footnote about hardware caches

book.tex

@@ -1022,16 +1022,22 @@ language to compile $R_1$.
 \section{The x86 Assembly Language}
 \label{sec:x86}
 
-An x86 program is a sequence of instructions. The program is stored in
-the computer's memory and the \emph{program counter} points to the
-address of the next instruction to be executed. For most instructions,
-once the instruction is executed, the program counter is incremented
-to point to the immediately following instruction in the program.
-Each instruction may refer to integer constants (called
-\emph{immediate values}), variables called \emph{registers}, and
-instructions may load and store values into memory.  For our purposes,
-we can think of the computer's memory as a mapping of 64-bit addresses
-to 64-bit values. Figure~\ref{fig:x86-a} defines the syntax for the
+An x86 program is a sequence of instructions. The program is stored in the
+computer's memory and the \emph{program counter} points to the address of the
+next instruction to be executed. For most instructions, once the instruction is
+executed, the program counter is incremented to point to the immediately
+following instruction in the program.  Each instruction may refer to integer
+constants (called \emph{immediate values}), variables called \emph{registers},
+and instructions may load and store values into memory.  For our purposes, we
+can think of the computer's memory as a mapping of 64-bit addresses to 64-bit
+%
+values\footnote{This simple story doesn't fully cover contemporary x86
+  processors, which combine multiple processing cores per silicon chip, together
+  with hardware memory caches.  The result is that, at some instants in real
+  time, different programs may hold conflicting cached values for a given memory
+  address.}.
+%
+Figure~\ref{fig:x86-a} defines the syntax for the
 subset of the x86 assembly language needed for this chapter.  (We use
 the AT\&T syntax expected by the GNU assembler inside \key{gcc}.)
 Also, Appendix~\ref{sec:x86-quick-reference} includes a