add bluescreen demo

c_os/user/demo/BlueScreenDemo.cc

@@ -0,0 +1,20 @@
+#include "user/demo/BlueScreenDemo.h"
+#include "kernel/Paging.h"
+
+void BlueScreenDemo::run() {
+    kout << "Trigger Bluescreen, if you can read this it didn't work" << endl;
+
+    // BlueScreen 1
+    // asm("int $3");
+
+    // BlueScreen 2
+    unsigned int* page = pg_alloc_page();
+    *page = 42;
+    pg_write_protect_page(page);
+    invalidate_tlb_entry(page); // If we don't invalidate after first access the write protection
+                                // won't work as no lookup is performed (address in tlb)
+    *page = 42; // We map logical to physical 1:1 so no need to do any lookup
+                // If tlb is invalidated this access produces a pagefault
+
+    scheduler.exit();
+}