hlt if no processes are runnable

diff --git a/proc.c b/proc.c
index bcdbfea..a32a672 100644
--- a/proc.c
+++ b/proc.c
@@ -257,18 +257,29 @@ wait(void)
 void
 scheduler(void)
 {
+  int no_runnable;
   struct proc *p;

-  for(;;){
+  for(no_runnable = 0;;){
     // Enable interrupts on this processor.
     sti();

+    // If no runnable processes were found last pass then halt
+    // the processor
+    if(no_runnable) hlt();
+
+    // Set no_runnable
+    no_runnable = 1;
+
     // Loop over process table looking for process to run.
     acquire(&ptable.lock);
     for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){
       if(p->state != RUNNABLE)
         continue;

+      // We found at least one runnable proc
+      no_runnable = 0;
+
       // Switch to chosen process.  It is the process's job
       // to release ptable.lock and then reacquire it
       // before jumping back to us.
diff --git a/x86.h b/x86.h
index 3949900..7d14145 100644
--- a/x86.h
+++ b/x86.h
@@ -117,6 +117,12 @@ sti(void)
   asm volatile("sti");
 }

+static inline void
+hlt(void)
+{
+  asm volatile("hlt");
+}
+
 static inline uint
 xchg(volatile uint *addr, uint newval)
 {