diff --git a/src/arch/x86/isa/specialize.isa b/src/arch/x86/isa/specialize.isa
index 3b4e09daa..fe3730b77 100644
--- a/src/arch/x86/isa/specialize.isa
+++ b/src/arch/x86/isa/specialize.isa
@@ -95,6 +95,9 @@ let {{
             self.size = match.group("size")
             self.rsize = match.group("rsize")
 
+    ModRMRegIndex = "(MODRM_REG | (REX_R << 3))"
+    ModRMRMIndex = "(MODRM_RM | (REX_B << 3))"
+
     # This function specializes the given piece of code to use a particular
     # set of argument types described by "opTypes".
     def specializeInst(Name, opTypes, env):
@@ -121,14 +124,14 @@ let {{
                 raise Exception, "Problem parsing operand tag: %s" % opType.tag
             elif opType.tag in ("C", "D", "G", "P", "S", "T", "V"):
                 # Use the "reg" field of the ModRM byte to select the register
-                env.addReg("(uint8_t)MODRM_REG")
+                env.addReg(ModRMRegIndex)
             elif opType.tag in ("E", "Q", "W"):
                 # This might refer to memory or to a register. We need to
                 # divide it up farther.
                 regTypes = copy.copy(opTypes)
                 regTypes.pop(0)
                 regEnv = copy.copy(env)
-                regEnv.addReg("(uint8_t)MODRM_RM")
+                regEnv.addReg(ModRMRMIndex)
                 # This needs to refer to memory, but we'll fill in the details
                 # later. It needs to take into account unaligned memory
                 # addresses.
@@ -149,7 +152,7 @@ let {{
             elif opType.tag in ("PR", "R", "VR"):
                 # There should probably be a check here to verify that mod
                 # is equal to 11b
-                env.addReg("(uint8_t)MODRM_RM")
+                env.addReg(ModRMRMIndex)
             else:
                 raise Exception, "Unrecognized tag %s." % opType.tag
             opTypes.pop(0)