diff --git a/boot/chs.inc b/boot/chs.inc
index 53c5c4b..1cceb56 100644
--- a/boot/chs.inc
+++ b/boot/chs.inc
@@ -4,10 +4,10 @@ fix_chs:
   jc chs_end ; skip if function does not exist
   inc dh
   mov [cs:fdc.nos], dh
-  mov ax, cx
-  and ax, 0x003F
-  mov [cs:fdc.spt], ax ; no adjustment because sectors are 1-indexed
-  mov ax, cx
+  push cx
+  and cx, 0x003F
+  mov [cs:fdc.spt], cx ; no adjustment because sectors are 1-indexed
+  pop ax
   xchg al, ah
   mov cl,6
   shr ah,cl