diff --git a/book/intro/chapter.tex b/book/intro/chapter.tex
index da49839..9b48443 100644
--- a/book/intro/chapter.tex
+++ b/book/intro/chapter.tex
@@ -143,7 +143,7 @@ to be simplified.  Thus reducing its complexity, power consumption and cost.}
 The \reg{pc} register is called the {\em program counter}.  The CPU uses it to
 remember the memory address where its program instructions are located.
 
-The term XLEN refer to the width of an integer register in bits (either 32 or 64).
+The term XLEN refer to the width of an integer register in bits (either 32, 64, or 128.)
 The number of bits in each register is defined by the \acrfull{isa}.
 
 \subsubsection{Harts}