diff --git a/tools/as b/tools/as
index 9779a6c..e927b77 100755
--- a/tools/as
+++ b/tools/as
@@ -3,9 +3,27 @@
 # use the v7 assembler to assemble some userland code into an 0405 binary.
 # Requires the first .s file to set ". = . + 40014".
 #
+# example, mount.s:
+#   exit  = 1.
+#   write = 4.
+#   mount = 21.
+#   . = . + 40014
+#   
+#        mov     $1,r0
+#        sys     write; rk0; 8.
+#        sys     write; usr; 4.
+#        sys mount; rk0; usr
+#        sys exit
+#        sys exit
+#   
+#   rk0:    </dev/rk0\0>
+#   usr:    </usr\0>
+#
+# $ tools/as mount.s
+# $ cp b.out mymount
+#
 
 . tools/assemv7.cfg
 export APOUT_ROOT
 $APOUT $APOUT_ROOT/bin/as "$@" && tools/fixaout.py
 
-