diff --git a/devices/ioctrl/heathrow.cpp b/devices/ioctrl/heathrow.cpp
index 5c08e65..6ccb089 100644
--- a/devices/ioctrl/heathrow.cpp
+++ b/devices/ioctrl/heathrow.cpp
@@ -341,7 +341,7 @@ void HeathrowIC::mio_ctrl_write(uint32_t offset, uint32_t value, int size) {
     case MIO_OHARE_FEAT_CTRL:
         this->feature_control(BYTESWAP_32(value));
         break;
-    case 0x3C:
+    case MIO_AUX_CTRL:
         LOG_F(9, "write %x to MIO:Aux_Ctrl register", value);
         this->aux_ctrl = value;
         break;
diff --git a/devices/ioctrl/macio.h b/devices/ioctrl/macio.h
index ec9ba8c..3bde0f2 100644
--- a/devices/ioctrl/macio.h
+++ b/devices/ioctrl/macio.h
@@ -263,6 +263,7 @@ private:
 enum {
     MIO_OHARE_ID        = 0x34, // IDs register
     MIO_OHARE_FEAT_CTRL = 0x38, // feature control register
+    MIO_AUX_CTRL        = 0x3C,
 };
 
 /** O'Hare/Heathrow DBDMA channels. */