diff --git a/Makefile b/Makefile
index 5ed973d..279ed6a 100644
--- a/Makefile
+++ b/Makefile
@@ -63,22 +63,25 @@ SPIKE_INF_LIB   := $(OBJ_DIR)/spike_interface.a
 
 
 #---------------------	user   -----------------------
-USER_LDS  := user/user.lds
-USER_CPPS 		:= user/*.c 
+USER_LDS0  := user/user0.lds
+USER_LDS1  := user/user1.lds
 
-USER_CPPS  		:= $(wildcard $(USER_CPPS))
-USER_OBJS  		:= $(addprefix $(OBJ_DIR)/, $(patsubst %.c,%.o,$(USER_CPPS)))
+USER_CPP0 		:= user/app0.c user/user_lib.c
+USER_CPP1 		:= user/app1.c user/user_lib.c
 
+USER_OBJ0  		:= $(addprefix $(OBJ_DIR)/, $(patsubst %.c,%.o,$(USER_CPP0)))
+USER_OBJ1  		:= $(addprefix $(OBJ_DIR)/, $(patsubst %.c,%.o,$(USER_CPP1)))
 
-
-USER_TARGET 	:= $(OBJ_DIR)/app_long_loop
+USER_TARGET0 	:= $(OBJ_DIR)/app0
+USER_TARGET1 	:= $(OBJ_DIR)/app1
 #------------------------targets------------------------
 $(OBJ_DIR):
 	@-mkdir -p $(OBJ_DIR)	
 	@-mkdir -p $(dir $(UTIL_OBJS))
 	@-mkdir -p $(dir $(SPIKE_INF_OBJS))
 	@-mkdir -p $(dir $(KERNEL_OBJS))
-	@-mkdir -p $(dir $(USER_OBJS))
+	@-mkdir -p $(dir $(USER_OBJ0))
+	@-mkdir -p $(dir $(USER_OBJ1))
 
 $(OBJ_DIR)/%.o : %.c
 	@echo "compiling" $<
@@ -103,9 +106,14 @@ $(KERNEL_TARGET): $(OBJ_DIR) $(UTIL_LIB) $(SPIKE_INF_LIB) $(KERNEL_OBJS) $(KERNE
 	@$(COMPILE) $(KERNEL_OBJS) $(UTIL_LIB) $(SPIKE_INF_LIB) -o $@ -T $(KERNEL_LDS)
 	@echo "PKE core has been built into" \"$@\"
 
-$(USER_TARGET): $(OBJ_DIR) $(UTIL_LIB) $(USER_OBJS) $(USER_LDS)
+$(USER_TARGET0): $(OBJ_DIR) $(UTIL_LIB) $(USER_OBJ0) $(USER_LDS0)
+	@echo "linking" $@	...	
+	@$(COMPILE) $(USER_OBJ0) $(UTIL_LIB) -o $@ -T $(USER_LDS0)
+	@echo "User app has been built into" \"$@\"
+	
+$(USER_TARGET1): $(OBJ_DIR) $(UTIL_LIB) $(USER_OBJ1) $(USER_LDS1)
 	@echo "linking" $@	...	
-	@$(COMPILE) $(USER_OBJS) $(UTIL_LIB) -o $@ -T $(USER_LDS)
+	@$(COMPILE) $(USER_OBJ1) $(UTIL_LIB) -o $@ -T $(USER_LDS1)
 	@echo "User app has been built into" \"$@\"
 
 -include $(wildcard $(OBJ_DIR)/*/*.d)
@@ -113,16 +121,16 @@ $(USER_TARGET): $(OBJ_DIR) $(UTIL_LIB) $(USER_OBJS) $(USER_LDS)
 
 .DEFAULT_GOAL := $(all)
 
-all: $(KERNEL_TARGET) $(USER_TARGET)
+all: $(KERNEL_TARGET) $(USER_TARGET0) $(USER_TARGET1)
 .PHONY:all
 
-run: $(KERNEL_TARGET) $(USER_TARGET)
+run: $(KERNEL_TARGET) $(USER_TARGET0) $(USER_TARGET1)
 	@echo "********************HUST PKE********************"
-	spike $(KERNEL_TARGET) $(USER_TARGET)
+	spike -p2 $(KERNEL_TARGET) $(USER_TARGET0) $(USER_TARGET1)
 
