tweak

book.tex

@@ -2521,8 +2521,9 @@ to save (in the prelude) and restore (in the conclusion) any callee
 save registers that get used during register allocation. The simplest
 approach is to save and restore all the callee save registers. The
 more efficient approach is to keep track of which callee save
-registers were used and only save and restore them.
-
+registers were used and only save and restore them. Either way, make
+sure to take this use of stack space into account when you round up
+the size of the frame to make sure it is a multiple of 16 bytes.
 
 \section{Challenge: Move Biasing$^{*}$}
 \label{sec:move-biasing}