8(u32 addr, u8 val)
         return;
 
     case 0x04000301:
-        if (val == 0x80) ARM7->Halt(1);
+        val & 0xC0;
+        if      (val == 0x40) printf("!! GBA MODE NOT SUPPORTED\n");
+        else if (val == 0x80) ARM7->Halt(1);
+        else if (val == 0xC0) EnterSleepMode();
         return;
     }
 
diff --git a/src/NDS.h b/src/NDS.h
index 2ffb35f6..bfb390f0 100644
--- a/src/NDS.h
+++ b/src/NDS.h
@@ -152,6 +152,8 @@ void ReleaseScreen();
 
 void SetKeyMask(u32 mask);
 
+void SetLidClosed(bool closed);
+
 void MicInputFrame(s16* data, int samples);
 
 void ScheduleEvent(u32 id, bool periodic, s32 delay, void (*func)(u32), u32 param);
diff --git a/src/libui_sdl/main.cpp b/src/libui_sdl/main.cpp
index 3b484be4..e3d981e9 100644
--- a/src/libui_sdl/main.cpp
+++ b/src/libui_sdl/main.cpp
@@ -108,6 +108,7 @@ uiDrawMatrix BottomScreenTrans;
 bool Touching = false;
 
 u32 KeyInputMask;
+bool LidCommand, LidStatus;
 SDL_Joystick* Joystick;
 
 const u32 kMicBufferSize = 2048; // must be power of two
@@ -290,6 +291,8 @@ int EmuThreadFunc(void* burp)
     ScreenDrawInited = false;
     Touching = false;
     KeyInputMask = 0xFFF;
+    LidCommand = false;
+    LidStatus = false;
 
     u32 nframes = 0;
     u32 starttick = SDL_GetTicks();
@@ -347,6 +350,12 @@ int EmuThreadFunc(void* burp)
             }
             NDS::SetKeyMask(keymask & joymask);
 
+            if (LidCommand)
+            {
+                NDS::SetLidClosed(LidStatus);
+                LidCommand = false;
+            }
+
             // microphone input
             if ((MicBufferReadPos + 735) > kMicBufferSize)
             {
@@ -620,6 +629,12 @@ int OnAreaKeyEvent(uiAreaHandler* handler, uiArea* area, uiAreaKeyEvent* evt)
             if (evt->Scancode == Config::KeyMapping[i])
                 KeyInputMask &= ~(1<<i);
 
+        if (evt->Scancode == 0x44) // F10, test
+        {
+            LidStatus = !LidStatus;
+            LidCommand = true;
+        }
+
         if (evt->Scancode == 0x57) // F11
             NDS::debug(0);
     }