diff --git a/src/arch/x86/isa/bitfields.isa b/src/arch/x86/isa/bitfields.isa
index 43fee5fef2..fb082f683c 100644
--- a/src/arch/x86/isa/bitfields.isa
+++ b/src/arch/x86/isa/bitfields.isa
@@ -60,6 +60,7 @@
 
 //REX prefix
 def bitfield REX rex;
+def bitfield REX_PRESENT rex.present;
 def bitfield REX_W rex.w;
 def bitfield REX_R rex.r;
 def bitfield REX_X rex.x;