Added magic number in the FindFirstFile structure to allow more robust

[wine/multimedia.git] / documentation / architecture.sgml

  <chapter id="architecture">
    <title>Overview</title>
    <para>Brief overview of Wine's architecture...</para>

    <sect1 id="basic-overview">
      <title>Wine Overview</title>

      <para>
        With the fundamental architecture of Wine stabilizing, and
        people starting to think that we might soon be ready to
        actually release this thing, it may be time to take a look at
        how Wine actually works and operates.
      </para>

      <sect2>
        <title>Foreword</title>
        <para>
          Wine is often used as a recursive acronym, standing for
          "Wine Is Not an Emulator". Sometimes it is also known to be
          used for "Windows Emulator". In a way, both meanings are
          correct, only seen from different perspectives. The first
          meaning says that Wine is not a virtual machine, it does not
          emulate a CPU, and you are not supposed to install 
          Windows nor any Windows device drivers on top of it; rather,
          Wine is an implementation of the Windows API, and can be
          used as a library to port Windows applications to Unix. The
          second meaning, obviously, is that to Windows binaries
          (<filename>.exe</filename> files), Wine does look like
          Windows, and emulates its behaviour and quirks rather
          closely.
        </para>
        <note>
          <title>"Emulator"</title>
          <para>
            The "Emulator" perspective should not be thought of as if
            Wine is a typical inefficient emulation layer that means
            Wine can't be anything but slow - the faithfulness to the
            badly designed Windows API may of course impose a minor
            overhead in some cases, but this is both balanced out by
            the higher efficiency of the Unix platforms Wine runs on,
            and that other possible abstraction libraries (like Motif,
            GTK+, CORBA, etc) has a runtime overhead typically
            comparable to Wine's.
          </para>
        </note>
      </sect2>

      <sect2>
        <title>Executables</title>
        <para>
          Wine's main task is to run Windows executables under non
          Windows operating systems. It supports different types of
          executables:
          <itemizedlist>
            <listitem>
              <para>
                DOS executable. Those are even older programs, using
                the DOS format (either <filename>.com</filename> or
                <filename>.exe</filename> (the later being also called
                MZ)).
              </para>
            </listitem>
            <listitem>
              <para>
                Windows NE executable, also called 16 bit. They were
                the native processes run by Windows 2.x and 3.x. NE
                stands for New Executable &lt;g&gt;.
              </para>
            </listitem>
            <listitem>
              <para>
                Windows PE executable. These are programs were
                introduced in Windows 95 (and became the native
                formats for all later Windows version), even if 16 bit
                applications were still supported. PE stands for
                Portable Executable, in a sense where the format of
                the executable (as a file) is independent of the CPU
                (even if the content of the file - the code - is CPU
                dependent). 
              </para>
            </listitem>
            <listitem>
              <para>
                WineLib executable. These are applications, written
                using the Windows API, but compiled as a Unix
                executable. Wine provides the tools to create such
                executables.
              </para>
            </listitem>
          </itemizedlist>
        </para>
        <para>
          Let's quickly review the main differences for the supported
          executables:
          <table>
            <title>Wine executables</title>
            <tgroup cols="5" align="left" colsep="1" rowsep="1">
              <thead>
                <row>
                  <entry></entry>
                  <entry>DOS (.COM or .EXE)</entry>
                  <entry>Win16 (NE)</entry>
                  <entry>Win32 (PE)</entry>
                  <entry>WineLib</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>Multitasking</entry>
                  <entry>Only one application at a time (except for TSR)</entry>
                  <entry>Cooperative</entry>
                  <entry>Preemptive</entry>
                  <entry>Preemptive</entry>
                </row>
                <row>
                  <entry>Address space</entry>
                  <entry>
                    One MB of memory, where each application is loaded
                    and unloaded.
                  </entry>
                  <entry>
                    All 16 bit applications share a single address
                    space, protected mode.
                  </entry>
                  <entry>
                    Each application has it's own address
                    space. Requires MMU support from CPU. 
                  </entry>
                  <entry>
                    Each application has it's own address
                    space. Requires MMU support from CPU. 
                  </entry>
                </row>
                <row>
                  <entry>Windows API</entry>
                  <entry>
                    No Windows API but the DOS API (like <function>Int
                      21h</function> traps).
                  </entry>
                  <entry>
                    Will call the 16 bit Windows API.
                  </entry>
                  <entry>
                    Will call the 32 bit Windows API.
                  </entry>
                  <entry>
                    Will call the 32 bit Windows API, and possibly
                    also the Unix APIs.
                  </entry>
                </row>
                <row>
                  <entry>Code (CPU level)</entry>
                  <entry>
                    Only available on x86 in real mode. Code and data
                    are in segmented forms, with 16 bit
                    offsets. Processor is in real mode.
                  </entry>
                  <entry>
                    Only available on IA-32 architectures, code and
                    data are in segmented forms, with 16 bit offsets
                    (hence the 16 bit name). Processor is in protected
                    mode.
                  </entry>
                  <entry>
                    Available (with NT) on several CPUs, including
                    IA-32. On this CPU, uses a flat memory model with
                    32 bit offsets (hence the 32 bit name).
                  </entry>
                  <entry>
                    Flat model, with 32 bit addresses.
                  </entry>
                </row>
                <row>
                  <entry>Multi-threading</entry>
                  <entry>Not available.</entry>
                  <entry>Not available.</entry>
                  <entry>
                    Available.
                  </entry>
                  <entry>
                    Available, but must use the Win32 APIs for
                    threading and synchronization, not the Unix ones.
                  </entry>
                </row>
              </tbody>
            </tgroup>
          </table>
        </para>

