interrupts during system calls

"It just works."

trap.c

@@ -20,7 +20,7 @@ tvinit(void)
 
   for(i = 0; i < 256; i++)
     SETGATE(idt[i], 0, SEG_KCODE<<3, vectors[i], 0);
-  SETGATE(idt[T_SYSCALL], 0, SEG_KCODE<<3, vectors[T_SYSCALL], DPL_USER);
+  SETGATE(idt[T_SYSCALL], 1, SEG_KCODE<<3, vectors[T_SYSCALL], DPL_USER);
   
   initlock(&tickslock, "time");
 }