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+/*  This file is part of the program psim.
+
+    Copyright (C) 1994-1997, Andrew Cagney <cagney@highland.com.au>
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+ 
+    You should have received a copy of the GNU General Public License
+    along with this program; if not, write to the Free Software
+    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ 
+    */
+
+
+#ifndef _EMUL_CHIRP_H_
+#define _EMUL_CHIRP_H_
+
+/* EMUL_CHIRP:
+
+   The emulation of the OpenBoot client interface (as defined in 1275)
+   illustrates how it is possible for PSIM to implement an interface
+   that is both running in virtual memory and is called using a
+   standard function call interface.
+
+   The OpenBoot client interface is implemented by using two
+   instructions:
+
+   	client_interface:
+		<emul_call>
+		blr
+
+   A client program makes a function call to `client_interface' using
+   the `bl' instruction.  The simulator will then execute the
+   <emul_call> instruction (which calls emul_chirp) and then the `blr'
+   which will return to the caller.
+
+   In addition to providing the `client_interface' entry point, while
+   a client request is being handled, emul_chirp patches (well it will
+   one day) the data access exception vector with a <emul_call>
+   instruction.  By doing this, emul_chirp is able to catch and handle
+   any invalid data accesses it makes while emulating a client call.
+
+   When such an exception occures, emul_chirp is able to recover by
+   restoring the processor and then calling the clients callback
+   interface so that the client can recover from the data exception.
+
+   Handling this are the emul_chirp states:
+
+                              serving---.
+                             /          |
+   Emulation compleated     ^           v Client makes call to
+     - restore int vectors  |           | emulated interface
+                            ^           v   - patch exception vectors
+                            |          /
+                            `-emulating-. emulating the request
+                             /          |
+                            |           v Emulation encounters
+   Client callback recovers ^           | data access exception
+   from data exception and  |           v   - re-enable vm
+   returns.                 ^           |   - call client callback
+     - restart request      |          /
+                            `--faulting
+   */
+
+
+extern const os_emul emul_chirp;
+
+#endif