diff --git a/Makefile b/Makefile
index 3c40d53..5ed973d 100644
--- a/Makefile
+++ b/Makefile
@@ -70,7 +70,8 @@ USER_CPPS  		:= $(wildcard $(USER_CPPS))
 USER_OBJS  		:= $(addprefix $(OBJ_DIR)/, $(patsubst %.c,%.o,$(USER_CPPS)))
 
 
-USER_TARGET 	:= $(OBJ_DIR)/app_illegal_instruction
+
+USER_TARGET 	:= $(OBJ_DIR)/app_long_loop
 #------------------------targets------------------------
 $(OBJ_DIR):
 	@-mkdir -p $(OBJ_DIR)	
diff --git a/kernel/config.h b/kernel/config.h
index 95a8668..156ef62 100644
--- a/kernel/config.h
+++ b/kernel/config.h
@@ -4,6 +4,9 @@
 // we use only one HART (cpu) in fundamental experiments
 #define NCPU 1
 
+//interval of timer interrupt. added @lab1_3
+#define TIMER_INTERVAL 1000000
+
 #define DRAM_BASE 0x80000000
 
 /* we use fixed physical (also logical) addresses for the stacks and trap frames as in
diff --git a/kernel/machine/minit.c b/kernel/machine/minit.c
index 82ec357..b258f71 100644
--- a/kernel/machine/minit.c
+++ b/kernel/machine/minit.c
@@ -76,6 +76,17 @@ static void delegate_traps() {
   assert(read_csr(medeleg) == exceptions);
 }
 
+//
+// enabling timer interrupt (irq) in Machine mode. added @lab1_3
+//
+void timerinit(uintptr_t hartid) {
+  // fire timer irq after TIMER_INTERVAL from now.
+  *(uint64*)CLINT_MTIMECMP(hartid) = *(uint64*)CLINT_MTIME + TIMER_INTERVAL;
+
+  // enable machine-mode timer irq in MIE (Machine Interrupt Enable) csr.
+  write_csr(mie, read_csr(mie) | MIE_MTIE);
+}
+
 //
 // m_start: machine mode C entry point.
 //
@@ -103,10 +114,19 @@ void m_start(uintptr_t hartid, uintptr_t dtb) {
   // setup trap handling vector for machine mode. added @lab1_2
   write_csr(mtvec, (uint64)mtrapvec);
 
+  // enable machine-mode interrupts. added @lab1_3
+  write_csr(mstatus, read_csr(mstatus) | MSTATUS_MIE);
+
   // delegate all interrupts and exceptions to supervisor mode.
   // delegate_traps() is defined above.
   delegate_traps();
 
+  // also enables interrupt handling in supervisor mode. added @lab1_3
+  write_csr(sie, read_csr(sie) | SIE_SEIE | SIE_STIE | SIE_SSIE);
+
+  // init timing. added @lab1_3
+  timerinit(hartid);
+
   // switch to supervisor mode (S mode) and jump to s_start(), i.e., set pc to mepc
   asm volatile("mret");
 }
diff --git a/kernel/machine/mtrap.c b/kernel/machine/mtrap.c
index 7a16011..72566ba 100644
--- a/kernel/machine/mtrap.c
+++ b/kernel/machine/mtrap.c
@@ -14,12 +14,25 @@ static void handle_misaligned_load() { panic("Misaligned Load!"); }
 
 static void handle_misaligned_store() { panic("Misaligned AMO!"); }
 
+// added @lab1_3
+static void handle_timer() {
+  int cpuid = 0;
+  // setup the timer fired at next time (TIMER_INTERVAL from now)
+  *(uint64*)CLINT_MTIMECMP(cpuid) = *(uint64*)CLINT_MTIMECMP(cpuid) + TIMER_INTERVAL;
+
+  // setup a soft interrupt in sip (S-mode Interrupt Pending) to be handled in S-mode
+  write_csr(sip, SIP_SSIP);
+}
+
 //
 // handle_mtrap calls a handling function according to the type of a machine mode interrupt (trap).
 //
 void handle_mtrap() {
   uint64 mcause = read_csr(mcause);
   switch (mcause) {
+    case CAUSE_MTIMER:
+      handle_timer();
+      break;
     case CAUSE_FETCH_ACCESS:
       handle_instruction_access_fault();
       break;
diff --git a/kernel/riscv.h b/kernel/riscv.h
index 6d8700c..419baac 100644
--- a/kernel/riscv.h
+++ b/kernel/riscv.h
@@ -52,6 +52,18 @@
 #define CAUSE_LOAD_PAGE_FAULT 0xd      // Load page fault
 #define CAUSE_STORE_PAGE_FAULT 0xf     // Store/AMO page fault
 
