diff --git a/reactos/hal/halx86/mca.c b/reactos/hal/halx86/mca.c
index 97ff15addf6..e19c93eb5a0 100644
--- a/reactos/hal/halx86/mca.c
+++ b/reactos/hal/halx86/mca.c
@@ -16,7 +16,7 @@
  *  along with this program; if not, write to the Free Software
  *  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
-/* $Id: mca.c,v 1.1 2002/10/02 19:32:57 ekohl Exp $
+/* $Id: mca.c,v 1.2 2002/10/03 09:11:00 ekohl Exp $
  *
  * COPYRIGHT:   See COPYING in the top level directory
  * PROJECT:     ReactOS kernel
@@ -25,6 +25,11 @@
  * PROGRAMMER:  Eric Kohl (ekohl@rz-online.de)
  */
 
+/*
+ * TODO:
+ *   What Adapter ID is read from an empty slot?
+ */
+
 /* INCLUDES *****************************************************************/
 
 #include <ddk/ntddk.h>
@@ -52,11 +57,15 @@ HalpGetMicroChannelData(PBUS_HANDLER BusHandler,
   DPRINT("  Offset 0x%lx\n", Offset);
   DPRINT("  Length 0x%lx\n", Length);
 
-  if ((BusNumber != 0) || (SlotNumber > 7) || (Length < sizeof(CM_MCA_POS_DATA)))
+  if ((BusNumber != 0) ||
+      (SlotNumber == 0) || (SlotNumber > 8) ||
+      (Length < sizeof(CM_MCA_POS_DATA)))
     return(0);
 
-  WRITE_PORT_UCHAR((PUCHAR)0x96, ((UCHAR)SlotNumber & 0x07) | 0x08);
+  /* Enter Setup-Mode for given slot */
+  WRITE_PORT_UCHAR((PUCHAR)0x96, ((UCHAR)(SlotNumber - 1) & 0x07) | 0x08);
 
+  /* Read POS data */
   PosData->AdapterId = (READ_PORT_UCHAR((PUCHAR)0x101) << 8) +
 			READ_PORT_UCHAR((PUCHAR)0x100);
   PosData->PosData1 = READ_PORT_UCHAR((PUCHAR)0x102);
@@ -64,6 +73,9 @@ HalpGetMicroChannelData(PBUS_HANDLER BusHandler,
   PosData->PosData3 = READ_PORT_UCHAR((PUCHAR)0x104);
   PosData->PosData4 = READ_PORT_UCHAR((PUCHAR)0x105);
 
+  /* Leave Setup-Mode for given slot */
+  WRITE_PORT_UCHAR((PUCHAR)0x96, (UCHAR)(SlotNumber - 1) & 0x07);
+
   return(sizeof(CM_MCA_POS_DATA));
 }