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diff --git a/vm/vm_external.h b/vm/vm_external.h
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+/*
+ * Mach Operating System
+ * Copyright (c) 1991,1990,1989 Carnegie Mellon University
+ * All Rights Reserved.
+ *
+ * Permission to use, copy, modify and distribute this software and its
+ * documentation is hereby granted, provided that both the copyright
+ * notice and this permission notice appear in all copies of the
+ * software, derivative works or modified versions, and any portions
+ * thereof, and that both notices appear in supporting documentation.
+ *
+ * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
+ * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR
+ * ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
+ *
+ * Carnegie Mellon requests users of this software to return to
+ *
+ *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
+ *  School of Computer Science
+ *  Carnegie Mellon University
+ *  Pittsburgh PA 15213-3890
+ *
+ * any improvements or extensions that they make and grant Carnegie Mellon
+ * the rights to redistribute these changes.
+ */
+
+#ifndef	_VM_VM_EXTERNAL_H_
+#define _VM_VM_EXTERNAL_H_
+
+/*
+ *	External page management hint technology
+ *
+ *	The data structure exported by this module maintains
+ *	a (potentially incomplete) map of the pages written
+ *	to external storage for a range of virtual memory.
+ */
+
+/*
+ *	The data structure representing the state of pages
+ *	on external storage.
+ */
+
+typedef struct vm_external {
+    	int		existence_size;	/* Size of the following bitmap */
+	char		*existence_map;	/* A bitmap of pages that have
+					 * been written to backing
+					 * storage.
+					 */
+#if 0
+	/* XXX: Currently, existence_count is not used.  I guess it
+	   could be useful to get rid of the map if the count drops to
+	   zero.  */
+	int		existence_count;/* Number of bits turned on in
+					 * existence_map.
+					 */
+#endif
+} *vm_external_t;
+
+#define	VM_EXTERNAL_NULL	((vm_external_t) 0)
+
+#define VM_EXTERNAL_SMALL_SIZE	128
+#define VM_EXTERNAL_LARGE_SIZE	8192
+
+/*
+ *	The states that may be recorded for a page of external storage.
+ */
+
+typedef int	vm_external_state_t;
+#define	VM_EXTERNAL_STATE_EXISTS		1
+#define	VM_EXTERNAL_STATE_UNKNOWN		2
+#define	VM_EXTERNAL_STATE_ABSENT		3
+
+
+/*
+ *	Routines exported by this module.
+ */
+
+/* Initialize the module */
+extern void		vm_external_module_initialize(void);
+/* Create a vm_external_t */
+extern vm_external_t	vm_external_create(vm_offset_t);
+/* Destroy one */
+extern void vm_external_destroy(vm_external_t);
+
+/* Set state of a page.  */
+extern void		vm_external_state_set(vm_external_t, vm_offset_t,
+					      vm_external_state_t);
+/* Retrieve the state for a given page, if known.  */
+#define	vm_external_state_get(e,offset)	(((e) != VM_EXTERNAL_NULL) ? \
+					  _vm_external_state_get(e, offset) : \
+					  VM_EXTERNAL_STATE_UNKNOWN)
+/* HIDDEN routine */
+extern vm_external_state_t _vm_external_state_get(vm_external_t, vm_offset_t);
+
+#endif	/* _VM_VM_EXTERNAL_H_ */