diff --git a/boot/startup32.S b/boot/startup32.S
index 801096d..38e95f7 100644
--- a/boot/startup32.S
+++ b/boot/startup32.S
@@ -80,7 +80,7 @@ startup:
 	# need to take the mutex to protect our use of the stack.
 
 	leal	(startup_stack_mutex - startup)(%edi), %eax
-0: lock bts	$0, (%eax)
+0: lock btsl	$0, (%eax)
 	jc	0b
 
 	leal	(startup_stack_top - startup)(%edi), %esp
diff --git a/boot/startup64.S b/boot/startup64.S
index b721c2c..0019354 100644
--- a/boot/startup64.S
+++ b/boot/startup64.S
@@ -145,7 +145,7 @@ startup:
 	# Use the startup stack until we pick the correct one. We
 	# need to take a mutex to protect our use of the stack.
 
-0: lock bts	$0, startup_stack_mutex(%rip)
+0: lock btsl	$0, startup_stack_mutex(%rip)
 	jc	0b
 	leaq	startup_stack_top(%rip), %rsp