run without lapic and ioapic, if they are not present

if no lapic available, use 8253pit for clock
now xv6 runs both on qemu (uniprocessor) and bochs (uniprocessor and MP)

1 files changed, 5 insertions, 2 deletions

diff --git a/main.c b/main.c
index dba8bdd..876c20a 100644
--- a/main.c
+++ b/main.c
@@ -71,8 +71,11 @@ main0(void)
   // start other CPUs
   mp_startthem();
 
-  // turn on timer and enable interrupts on the local APIC
-  lapic_timerinit();
+  // turn on timer
+  if (ismp) lapic_timerinit();
+  else pit8253_timerinit();
+
+  // enable interrupts on the local APIC
   lapic_enableintr();
 
   // Enable interrupts on this processor.