sim/ppc/emul_chirp.h

   1 /*  This file is part of the program psim.
   2
   3     Copyright (C) 1994-1997, Andrew Cagney <cagney@highland.com.au>
   4
   5     This program is free software; you can redistribute it and/or modify
   6     it under the terms of the GNU General Public License as published by
   7     the Free Software Foundation; either version 2 of the License, or
   8     (at your option) any later version.
   9
  10     This program is distributed in the hope that it will be useful,
  11     but WITHOUT ANY WARRANTY; without even the implied warranty of
  12     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13     GNU General Public License for more details.
  14
  15     You should have received a copy of the GNU General Public License
  16     along with this program; if not, write to the Free Software
  17     Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  18
  19     */
  20
  21
  22 #ifndef _EMUL_CHIRP_H_
  23 #define _EMUL_CHIRP_H_
  24
  25 /* EMUL_CHIRP:
  26
  27    The emulation of the OpenBoot client interface (as defined in 1275)
  28    illustrates how it is possible for PSIM to implement an interface
  29    that is both running in virtual memory and is called using a
  30    standard function call interface.
  31
  32    The OpenBoot client interface is implemented by using two
  33    instructions:
  34
  35         client_interface:
  36                 <emul_call>
  37                 blr
  38
  39    A client program makes a function call to `client_interface' using
  40    the `bl' instruction.  The simulator will then execute the
  41    <emul_call> instruction (which calls emul_chirp) and then the `blr'
  42    which will return to the caller.
  43
  44    In addition to providing the `client_interface' entry point, while
  45    a client request is being handled, emul_chirp patches (well it will
  46    one day) the data access exception vector with a <emul_call>
  47    instruction.  By doing this, emul_chirp is able to catch and handle
  48    any invalid data accesses it makes while emulating a client call.
  49
  50    When such an exception occures, emul_chirp is able to recover by
  51    restoring the processor and then calling the clients callback
  52    interface so that the client can recover from the data exception.
  53
  54    Handling this are the emul_chirp states:
  55
  56                               serving---.
  57                              /          |
  58    Emulation compleated     ^           v Client makes call to
  59      - restore int vectors  |           | emulated interface
  60                             ^           v   - patch exception vectors
  61                             |          /
  62                             `-emulating-. emulating the request
  63                              /          |
  64                             |           v Emulation encounters
  65    Client callback recovers ^           | data access exception
  66    from data exception and  |           v   - re-enable vm
  67    returns.                 ^           |   - call client callback
  68      - restart request      |          /
  69                             `--faulting
  70    */
  71
  72
  73 extern const os_emul emul_chirp;
  74
  75 #endif