 # need openocd!
-gdb:$(KERNEL_TARGET) $(USER_TARGET)
-	spike --rbb-port=9824 -H $(KERNEL_TARGET) $(USER_TARGET) &
+gdb:$(KERNEL_TARGET) $(USER_TARGET0) $(USER_TARGET1)
+	spike --rbb-port=9824 -H -p2 $(KERNEL_TARGET) $(USER_TARGET0) $(USER_TARGET1) &
 	@sleep 1
 	openocd -f ./.spike.cfg &
 	@sleep 1
@@ -136,7 +144,8 @@ gdb_clean:
 
 objdump:
 	riscv64-unknown-elf-objdump -d $(KERNEL_TARGET) > $(OBJ_DIR)/kernel_dump
-	riscv64-unknown-elf-objdump -d $(USER_TARGET) > $(OBJ_DIR)/user_dump
+	riscv64-unknown-elf-objdump -d $(USER_TARGET0) > $(OBJ_DIR)/app0_dump
+	riscv64-unknown-elf-objdump -d $(USER_TARGET1) > $(OBJ_DIR)/app1_dump
 
 cscope:
 	find ./ -name "*.c" > cscope.files
@@ -149,4 +158,4 @@ format:
 	@python ./format.py ./
 
 clean:
-	rm -fr ${OBJ_DIR}
+	rm -fr ${OBJ_DIR}
\ No newline at end of file
diff --git a/kernel/config.h b/kernel/config.h
index 156ef62..1c1fe15 100644
--- a/kernel/config.h
+++ b/kernel/config.h
@@ -1,8 +1,8 @@
 #ifndef _CONFIG_H_
 #define _CONFIG_H_
 
-// we use only one HART (cpu) in fundamental experiments
-#define NCPU 1
+// we use two HART (cpu) in challenge3
+#define NCPU 2
 
 //interval of timer interrupt. added @lab1_3
 #define TIMER_INTERVAL 1000000
diff --git a/kernel/elf.c b/kernel/elf.c
index beb3dff..5963121 100644
--- a/kernel/elf.c
+++ b/kernel/elf.c
@@ -111,7 +111,7 @@ void load_bincode_from_host_elf(process *p) {
   size_t argc = parse_args(&arg_bug_msg);
   if (!argc) panic("You need to specify the application program!\n");
 
-  sprint("Application: %s\n", arg_bug_msg.argv[0]);
+  sprint("hartid = ?: Application: %s\n", arg_bug_msg.argv[0]);
 
   //elf loading. elf_ctx is defined in kernel/elf.h, used to track the loading process.
   elf_ctx elfloader;
@@ -136,5 +136,5 @@ void load_bincode_from_host_elf(process *p) {
   // close the host spike file
   spike_file_close( info.f );
 
-  sprint("Application program entry point (virtual address): 0x%lx\n", p->trapframe->epc);
+  sprint("hartid = ?: Application program entry point (virtual address): 0x%lx\n", p->trapframe->epc);
 }
diff --git a/kernel/kernel.c b/kernel/kernel.c
index ec07e94..c9800ff 100644
--- a/kernel/kernel.c
+++ b/kernel/kernel.c
@@ -32,7 +32,7 @@ void load_user_program(process *proc) {
 // s_start: S-mode entry point of riscv-pke OS kernel.
 //
 int s_start(void) {
-  sprint("Enter supervisor mode...\n");
+  sprint("hartid = ?: Enter supervisor mode...\n");
   // Note: we use direct (i.e., Bare mode) for memory mapping in lab1.
   // which means: Virtual Address = Physical Address
   // therefore, we need to set satp to be 0 for now. we will enable paging in lab2_x.
@@ -43,7 +43,7 @@ int s_start(void) {
   // the application code (elf) is first loaded into memory, and then put into execution
   load_user_program(&user_app);
 
-  sprint("Switch to user mode...\n");
+  sprint("hartid = ?: Switch to user mode...\n");
   // switch_to() is defined in kernel/process.c
   switch_to(&user_app);
 
diff --git a/kernel/machine/mtrap.c b/kernel/machine/mtrap.c
index 72566ba..e85069f 100644
--- a/kernel/machine/mtrap.c
+++ b/kernel/machine/mtrap.c
@@ -16,7 +16,7 @@ static void handle_misaligned_store() { panic("Misaligned AMO!"); }
 
