diff --git a/code/os/05-traps/start.S b/code/os/05-traps/start.S
index c7ad5e4fa7cf5d5055508a12a385d5571dae3f96..7c19fa8adb6078030d98f30964ecf74ef9b4dd73 100644
--- a/code/os/05-traps/start.S
+++ b/code/os/05-traps/start.S
@@ -30,12 +30,18 @@ _start:
 	add	sp, sp, t0		# move the current hart stack pointer
 					# to its place in the stack space
 
+	la	t0, trap_stack
+	csrw	mscratch, t0
+
 	j	start_kernel		# hart 0 jump to c
 
 park:
 	wfi
 	j	park
 
+trap_stack:
+	.skip	STACK_SIZE
+
 stacks:
 	.skip	STACK_SIZE * MAXNUM_CPU # allocate space for all the harts stacks
 
diff --git a/code/os/06-interrupts/start.S b/code/os/06-interrupts/start.S
index c7ad5e4fa7cf5d5055508a12a385d5571dae3f96..7c19fa8adb6078030d98f30964ecf74ef9b4dd73 100644
--- a/code/os/06-interrupts/start.S
+++ b/code/os/06-interrupts/start.S
@@ -30,12 +30,18 @@ _start:
 	add	sp, sp, t0		# move the current hart stack pointer
 					# to its place in the stack space
 
+	la	t0, trap_stack
+	csrw	mscratch, t0
+
 	j	start_kernel		# hart 0 jump to c
 
 park:
 	wfi
 	j	park
 
+trap_stack:
+	.skip	STACK_SIZE
+
 stacks:
 	.skip	STACK_SIZE * MAXNUM_CPU # allocate space for all the harts stacks
 
diff --git a/code/os/07-hwtimer/start.S b/code/os/07-hwtimer/start.S
index c7ad5e4fa7cf5d5055508a12a385d5571dae3f96..7c19fa8adb6078030d98f30964ecf74ef9b4dd73 100644
--- a/code/os/07-hwtimer/start.S
+++ b/code/os/07-hwtimer/start.S
@@ -30,12 +30,18 @@ _start:
 	add	sp, sp, t0		# move the current hart stack pointer
 					# to its place in the stack space
 
+	la	t0, trap_stack
+	csrw	mscratch, t0
+
 	j	start_kernel		# hart 0 jump to c
 
 park:
 	wfi
 	j	park
 
+trap_stack:
+	.skip	STACK_SIZE
+
 stacks:
 	.skip	STACK_SIZE * MAXNUM_CPU # allocate space for all the harts stacks
 
diff --git a/code/os/08-preemptive/start.S b/code/os/08-preemptive/start.S
index 0b730a9fe88869d4424e292f34584934d67a3b59..e127424fdaf974f9e6e705b879e9d21ceb1b0ffc 100644
--- a/code/os/08-preemptive/start.S
+++ b/code/os/08-preemptive/start.S
@@ -40,12 +40,18 @@ _start:
 	or	t0, t0, a1
 	csrw	mstatus, t0
 
+	la	t0, trap_stack
+	csrw	mscratch, t0
+
 	j	start_kernel		# hart 0 jump to c
 
 park:
 	wfi
 	j	park
 
+trap_stack:
+	.skip	STACK_SIZE
+
 stacks:
 	.skip	STACK_SIZE * MAXNUM_CPU # allocate space for all the harts stacks
 
diff --git a/code/os/09-lock/start.S b/code/os/09-lock/start.S
index 0b730a9fe88869d4424e292f34584934d67a3b59..e127424fdaf974f9e6e705b879e9d21ceb1b0ffc 100644
--- a/code/os/09-lock/start.S
+++ b/code/os/09-lock/start.S
@@ -40,12 +40,18 @@ _start:
 	or	t0, t0, a1
 	csrw	mstatus, t0
 
+	la	t0, trap_stack
+	csrw	mscratch, t0
+
 	j	start_kernel		# hart 0 jump to c
 
 park:
 	wfi
 	j	park
 
+trap_stack:
+	.skip	STACK_SIZE
+
 stacks:
 	.skip	STACK_SIZE * MAXNUM_CPU # allocate space for all the harts stacks
 
diff --git a/code/os/10-swtimer/start.S b/code/os/10-swtimer/start.S
index 0b730a9fe88869d4424e292f34584934d67a3b59..9a9e5fcbf62ba4a5c06d2783e8c9be011e317c4b 100644
--- a/code/os/10-swtimer/start.S
+++ b/code/os/10-swtimer/start.S
@@ -40,8 +40,14 @@ _start:
 	or	t0, t0, a1
 	csrw	mstatus, t0
 
+	la	t0, trap_stack
+	csrw	mscratch, t0
+
 	j	start_kernel		# hart 0 jump to c
 
+trap_stack:
+	.skip	STACK_SIZE
+
 park:
 	wfi
 	j	park
diff --git a/code/os/11-syscall/start.S b/code/os/11-syscall/start.S
index ba34f12c77b58610b8dd7a0707e4a099af4d71ba..d595d7c1394b579718181dbc51e3c65088c3f5f6 100644
--- a/code/os/11-syscall/start.S
+++ b/code/os/11-syscall/start.S
@@ -64,12 +64,18 @@ _start:
 	or	t0, t0, a1
 	csrw	mstatus, t0
 
+	la	t0, trap_stack
+	csrw	mscratch, t0
+
 	j	start_kernel		# hart 0 jump to c
 
 park:
 	wfi
 	j	park
 
+trap_stack:
+	.skip	STACK_SIZE
+
 stacks:
 	.skip	STACK_SIZE * MAXNUM_CPU # allocate space for all the harts stacks