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diff --git a/kern/ast.h b/kern/ast.h
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+/*
+ * Mach Operating System
+ * Copyright (c) 1991,1990,1989 Carnegie Mellon University.
+ * Copyright (c) 1993,1994 The University of Utah and
+ * the Computer Systems Laboratory (CSL).
+ * 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, THE UNIVERSITY OF UTAH AND CSL ALLOW FREE USE OF
+ * THIS SOFTWARE IN ITS "AS IS" CONDITION, AND DISCLAIM 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.
+ */
+/*
+ *	kern/ast.h: Definitions for Asynchronous System Traps.
+ */
+
+#ifndef	_KERN_AST_H_
+#define _KERN_AST_H_
+
+/*
+ *	A CPU takes an AST when it is about to return to user code.
+ *	Instead of going back to user code, it calls ast_taken.
+ *	Machine-dependent code is responsible for maintaining
+ *	a set of reasons for an AST, and passing this set to ast_taken.
+ */
+
+#include <cpus.h>
+
+#include "cpu_number.h"
+#include <kern/macro_help.h>
+#include <machine/ast.h>
+
+/*
+ *	Bits for reasons
+ */
+
+#define	AST_ZILCH	0x0
+#define AST_HALT	0x1
+#define AST_TERMINATE	0x2
+#define AST_BLOCK	0x4
+#define AST_NETWORK	0x8
+#define AST_NETIPC	0x10
+
+#define	AST_SCHEDULING	(AST_HALT|AST_TERMINATE|AST_BLOCK)
+
+/*
+ * Per-thread ASTs are reset at context-switch time.
+ * machine/ast.h can define MACHINE_AST_PER_THREAD.
+ */
+
+#ifndef	MACHINE_AST_PER_THREAD
+#define	MACHINE_AST_PER_THREAD	0
+#endif
+
+#define AST_PER_THREAD  (AST_HALT | AST_TERMINATE | MACHINE_AST_PER_THREAD)
+
+typedef unsigned int ast_t;
+
+extern volatile ast_t need_ast[NCPUS];
+
+#ifdef	MACHINE_AST
+/*
+ *	machine/ast.h is responsible for defining aston and astoff.
+ */
+#else	MACHINE_AST
+
+#define aston(mycpu)
+#define astoff(mycpu)
+
+#endif	MACHINE_AST
+
+extern void ast_taken();
+
+/*
+ *	ast_needed, ast_on, ast_off, ast_context, and ast_propagate
+ *	assume splsched.  mycpu is always cpu_number().  It is an
+ *	argument in case cpu_number() is expensive.
+ */
+
+#define ast_needed(mycpu)		need_ast[mycpu]
+
+#define ast_on(mycpu, reasons)						\
+MACRO_BEGIN								\
+	if ((need_ast[mycpu] |= (reasons)) != AST_ZILCH)		\
+		{ aston(mycpu); }					\
+MACRO_END
+
+#define ast_off(mycpu, reasons)						\
+MACRO_BEGIN								\
+	if ((need_ast[mycpu] &= ~(reasons)) == AST_ZILCH)		\
+		{ astoff(mycpu); } 					\
+MACRO_END
+
+#define ast_propagate(thread, mycpu)	ast_on((mycpu), (thread)->ast)
+
+#define ast_context(thread, mycpu)					\
+MACRO_BEGIN								\
+	if ((need_ast[mycpu] =						\
+	     (need_ast[mycpu] &~ AST_PER_THREAD) | (thread)->ast)	\
+					!= AST_ZILCH)			\
+		{ aston(mycpu);	}					\
+	else								\
+		{ astoff(mycpu); }					\
+MACRO_END
+
+
+#define	thread_ast_set(thread, reason)		(thread)->ast |= (reason)
+#define thread_ast_clear(thread, reason)	(thread)->ast &= ~(reason)
+#define thread_ast_clear_all(thread)		(thread)->ast = AST_ZILCH
+
+/*
+ *	NOTE: if thread is the current thread, thread_ast_set should
+ *	be followed by ast_propagate().
+ */
+
+#endif	_KERN_AST_H_