diff --git a/example-os/Makefile b/example-os/Makefile
index 15516ab..0a4b3ea 100644
--- a/example-os/Makefile
+++ b/example-os/Makefile
@@ -8,8 +8,9 @@ all: $(os_filename) os.ndisasm.disasm os.reasm.asm os.reasm .gitignore test Make
 	mkdir -p $@
 
 $(os_filename): os.asm ../deploy-screenshots Makefile
+	rm -f $@
 	nasm -o $@ $<
-	chmod a+x $@
+	chmod a+x-w $@
 
 os.ndisasm.disasm: $(os_filename) Makefile
 	../utils/compact-ndisasm.sh $< $@
diff --git a/test/qemu-system-i386.sh b/test/qemu-system-i386.sh
index 14f02ca..9b0e7b6 100755
--- a/test/qemu-system-i386.sh
+++ b/test/qemu-system-i386.sh
@@ -6,7 +6,7 @@ if test $# -ne 1 || test "$1" = '-h' -o "$1" = '--help'; then
 fi
 os_filename="$1"
 
-qemu-system-i386 -drive format=raw,file=${os_filename},index=0,if=floppy &
+qemu-system-i386 -drive format=raw,readonly,file=${os_filename},index=0,if=floppy &
 pid=$!
 runsikulix -r test/check-gradient.sikuli && exitcode=$? || exitcode=$?