update preemption demo

c_os/user/demo/PreemptiveThreadDemo.cc

@@ -0,0 +1,36 @@
+#include "user/demo/PreemptiveThreadDemo.h"
+
+void PreemptiveLoopThread::run() {
+    int cnt = 0;
+    while (true) {
+        // Basic synchronization by semaphore
+        sem->p();
+
+        kout.setpos(55, this->id);
+        kout << this->id << ": " << dec << cnt++ << endl;
+
+        sem->v();
+    }
+}
+
+void PreemptiveThreadDemo::run() {
+    kout << "PreemptiveThreadDemo ====================================" << endl;
+    kout << "Initializing Semaphore" << endl;
+    Semaphore* sem = new Semaphore(1);  // Create this semaphore on the heap as this thread exits itself,
+                                        // so stack allocated objects will be lost
+
+    kout << "Allocating LoopThread A" << endl;
+    Thread* cntA = new PreemptiveLoopThread(0, sem);
+    kout << "Allocating LoopThread B" << endl;
+    Thread* cntB = new PreemptiveLoopThread(1, sem);
+    kout << "Allocating LoopThread C" << endl;
+    Thread* cntC = new PreemptiveLoopThread(2, sem);
+
+    kout << "Adding threads to ready queue" << endl;
+    scheduler.ready(cntA);
+    scheduler.ready(cntB);
+    scheduler.ready(cntC);
+
+    kout << "Exiting main thread" << endl;
+    scheduler.exit();
+}