        <para>
          Wine deals with this issue by launching a separate Wine
          process (which is in fact a Unix process) for each Win32
          process, but not for Win16 tasks. Win16 tasks (as well as
          DOS programs) are run as different intersynchronized
          Unix-threads in the same dedicated Wine process; this Wine
          process is commonly known as a <firstterm>WOW</firstterm>
          process (Windows on Windows), referring to a similar
          mechanism used by Windows NT.
        </para>
        <para>
          Synchronization between the Win16 tasks running in the WOW
          process is normally done through the Win16 mutex - whenever
          one of them is running, it holds the Win16 mutex, keeping
          the others from running. When the task wishes to let the
          other tasks run, the thread releases the Win16 mutex, and
          one of the waiting threads will then acquire it and let its
          task run.
        </para>
      </sect2>
    </sect1>

    <sect1>
      <title>Standard Windows Architectures</title>

      <sect2>
        <title>Windows 9x architecture</title>

        <para>
          The windows architecture (Win 9x way) looks like this:
          <screen>
+---------------------+        \
|     Windows EXE     |         } application
+---------------------+        /

+---------+ +---------+        \
| Windows | | Windows |         \ application & system DLLs
|   DLL   | |   DLL   |         /
+---------+ +---------+        /

+---------+ +---------+      \
|  GDI32  | | USER32  |       \
|   DLL   | |   DLL   |        \
+---------+ +---------+         } core system DLLs
+---------------------+        /
|     Kernel32 DLL    |       /
+---------------------+      / 

+---------------------+        \
|     Win9x kernel    |         } kernel space
+---------------------+        /

+---------------------+       \
|  Windows low-level  |        \ drivers (kernel space)
|       drivers       |        /
+---------------------+       /
          </screen>
        </para>
      </sect2>
      <sect2>
        <title>Windows NT architecture</title>

        <para>
          The windows architecture (Windows NT way) looks like the
          following drawing. Note the new DLL (NTDLL) which allows
          implementing different subsystems (as win32); kernel32 in NT
          architecture implements the Win32 subsystem on top of NTDLL.
          <screen>
+---------------------+                      \
|     Windows EXE     |                       } application
+---------------------+                      /

+---------+ +---------+                      \
| Windows | | Windows |                       \ application & system DLLs
|   DLL   | |   DLL   |                       /
+---------+ +---------+                      /

+---------+ +---------+     +-----------+   \
|  GDI32  | |  USER32 |     |           |    \
|   DLL   | |   DLL   |     |           |     \
+---------+ +---------+     |           |      \ core system DLLs
+---------------------+     |           |      / (on the left side)
|    Kernel32 DLL     |     | Subsystem |     /
|  (Win32 subsystem)  |     |Posix, OS/2|    /
+---------------------+     +-----------+   / 

+---------------------------------------+
|               NTDLL.DLL               |
+---------------------------------------+

+---------------------------------------+     \
|               NT kernel               |      } NT kernel (kernel space)
+---------------------------------------+     /
+---------------------------------------+     \
|       Windows low-level drivers       |      } drivers (kernel space)
+---------------------------------------+     /
          </screen>
        </para>
        <para>
          Note also (not depicted in schema above) that the 16 bit
          applications are supported in a specific subsystem.
          Some basic differences between the Win9x and the NT
          architectures include:
          <itemizedlist>
            <listitem>
              <para>
                Several subsystems (Win32, Posix...) can be run on NT,
                while not on Win 9x
              </para>
            </listitem>
            <listitem>
              <para>
                Win 9x roots its architecture in 16 bit systems, while
                NT is truely a 32 bit system.
              </para>
            </listitem>
            <listitem>
              <para>
                The drivers model and interfaces in Win 9x and NT are
                different (even if Microsoft tried to bridge the gap
                with some support of WDM drivers in Win 98 and above).
              </para>
            </listitem>
          </itemizedlist>
        </para>
      </sect2>
    </sect1>

    <sect1>
      <title>Wine architecture</title>

      <sect2>
        <title>Global picture</title>

        <para>
          Wine implementation is closer to the Windows NT
          architecture, even if several subsystems are not implemented
          yet (remind also that 16bit support is implemented in a 32-bit
          Windows EXE, not as a subsystem). Here's the overall picture:
          <screen>
+---------------------+                                  \
|     Windows EXE     |                                   } application
+---------------------+                                  /

+---------+ +---------+                                  \
| Windows | | Windows |                                   \ application & system DLLs
|   DLL   | |   DLL   |                                   /
+---------+ +---------+                                  /

