diff --git a/src/dev/x86/cmos.cc b/src/dev/x86/cmos.cc
index 19fab68078..d1419221c3 100644
--- a/src/dev/x86/cmos.cc
+++ b/src/dev/x86/cmos.cc
@@ -56,7 +56,7 @@ X86ISA::Cmos::read(PacketPtr pkt)
         pkt->setLE(address);
         break;
       case 0x1:
-        pkt->setLE(readRegister(address));
+        pkt->setLE(readRegister(address.regNum));
         break;
       default:
         panic("Read from undefined CMOS port.\n");
@@ -75,7 +75,8 @@ X86ISA::Cmos::write(PacketPtr pkt)
         address = pkt->getLE<uint8_t>();
         break;
       case 0x1:
-        writeRegister(address, pkt->getLE<uint8_t>());
+        // Ignore the NMI mask bit since we never try to generate one anyway.
+        writeRegister(address.regNum, pkt->getLE<uint8_t>());
         break;
       default:
         panic("Write to undefined CMOS port.\n");
diff --git a/src/dev/x86/cmos.hh b/src/dev/x86/cmos.hh
index 72937af372..de64d5acd4 100644
--- a/src/dev/x86/cmos.hh
+++ b/src/dev/x86/cmos.hh
@@ -29,6 +29,7 @@
 #ifndef __DEV_X86_CMOS_HH__
 #define __DEV_X86_CMOS_HH__
 
+#include "base/bitunion.hh"
 #include "dev/intpin.hh"
 #include "dev/io_device.hh"
 #include "dev/mc146818.hh"
@@ -45,7 +46,12 @@ class Cmos : public BasicPioDevice
   protected:
     Tick latency;
 
-    uint8_t address;
+    BitUnion8(CmosAddress)
+        Bitfield<6, 0> regNum;
+        Bitfield<7> nmiMask;
+    EndBitUnion(CmosAddress)
+
+    CmosAddress address;
 
     static const int numRegs = 128;