 // added @lab1_3
 static void handle_timer() {
-  int cpuid = 0;
+  int cpuid = read_csr(mhartid);
   // setup the timer fired at next time (TIMER_INTERVAL from now)
   *(uint64*)CLINT_MTIMECMP(cpuid) = *(uint64*)CLINT_MTIMECMP(cpuid) + TIMER_INTERVAL;
 
diff --git a/kernel/sync_utils.h b/kernel/sync_utils.h
new file mode 100644
index 0000000..988fc4c
--- /dev/null
+++ b/kernel/sync_utils.h
@@ -0,0 +1,20 @@
+#ifndef _SYNC_UTILS_H_
+#define _SYNC_UTILS_H_
+
+static inline void sync_barrier(volatile int *counter, int all) {
+
+  int local;
+
+  asm volatile("amoadd.w %0, %2, (%1)\n"
+               : "=r"(local)
+               : "r"(counter), "r"(1)
+               : "memory");
+
+  if (local + 1 < all) {
+    do {
+      asm volatile("lw %0, (%1)\n" : "=r"(local) : "r"(counter) : "memory");
+    } while (local < all);
+  }
+}
+
+#endif
\ No newline at end of file
diff --git a/kernel/syscall.c b/kernel/syscall.c
index 562245a..7ee9f03 100644
--- a/kernel/syscall.c
+++ b/kernel/syscall.c
@@ -17,7 +17,7 @@
 // implement the SYS_user_print syscall
 //
 ssize_t sys_user_print(const char* buf, size_t n) {
-  sprint(buf);
+  sprint("hartid = ?: %s\n", buf);
   return 0;
 }
 
@@ -25,9 +25,10 @@ ssize_t sys_user_print(const char* buf, size_t n) {
 // implement the SYS_user_exit syscall
 //
 ssize_t sys_user_exit(uint64 code) {
-  sprint("User exit with code:%d.\n", code);
+  sprint("hartid = ?: User exit with code:%d.\n", code);
   // in lab1, PKE considers only one app (one process). 
   // therefore, shutdown the system when the app calls exit()
+  sprint("hartid = ?: shutdown with code:%d.\n", code);
   shutdown(code);
 }
 
diff --git a/user/app0.c b/user/app0.c
new file mode 100644
index 0000000..c1a3704
--- /dev/null
+++ b/user/app0.c
@@ -0,0 +1,7 @@
+#include "user_lib.h"
+#include "util/types.h"
+
+int main(void) {
+  printu(">>> app0 is expected to be executed by hart0\n");
+  exit(0);
+}
\ No newline at end of file
diff --git a/user/app1.c b/user/app1.c
new file mode 100644
index 0000000..89578f9
--- /dev/null
+++ b/user/app1.c
@@ -0,0 +1,7 @@
+#include "user_lib.h"
+#include "util/types.h"
+
+int main(void) {
+  printu(">>> app1 is expected to be executed by hart1\n");
+  exit(0);
+}
\ No newline at end of file
diff --git a/user/app_long_loop.c b/user/app_long_loop.c
deleted file mode 100644
index 8b4d873..0000000
--- a/user/app_long_loop.c
+++ /dev/null
@@ -1,20 +0,0 @@
-/*
- * 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;
-}
diff --git a/user/user.lds b/user/user0.lds
similarity index 98%
rename from user/user.lds
rename to user/user0.lds
index 59a3bb8..acd0546 100644
--- a/user/user.lds
+++ b/user/user0.lds
@@ -11,4 +11,4 @@ SECTIONS
   .data : { *(.data) }
   . = ALIGN(16);
   .bss : { *(.bss) }
-}
+}
\ No newline at end of file
diff --git a/user/user1.lds b/user/user1.lds
new file mode 100644
index 0000000..71e1d27
--- /dev/null
+++ b/user/user1.lds
@@ -0,0 +1,14 @@
+OUTPUT_ARCH( "riscv" )
+
+ENTRY(main)
+
+SECTIONS
+{
+  . = 0x85000000;
+  . = ALIGN(0x1000);
+  .text : { *(.text) }
+  . = ALIGN(16);
+  .data : { *(.data) }
+  . = ALIGN(16);
+  .bss : { *(.bss) }
+}
\ No newline at end of file