+---------+ +---------+     +-----------+  +--------+  \
|  GDI32  | |  USER32 |     |           |  |        |   \
|   DLL   | |   DLL   |     |           |  |  Wine  |    \
+---------+ +---------+     |           |  | Server |     \ core system DLLs
+---------------------+     |           |  |        |     / (on the left side)
|    Kernel32 DLL     |     | Subsystem |  | NT-like|    /
|  (Win32 subsystem)  |     |Posix, OS/2|  | Kernel |   /
+---------------------+     +-----------+  |        |  / 
                                           |        |
+---------------------------------------+  |        |
|                 NTDLL                 |  |        |
+---------------------------------------+  +--------+

+---------------------------------------+               \
|   Wine executable (wine-?thread)      |                } unix executable
+---------------------------------------+               /
+---------------------------------------------------+   \
|                   Wine drivers                    |    } Wine specific DLLs
+---------------------------------------------------+   /

+------------+    +------------+     +--------------+   \
|    libc    |    |   libX11   |     |  other libs  |    } unix shared libraries
+------------+    +------------+     +--------------+   /  (user space)

+---------------------------------------------------+   \
|         Unix kernel (Linux,*BSD,Solaris,OS/X)     |    } (Unix) kernel space
+---------------------------------------------------+   /
+---------------------------------------------------+   \
|                 Unix device drivers               |    } Unix drivers (kernel space)
+---------------------------------------------------+   /
          </screen>
        </para>

        <para>
          Wine must at least completely replace the "Big Three" DLLs
          (KERNEL/KERNEL32, GDI/GDI32, and USER/USER32), which all
          other DLLs are layered on top of. But since Wine is (for
          various reasons) leaning towards the NT way of implementing
          things, the NTDLL is another core DLL to be implemented in
          Wine, and many KERNEL32 and ADVAPI32 features will be
          implemented through the NTDLL.
        </para>
        <para>
          As of today, no real subsystem (apart the Win32 one) has
          been implemented in Wine. 
        </para>
        <para>
          The Wine server provides the backbone for the implementation
          of the core DLLs. It mainly implementents inter-process
          synchronization and object sharing. It can be seen, from a
          functional point of view, as a NT kernel (even if the APIs
          and protocols used between Wine's DLL and the Wine server
          are Wine specific).
        </para>
        <para>
          Wine uses the Unix drivers to access the various hardware
          pieces on the box. However, in some cases, Wine will
          provide a driver (in Windows sense) to a physical hardware
          device. This driver will be a proxy to the Unix driver
          (this is the case, for example, for the graphical part
          with X11 or SDL drivers, audio with OSS or ALSA drivers...).
        </para>
        <para>
          All DLLs provided by Wine try to stick as much as possible
          to the exported APIs from the Windows platforms. There are
          rare cases where this is not the case, and have been
          propertly documented (Wine DLLs export some Wine specific
          APIs). Usually, those are prefixed with
          <function>__wine</function>.
        </para>
        <para>
          Let's now review in greater details all of those components.
        </para>
      </sect2>

      <sect2>
        <title>The Wine server</title>
        <para>
          The Wine server is among the most confusing concepts in
          Wine. What is its function in Wine? Well, to be brief, it
          provides Inter-Process Communication (IPC),
          synchronization, and process/thread management. When the
          Wine server launches, it creates a Unix socket for the
          current host based on (see below) your home directory's
          <filename>.wine</filename> subdirectory (or wherever the
          <constant>WINEPREFIX</constant> environment variable
          points to) - all Wine processes launched later connects to
          the Wine server using this socket. (If a Wine server was
          not already running, the first Wine process will start up
          the Wine server in auto-terminate mode (i.e. the Wine
          server will then terminate itself once the last Wine
          process has terminated).)
        </para>
        <para>
          In earlier versions of Wine the master socket mentioned
          above was actually created in the configuration directory;
          either your home directory's <filename>/wine</filename>
          subdirectory or wherever the
          <constant>WINEPREFIX</constant> environment variable
          points>. Since that might not be possible the socket is
          actually created within the <filename>/tmp</filename>
          directory with a name that reflects the configuration
          directory. This means that there can actually be several
          separate copies of the Wine server running; one per
          combination of user and configuration directory. Note that
          you should not have several users using the same
          configuration directory at the same time; they will have
          different copies of the Wine server running and this could
          well lead to problems with the registry information that
          they are sharing.
        </para>
        <para>
          Every thread in each Wine process has its own request
          buffer, which is shared with the Wine server. When a
          thread needs to synchronize or communicate with any other
          thread or process, it fills out its request buffer, then
          writes a command code through the socket. The Wine server
          handles the command as appropriate, while the client
          thread waits for a reply. In some cases, like with the
          various <function>WaitFor???</function> synchronization
          primitives, the server handles it by marking the client
          thread as waiting and does not send it a reply before the
          wait condition has been satisfied.
        </para>
        <para>
          The Wine server itself is a single and separate Unix
          process and does not have its own threading - instead, it
          is built on top of a large <function>poll()</function>
          loop that alerts the Wine server whenever anything
          happens, such as a client having sent a command, or a wait
          condition having been satisfied. There is thus no danger
          of race conditions inside the Wine server itself - it is
          often called upon to do operations that look completely
          atomic to its clients.
        </para>
        <para>
          Because the Wine server needs to manage processes,
          threads, shared handles, synchronization, and any related
          issues, all the clients' Win32 objects are also managed by
          the Wine server, and the clients must send requests to the
          Wine server whenever they need to know any Win32 object
          handle's associated Unix file descriptor (in which case
          the Wine server duplicates the file descriptor, transmits
          it back to the client, and leaves it to the client to
          close the duplicate when the client has finished with
          it).
        </para>
      </sect2>

