diff --git a/reactos/hal/halx86/generic/acpi/halacpi.c b/reactos/hal/halx86/generic/acpi/halacpi.c
index b6b6ecaa805..4f8642edf86 100644
--- a/reactos/hal/halx86/generic/acpi/halacpi.c
+++ b/reactos/hal/halx86/generic/acpi/halacpi.c
@@ -422,7 +422,7 @@ HalpAcpiGetTable(IN PLOADER_PARAMETER_BLOCK LoaderBlock,
             else
             {
                 /* Phase 1, use Mm */
-                MmUnmapIoSpace(TableAddress, PageCount << 12);
+                MmUnmapIoSpace(TableAddress, PageCount << PAGE_SHIFT);
             }
             
             /* Cache the bios copy */