+// irqs (interrupts). added @lab1_3
+#define CAUSE_MTIMER 0x8000000000000007
+#define CAUSE_MTIMER_S_TRAP 0x8000000000000001
+
+//Supervisor interrupt-pending register
+#define SIP_SSIP (1L << 1)
+
+// core local interruptor (CLINT), which contains the timer.
+#define CLINT 0x2000000L
+#define CLINT_MTIMECMP(hartid) (CLINT + 0x4000 + 8 * (hartid))
+#define CLINT_MTIME (CLINT + 0xBFF8)  // cycles since boot.
+
 // fields of sstatus, the Supervisor mode Status register
 #define SSTATUS_SPP (1L << 8)   // Previous mode, 1=Supervisor, 0=User
 #define SSTATUS_SPIE (1L << 5)  // Supervisor Previous Interrupt Enable
diff --git a/kernel/strap.c b/kernel/strap.c
index e1befad..8f3086b 100644
--- a/kernel/strap.c
+++ b/kernel/strap.c
@@ -26,6 +26,21 @@ static void handle_syscall(trapframe *tf) {
 
 }
 
+//
+// global variable that store the recorded "ticks". added @lab1_3
+static uint64 g_ticks = 0;
+//
+// added @lab1_3
+//
+void handle_mtimer_trap() {
+  sprint("Ticks %d\n", g_ticks);
+  // TODO (lab1_3): increase g_ticks to record this "tick", and then clear the "SIP"
+  // field in sip register.
+  // hint: use write_csr to disable the SIP_SSIP bit in sip.
+  panic( "lab1_3: increase g_ticks by one, and clear SIP field in sip register.\n" );
+
+}
+
 //
 // kernel/smode_trap.S will pass control to smode_trap_handler, when a trap happens
 // in S-mode.
@@ -41,8 +56,13 @@ void smode_trap_handler(void) {
 
   // if the cause of trap is syscall from user application.
   // read_csr() and CAUSE_USER_ECALL are macros defined in kernel/riscv.h
-  if (read_csr(scause) == CAUSE_USER_ECALL) {
+  uint64 cause = read_csr(scause);
+
+  // we need to handle the timer trap @lab1_3.
+  if (cause == CAUSE_USER_ECALL) {
     handle_syscall(current->trapframe);
+  } else if (cause == CAUSE_MTIMER_S_TRAP) {  //soft trap generated by timer interrupt in M mode
+    handle_mtimer_trap();
   } else {
     sprint("smode_trap_handler(): unexpected scause %p\n", read_csr(scause));
     sprint("            sepc=%p stval=%p\n", read_csr(sepc), read_csr(stval));
diff --git a/user/app_illegal_instruction.c b/user/app_illegal_instruction.c
deleted file mode 100644
index 3ff98cb..0000000
--- a/user/app_illegal_instruction.c
+++ /dev/null
@@ -1,15 +0,0 @@
-/*
- * Below is the given application for lab1_2.
- * This app attempts to issue M-mode instruction in U-mode, and consequently raises an exception.
- */
-
-#include "user_lib.h"
-#include "util/types.h"
-
-int main(void) {
-  printu("Going to hack the system by running privilege instructions.\n");
-  // we are now in U(user)-mode, but the "csrw" instruction requires M-mode privilege.
-  // Attempting to execute such instruction will raise illegal instruction exception.
-  asm volatile("csrw sscratch, 0");
-  exit(0);
-}
diff --git a/user/app_long_loop.c b/user/app_long_loop.c
new file mode 100644
index 0000000..8b4d873
--- /dev/null
+++ b/user/app_long_loop.c
@@ -0,0 +1,20 @@
+/*
+ * Below is the given application for lab1_3.
+ * This app performs a long loop, during which, timers are
+ * generated and pop messages to our screen.
+ */
+
+#include "user_lib.h"
+#include "util/types.h"
+
+int main(void) {
+  printu("Hello world!\n");
+  int i;
+  for (i = 0; i < 100000000; ++i) {
+    if (i % 5000000 == 0) printu("wait %d\n", i);
+  }
+
+  exit(0);
+
+  return 0;
+}