      <sect2>
        <title>
          Wine builtin DLLs: about Relays, Thunks, and DLL
          descriptors
        </title>
        <para>
          This section mainly applies to builtin DLLs (DLLs provided
          by Wine). See section <xref linkend="arch-dlls"> for the
          details on native vs. builtin DLL handling.
        </para>
        <para>
          Loading a Windows binary into memory isn't that hard by
          itself, the hard part is all those various DLLs and entry
          points it imports and expects to be there and function as
          expected; this is, obviously, what the entire Wine
          implementation is all about. Wine contains a range of DLL
          implementations. You can find the DLLs implementation in the
          <filename>dlls/</filename> directory. 
        </para>
        <para>
          Each DLL (at least, the 32 bit version, see below) is
          implemented in a Unix shared library. The file name of this
          shared library is the module name of the DLL with a
          <filename>.dll.so</filename> suffix (or
          <filename>.drv.so</filename> or any other relevant extension
          depending on the DLL type). This shared library contains the
          code itself for the DLL, as well as some more information,
          as the DLL resources and a Wine specific DLL descriptor.
        </para>
        <para>
          The DLL descriptor, when the DLL is instanciated, is used to
          create an in-memory PE header, which will provide access to
          various information about the DLL, including but not limited
          to its entry point, its resources, its sections, its debug
          information...
        </para>
        <para>
          The DLL descriptor and entry point table is generated by
          the <command>winebuild</command> tool (previously just
          named <command>build</command>), taking DLL specification
          files with the extension <filename>.spec</filename> as
          input. Resources (after compilation by
          <command>wrc</command>) or message tables (after
          compilation by <command>wmc</command>) are also added to
          the descriptor by <command>winebuild</command>.
        </para>
        <para>
          Once an application module wants to import a DLL, Wine
          will look at:
          <itemizedlist>
            <listitem>
              <para>
                through its list of registered DLLs (in fact, both
                the already loaded DLLs, and the already loaded
                shared libraries which has registered a DLL
                descriptor). Since, the DLL descriptor is
                automatically registered when the shared library is
                loaded - remember, registration call is put inside a
                shared library constructor - using the
                <constant>PRELOAD</constant> environment variable
                when running a Wine process can force the
                registration of some DLL descriptors.
              </para>
            </listitem>
            <listitem>
              <para>
                If it's not registered, Wine will look for it on
                disk, building the shared library name from the DLL
                module name. Directory searched for are specified by
                the <constant>WINEDLLPATH</constant> environment
                variable.
              </para>
            </listitem>
            <listitem>
              <para>
                Failing that, it will look for a real Windows
                <filename>.DLL</filename> file to use, and look
                through its imports, etc) and use the loading of
                native DLLs.
              </para>
            </listitem>
          </itemizedlist>
        </para>
        <para>
          After the DLL has been identified (assuming it's still a
          native one), it's mapped into memory using a
          <function>dlopen()</function> call. Note, that Wine doesn't
          use the shared library mechanisms for resolving and/or
          importing functions between two shared libraries (for two
          DLLs). The shared library is only used for providing a way
          to load a piece of code on demand. This piece of code,
          thanks the DLL descriptor, will provide the same type of
          information a native DLL would. Wine can then use the same
          code for native and builtin DLL to handle imports/exports.
        </para>
        <para>
          Wine also relies on the dynamic loading features of the Unix
          shared libraries to relocate the DLLs if needed (the same
          DLL can be loaded at different address in two different
          processes, and even in two consecutive run of the same
          executable if the order of loading the DLLs differ). 
        </para>
        <para>
          The DLL descriptor is registered in the Wine realm using
          some tricks. The <command>winebuild</command> tool, while
          creating the code for DLL descriptor, also creates a
          constructor, that will be called when the shared library is
          loaded into memory. This constructor will actually register
          the descriptor to the Wine DLL loader. Hence, before the
          <function>dlopen</function> call returns, the DLL descriptor
          will be known and registered. This also helps to deal with
          the cases where there's still dependencies (at the ELF
          shared lib level, not at the embedded DLL level) between
          different shared libraries: the embedded DLLs will be
          properly registered, and even loaded (from a Windows point
          of view).
        </para>
        <para>
          Since Wine is 32-bit code itself, and if the compiler
          supports Windows' calling convention, <type>stdcall</type>
          (<command>gcc</command> does), Wine can resolve imports
          into Win32 code by substituting the addresses of the Wine
          handlers directly without any thunking layer in
          between. This eliminates the overhead most people
          associate with "emulation", and is what the applications
          expect anyway.
        </para>
        <para>
          However, if the user specified <parameter>WINEDEBUG=+relay
          </parameter>, a thunk layer is inserted between the
          application imports and the Wine handlers (actually the
          export table of the DLL is modified, and a thunk is
          inserted in the table); this layer is known as "relay"
          because all it does is print out the arguments/return
          values (by using the argument lists in the DLL
          descriptor's entry point table), then pass the call on,
          but it's invaluable for debugging misbehaving calls into
          Wine code. A similar mechanism also exists between Windows
          DLLs - Wine can optionally insert thunk layers between
          them, by using <parameter>WINEDEBUG=+snoop</parameter>,
          but since no DLL descriptor information exists for
          non-Wine DLLs, this is less reliable and may lead to
          crashes.
        </para>
        <para>
          For Win16 code, there is no way around thunking - Wine
          needs to relay between 16-bit and 32-bit code. These
          thunks switch between the app's 16-bit stack and Wine's
          32-bit stack, copies and converts arguments as appropriate
          (an int is 16 bit 16-bit and 32 bits in 32-bit, pointers
          are segmented in 16 bit (and also near or far) but are 32
          bit linear values in 32 bit), and handles the Win16
          mutex. Some finer control can be obtained on the
          conversion, see <command>winebuild</command> reference
          manual for the details. Suffice to say that the kind of
          intricate stack content juggling this results in, is not
          exactly suitable study material for beginners.
        </para>
        <para>
          A DLL descriptor is also created for every 16 bit
          DLL. However, this DLL normally paired with a 32 bit
          DLL. Either, it's the 16 bit counterpart of the 16 bit DLL
          (KRNL386.EXE for KERNEL32, USER for USER32...), or a 16
          bit DLL directly linked to a 32 bit DLL (like SYSTEM for
          KERNEL32, or DDEML for USER32). In those cases, the 16 bit
          descriptor(s) is (are) inserted in the same shared library
          as the the corresponding 32 bit DLL. Wine will also create
          symbolic links between kernel32.dll.so and system.dll.so
          so that loading of either
          <filename>kernel32.dll</filename> or
          <filename>system.dll</filename> will end up on the same
          shared library.
        </para>
      </sect2>
      <sect2 id="arch-mem">
        <title>Memory management</title>
        <para>
          Every Win32 process in Wine has its own dedicated native
          process on the host system, and therefore its own address
          space. This section explores the layout of the Windows
          address space and how it is emulated.
        </para>

