diff --git a/sys/dev/acpica/Osd/OsdSynch.c b/sys/dev/acpica/Osd/OsdSynch.c
index d27346784818..9c80485c7aeb 100644
--- a/sys/dev/acpica/Osd/OsdSynch.c
+++ b/sys/dev/acpica/Osd/OsdSynch.c
@@ -35,22 +35,37 @@
 
 #include "opt_acpi.h"
 #include <sys/kernel.h>
-#include <sys/lock.h>
 #include <sys/malloc.h>
-#include <sys/mutex.h>
 #include <sys/sysctl.h>
+#if __FreeBSD_version >= 500000
+#include <sys/lock.h>
+#include <sys/mutex.h>
+#endif
 
 #define _COMPONENT	ACPI_OS_SERVICES
 ACPI_MODULE_NAME("SYNCH")
 
 static MALLOC_DEFINE(M_ACPISEM, "acpisem", "ACPI semaphore");
 
+#if __FreeBSD_version < 500000
+# define AS_LOCK(as)		s = splhigh()
+# define AS_UNLOCK(as)		splx(s)
+# define AS_LOCK_DECL		int s
+# define msleep(a, b, c, d, e)	tsleep(a, c, d, e)
+#else
+# define AS_LOCK(as)		mtx_lock(&(as)->as_mtx)
+# define AS_UNLOCK(as)		mtx_unlock(&(as)->as_mtx)
+# define AS_LOCK_DECL
+#endif
+
 /*
  * Simple counting semaphore implemented using a mutex. (Subsequently used
  * in the OSI code to implement a mutex.  Go figure.)
  */
 struct acpi_semaphore {
+#if __FreeBSD_version >= 500000
     struct mtx	as_mtx;
+#endif
     UINT32	as_units;
     UINT32	as_maxunits;
     UINT32	as_pendings;
@@ -85,7 +100,9 @@ AcpiOsCreateSemaphore(UINT32 MaxUnits, UINT32 InitialUnits, ACPI_HANDLE *OutHand
 	return_ACPI_STATUS(AE_NO_MEMORY);
 
     bzero(as, sizeof(*as));
+#if __FreeBSD_version >= 500000
     mtx_init(&as->as_mtx, "ACPI semaphore", NULL, MTX_DEF);
+#endif
     as->as_units = InitialUnits;
     as->as_maxunits = MaxUnits;
     as->as_pendings = as->as_resetting = as->as_timeouts = 0;
@@ -111,7 +128,9 @@ AcpiOsDeleteSemaphore (ACPI_HANDLE Handle)
     ACPI_FUNCTION_TRACE((char *)(uintptr_t)__func__);
 
     ACPI_DEBUG_PRINT((ACPI_DB_MUTEX, "destroyed semaphore %p\n", as));
+#if __FreeBSD_version >= 500000
     mtx_destroy(&as->as_mtx);
+#endif
     free(Handle, M_ACPISEM);
     return_ACPI_STATUS(AE_OK);
 #else
@@ -132,6 +151,7 @@ AcpiOsWaitSemaphore(ACPI_HANDLE Handle, UINT32 Units, UINT32 Timeout)
     ACPI_STATUS			result;
     int				rv, tmo;
     struct timeval		timeouttv, currenttv, timelefttv;
+    AS_LOCK_DECL;
 
     ACPI_FUNCTION_TRACE((char *)(uintptr_t)__func__);
 
@@ -144,13 +164,13 @@ AcpiOsWaitSemaphore(ACPI_HANDLE Handle, UINT32 Units, UINT32 Timeout)
 #if 0
     if (as->as_units < Units && as->as_timeouts > 10) {
 	printf("%s: semaphore %p too many timeouts, resetting\n", __func__, as);
-	mtx_lock(&as->as_mtx);
+	AS_LOCK(as);
 	as->as_units = as->as_maxunits;
 	if (as->as_pendings)
 	    as->as_resetting = 1;
 	as->as_timeouts = 0;
 	wakeup(as);
-	mtx_unlock(&as->as_mtx);
+	AS_UNLOCK(as);
 	return_ACPI_STATUS(AE_TIME);
     }
 
@@ -177,7 +197,7 @@ AcpiOsWaitSemaphore(ACPI_HANDLE Handle, UINT32 Units, UINT32 Timeout)
     getmicrotime(&currenttv);
     timevaladd(&timeouttv, &currenttv);
 
-    mtx_lock(&as->as_mtx);
+    AS_LOCK(as);
     ACPI_DEBUG_PRINT((ACPI_DB_MUTEX,
 	"get %d units from semaphore %p (has %d), timeout %d\n",
 	Units, as, as->as_units, Timeout));
@@ -204,9 +224,11 @@ AcpiOsWaitSemaphore(ACPI_HANDLE Handle, UINT32 Units, UINT32 Timeout)
 	    break;
 	}
 
+#if __FreeBSD_version >= 500000
 	ACPI_DEBUG_PRINT((ACPI_DB_MUTEX,
 	    "semaphore blocked, calling msleep(%p, %p, %d, \"acsem\", %d)\n",
 	    as, &as->as_mtx, PCATCH, tmo));
+#endif
 
 	as->as_pendings++;
 
@@ -274,7 +296,7 @@ AcpiOsWaitSemaphore(ACPI_HANDLE Handle, UINT32 Units, UINT32 Timeout)
 	as->as_timeouts = 0;
     }
 
-    mtx_unlock(&as->as_mtx);
+    AS_UNLOCK(as);
 
     return_ACPI_STATUS(result);
 #else
@@ -287,13 +309,14 @@ AcpiOsSignalSemaphore(ACPI_HANDLE Handle, UINT32 Units)
 {
 #ifndef ACPI_NO_SEMAPHORES
     struct acpi_semaphore	*as = (struct acpi_semaphore *)Handle;
+    AS_LOCK_DECL;
 
     ACPI_FUNCTION_TRACE((char *)(uintptr_t)__func__);
 
     if (as == NULL)
 	return_ACPI_STATUS(AE_BAD_PARAMETER);
 
-    mtx_lock(&as->as_mtx);
+    AS_LOCK(as);
     ACPI_DEBUG_PRINT((ACPI_DB_MUTEX,
 	"return %d units to semaphore %p (has %d)\n",
 	Units, as, as->as_units));
@@ -309,7 +332,7 @@ AcpiOsSignalSemaphore(ACPI_HANDLE Handle, UINT32 Units)
     }
 
     wakeup(as);
-    mtx_unlock(&as->as_mtx);
+    AS_UNLOCK(as);
     return_ACPI_STATUS(AE_OK);
 #else
     return(AE_OK);