        <para>
          Firstly, a quick recap of how virtual memory works. Physical
          memory in RAM chips is split into
          <emphasis>frames</emphasis>, and the memory that each
          process sees is split into <emphasis>pages</emphasis>. Each
          process has its own 4 gigabytes of address space (4gig being
          the maximum space addressable with a 32 bit pointer). Pages
          can be mapped or unmapped: attempts to access an unmapped
          page cause an
          <constant>EXCEPTION_ACCESS_VIOLATION</constant> which has
          the easily recognizable code of
          <constant>0xC0000005</constant>. Any page can be mapped to
          any frame, therefore you can have multiple addresses which
          actually "contain" the same memory. Pages can also be mapped
          to things like files or swap space, in which case accessing
          that page will cause a disk access to read the contents into
          a free frame.
        </para>

        <sect3>
          <title>Initial layout (in Windows)</title>
          <para>
            When a Win32 process starts, it does not have a clear
            address space to use as it pleases. Many pages are already
            mapped by the operating system. In particular, the EXE
            file itself and any DLLs it needs are mapped into memory,
            and space has been reserved for the stack and a couple of
            heaps (zones used to allocate memory to the app
            from). Some of these things need to be at a fixed address,
            and others can be placed anywhere.
          </para>

          <para>
            The EXE file itself is usually mapped at address 0x400000
            and up: indeed, most EXEs have their relocation records
            stripped which means they must be loaded at their base
            address and cannot be loaded at any other address.
          </para>

          <para>
            DLLs are internally much the same as EXE files but they
            have relocation records, which means that they can be
            mapped at any address in the address space. Remember we
            are not dealing with physical memory here, but rather
            virtual memory which is different for each
            process. Therefore <filename>OLEAUT32.DLL</filename> may
            be loaded at one address in one process, and a totally
            different one in another. Ensuring all the functions
            loaded into memory can find each other is the job of the
            Windows dynamic linker, which is a part of NTDLL.
          </para>
          <para>
            So, we have the EXE and its DLLs mapped into memory. Two
            other very important regions also exist: the stack and the
            process heap. The process heap is simply the equivalent of
            the libc <function>malloc</function> arena on UNIX: it's a
            region of memory managed by the OS which
            <function>malloc</function>/<function>HeapAlloc</function>
            partitions and hands out to the application. Windows
            applications can create several heaps but the process heap
            always exists.
          </para>
          <para>
            Windows 9x also implements another kind of heap: the
            shared heap. The shared heap is unusual in that
            anything allocated from it will be visible in every other
            process.
          </para>
        </sect3>

        <sect3>
          <title>Comparison</title>
          <para>
            So far we've assumed the entire 4 gigs of address space is
            available for the application. In fact that's not so: only
            the lower 2 gigs are available, the upper 2 gigs are on
            Windows NT used by the operating system and hold the
            kernel (from 0x80000000). Why is the kernel mapped into
            every address space?  Mostly for performance: while it's
            possible to give the kernel its own address space too -
            this is what Ingo Molnars 4G/4G VM split patch does for
            Linux - it requires that every system call into the kernel
            switches address space. As that is a fairly expensive
            operation (requires flushing the translation lookaside
            buffers etc) and syscalls are made frequently it's best
            avoided by keeping the kernel mapped at a constant
            position in every processes address space.
          </para>
          
          <para>
            Basically, the comparison of memory mappings looks as
            follows:
            <table>
              <title>Memory layout (Windows and Wine)</title>
              <tgroup cols="4" align="left" colsep="1" rowsep="1">
                <thead>
                  <row>
                    <entry>Address</entry>
                    <entry>Windows 9x</entry>
                    <entry>Windows NT</entry>
                    <entry>Linux</entry>
                  </row>
                </thead>
                <tbody>
                  <row>
                    <entry>00000000-7fffffff</entry>
                    <entry>User</entry>
                    <entry>User</entry>
                    <entry>User</entry>
                  </row>
                  <row>
                    <entry>80000000-bfffffff</entry>
                    <entry>Shared</entry>
                    <entry>User</entry>
                    <entry>User</entry>
                  </row>
                  <row>
                    <entry>c0000000-ffffffff</entry>
                    <entry>Kernel</entry>
                    <entry>Kernel</entry>
                    <entry>Kernel</entry>
                  </row>
                </tbody>
              </tgroup>
            </table>
          </para>

          <para>
            On Windows 9x, in fact only the upper gigabyte
            (<constant>0xC0000000</constant> and up) is used by the
            kernel, the region from 2 to 3 gigs is a shared area used
            for loading system DLLs and for file mappings. The bottom
            2 gigs on both NT and 9x are available for the programs
            memory allocation and stack.
          </para>
        </sect3>

        <sect3>
          <title>Implementation</title>
          <para>
            Wine (with a bit of black magic) is able to map all items
            at the correct locations as depicted above.
          </para>
          <para>
            Wine also implements the shared heap so native win9x DLLs
            can be used. This heap is always created at the
            <constant>SYSTEM_HEAP_BASE</constant> address or
            <constant>0x80000000</constant> and defaults to 16
            megabytes in size.
          </para>
          <para>
            There are a few other magic locations. The bottom 64k of
            memory is deliberately left unmapped to catch null pointer
            dereferences. The region from 64k to 1mb+64k are reserved
            for DOS compatibility and contain various DOS data
            structures. Finally, the address space also contains
            mappings for the Wine binary itself, any native libaries
            Wine is using, the glibc malloc arena and so on.
          </para>
        </sect3>
      </sect2>
      <sect2>
        <title>Processes</title>
        <para>
          Let's take a closer look at the way Wine loads and run
          processes in memory.
        </para>
        <sect3>
          <title>Starting a process from command line</title>
          <para>
            When starting a Wine process from command line (we'll get
            later on to the differences between NE, PE and Winelib
            executables), there are a couple of things Wine need to do
            first. A first executable is run to check the threading
            model of the underlying OS (see <xref linkend="threading">
            for the details) and will start the real Wine loader
            corresponding to the choosen threading model.
          </para>
          <para>
            Then Wine graps a few elements from the Unix world: the
            environment, the program arguments. Then the
            <filename>ntdll.dll.so</filename> is loaded into memory
            using the standard shared library dynamic loader. When
            loaded, NTDLL will mainly first create a decent Windows
            environment:
            <itemizedlist>
              <listitem>
                <para>create a PEB and a TEB</para>
              </listitem>
              <listitem>
                <para>
                  set up the connection to the Wine server - and
                  eventually launching the Wine server if none runs
                </para>
              </listitem>
              <listitem>
                <para>create the process heap</para>
              </listitem>
            </itemizedlist>
          </para>
          <para>
            Then <filename>Kernel32</filename> is loaded (but now
            using the Windows dynamic loading capabilities) and a Wine
            specific entry point is called
            <function>__wine_kernel_init</function>. This function
            will actually handle all the logic of the process loading
            and execution, and will never return from it's call.
          </para>
          <para>
            <function>__wine_kernel_init</function> will undergo the
            following tasks:
            <itemizedlist>
              <listitem>
                <para>
                  initialization of program arguments from Unix
                  program arguments
                </para>
              </listitem>
              <listitem>
                <para>
                  lookup of executable in the file system
                </para>
              </listitem>
              <listitem>
                <para>
                  If the file is not found, then an error is printed
                  and the Wine loader stops.
                </para>
              </listitem>
              <listitem>
                <para>
                  We'll cover the non-PE file type later on, so assume
                  for now it's a PE file. The PE module is loaded in
                  memory using the Windows shared library
                  mechanism. Note that the dependencies on the module
                  are not resolved at this point.
                </para>
              </listitem>
              <listitem>
                <para>
                  A new stack is created, which size is given in the
                  PE header, and this stack is made the one of the
                  running thread (which is still the only one in the
                  process). The stack used at startup will no longer
                  be used.
                </para>
              </listitem>
              <listitem>
                <para>
                  Which this new stack,
                  <function>ntdll.LdrInitializeThunk</function> is
                  called which performs the remaining initialization
                  parts, including resolving all the DLL imports on
                  the PE module, and doing the init of the TLS slots.
                </para>
              </listitem>
              <listitem>
                <para>
                  Control can now be passed to the
                  <function>EntryPoint</function> of the PE module,
                  which will let the executable run.
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </sect3>
        <sect3>
          <title>Creating a child process from a running process</title>
          <para>
            The steps used are closely link to what is done in the
            previous case.
          </para>
          <para>
            There are however a few points to look at a bit more
            closely. The inner implementation creates the child
            process using the <function>fork()</function> and
            <function>exec()</function> calls. This means that we
            don't need to check again for the threading model, we can
            use what the parent (or the grand-parent process...)
            started from command line has found.
          </para>
          <para>
            The Win32 process creation allows to pass a lot of
            information between the parent and the child. This
            includes object handles, windows title, console
            parameters, environment strings... Wine makes use of both
            the standard Unix inheritance mechanisms (for environment
            for example) and the Wine server (to pass from parent to
            child a chunk of data containing the relevant information).
          </para>
          <para>
            The previously described loading mechanism will check in
            the Wine server if such a chunk exists, and, if so, will
            perform the relevant initialization.
          </para>
          <para>
            Some further synchronization is also put in place: a
            parent will wait until the child has started, or has
            failed. The Wine server is also used to perform those
            tasks.
          </para>
        </sect3>
        <sect3>
          <title>Starting a Winelib process</title>
          <para>
            Before going into the gory details, let's first go back to
            what a Winelib application is. It can be either a regular
            Unix executable, or a more specific Wine beast. This later
            form in fact creates two files for a given executable (say
            <filename>foo.exe</filename>). The first one, named
            <filename>foo</filename> will be a symbolic link to the
            Wine loader (<filename>wine</filename>). The second one,
            named <filename>foo.exe.so</filename>, is the equivalent
            of the <filename>.dll.so</filename> files we've already
            described for DLLs. As in Windows, an executable is, among
            other things, a module with its import and export
            information, as any DLL, it makes sense Wine uses the same
            mechanisms for loading native executables and DLLs.
          </para>
          <para>
            When starting a Winelib application from the command line
            (say with <command>foo arg1 arg2</command>), the Unix
            shell will execute <command>foo</command> as a Unix
            executable. Since this is in fact the Wine loader, Wine
            will fire up. However, will notice that it hasn't been
            started as <command>wine</command> but as
            <command>foo</command>, and hence, will try to load (using
            Unix shared library mechanism) the second file
            <filename>foo.exe.so</filename>. Wine will recognize a 32
            bit module (with its descriptor) embedded in the shared
            library, and once the shared library loaded, it will
            proceed the same path as when loading a standard native PE
            executable.
          </para>
          <para>
            Wine needs to implement this second form of executable in
            order to maintain the order of initialization of some
            elements in the executable. One particular issue is when
            dealing with global C++ objects. In standard Unix
            executable, the call of the constructor to such objects is
            stored in the specific section of the executable
            (<function>.init</function> not to name it). All
            constructors in this section are called before the
            <function>main</function> function is called. Creating a
            Wine executable using the first form mentionned above will
            let those constructors being called before Wine gets a
            chance to initialize itself. So, any constructor using a
            Windows API will fail, because Wine infrastructure isn't
            in place. The use of the second form for Winelib
            executables ensures that we do the initialization using
            the following steps:
            <itemizedlist>
              <listitem>
                <para>
                  initialize the Wine infrastructure
                </para>
              </listitem>
              <listitem>
                <para>
                  load the executable into memory
                </para>
              </listitem>
              <listitem>
                <para>
                  handle the import sections for the executable
                </para>
              </listitem>
              <listitem>
                <para>
                  call the global object constructors (if any). They
                  now can properly call the Windows APIs
                </para>
              </listitem>
              <listitem>
                <para>
                  call the executable entry point
                </para>
              </listitem>
            </itemizedlist>
          </para>
          <para>
            The attentive reader would have noted that the resolution
            of imports for the executable is done, as for a DLL, when
            the executable/DLL descriptor is registered. However, this
            is done also by adding a specific constructor in the
            <function>.init</function> section. For the above describe
            scheme to function properly, this constructor must be the
            first constructor to be called, before all the other
            constructors, generated by the executable itself. The Wine
            build chain takes care of that, and also generating the
            executable/DLL descriptor for the Winelib executable.
          </para>
        </sect3>
        <sect3>
          <title>Starting a NE (Win16) process</title>
          <para>
            When Wine is requested to run a NE (Win 16 process), it
            will in fact hand over the execution of it to a specific
            executable <filename>winevdm</filename>. VDM stands for
            Virtual DOS Machine. This <filename>winevdm</filename>
            will in fact set up the correct 16 bit environment to run
            the executable. Any new 16 bit process created by this
            executable (or its children) will run into the same
            <filename>winevdm</filename> instance. Among one instance,
            several functionalities will be provided to those 16 bit
            processes, including the cooperative multitasking, sharing
            the same address space, managing the selectors for the 16
            bit segments needed for code, data and stack.
          </para>
          <para>
            Note that several <filename>winevdm</filename> instances
            can run in the same Wine session, but the functionalities
            described above are only shared among a given instance,
            not among all the instances. <filename>winevdm</filename>
            is built as Winelib application, and hence has access to
            any facility a 32 bit application has.
          </para>
          <para>
            The behaviour we just described also applies to DOS
            executables, which are handled the same way by
            <filename>winevdm</filename>.
          </para>
        </sect3>
      </sect2>
      <sect2>
        <title>Wine drivers</title>
        <para>
          Wine will not allow running native Windows drivers under
          Unix. This comes mainly because (look at the generic
          architecture schemas) Wine doesn't implement the kernel
          features of Windows (kernel here really means the kernel,
          not the KERNEL32 DLL), but rather sets up a proxy layer on
          top of the Unix kernel to provide the NTDLL and KERNEL32
          features. This means that Wine doesn't provide the inner
          infrastructure to run native drivers, either from the Win9x
          family or from the NT family.
        </para>
        <para>
          In other words, Wine will only be able to provide access to
          a specific device, if and only if, 1/ this device is
          supported in Unix (there is Unix-driver to talk to it), 2/
          Wine has implemented the proxy code to make the glue between
          the API of a Windows driver, and the Unix interface of the
          Unix driver.
        </para>
        <para>
          Wine, however, tries to implement in the various DLLs
          needing to access devices to do it through the standard
          Windows APIs for device drivers in user space. This is for
          example the case for the multimedia drivers, where Wine
          loads Wine builtin DLLs to talk to the OSS interface, or the
          ALSA interface. Those DLLs implement the same interface as
          any user space audio driver in Windows.
        </para>
      </sect2>
    </sect1>

    <sect1 id="module-overview">
      
      <title>Module Overview</title>
      <sect2>
        <title>NTDLL Module</title>
        <para>
          NTDLL provides most of the services you'd expect from a
          kernel.
        </para>
        <para>
          Process and thread management are part of them (even if
          process management is still mainly done in KERNEL32, unlike
          NT). A Windows process runs as a Unix process, and a Windows
          thread runs as a Unix thread.
        </para>
        <para>
          Wine also provide fibers (which is the Windows name of
          co-routines).
        </para>
        <para>
          Most of the Windows memory handling (Heap, Global and Local
          functions, virtual memory...) are easily mapped upon their
          Unix equivalents. Note the NTDLL doesn't know about 16 bit
          memory, which is only handled in KERNEL32/KRNL386.EXE (and
          also the DOS routines).
        </para>

        <sect3>
          <title>File management</title>
          <para>
            Wine uses some configuration in order to map Windows
            filenames (either defined with drive letters, or as UNC
            names) to the unix filenames. Wine also uses some
            incantation so that most of file related APIs can also
            take full unix names. This is handy when passing filenames
            on the command line.
          </para>
          <para>
            File handles can be waitable objects, as Windows define
            them.
          </para>
          <para>
            Asynchronous I/O is implemented on file handles by
            queueing pseudo APC. They are not real APC in the sense
            that they have the same priority as the threads in the
            considered process (while APCs on NT have normally a
            higher priority). These APCs get called when invoking
            Wine server (which should lead to correct behavior when the
            program ends up waiting on some object - waiting always
            implies calling Wine server).
          </para>
          <para>
            FIXME: this should be enhanced and updated to latest work
            on FS.
          </para>
        </sect3>

        <sect3>
          <title>Synchronization</title>
          <para>
            Most of the synchronization (between threads or processes)
            is done in Wine server, which handles both the waiting
            operation (on a single object or a set of objects) and the
            signaling of objects. 
          </para>
        </sect3>

        <sect3>
          <title>Module (DLL) loading</title>
          <para>
            Wine is able to load any NE and PE module. In all cases,
            the module's binary code is directly executed by the
            processor.
          </para>
        </sect3>

        <sect3>
          <title>Device management</title>
          <para>
            Wine allows usage a wide variety of devices:
            <itemizedlist>
              <listitem>
                <para>
                  Communication ports are mapped to Unix
                  communication ports (if they have sufficient
                  permissions).
                </para>
              </listitem>
              <listitem>
                <para>
                  Parallel ports are mapped to Unix parallel ports (if
                  they have sufficient permissions).
                </para>
              </listitem>
              <listitem>
                <para>
                  CDROM: the Windows device I/O control calls are
                  mapped onto Unix <function>ioctl()</function>.
                </para>
              </listitem>
              <listitem>
                <para>
                  Some Win9x VxDs are supported, by rewriting some of
                  their internal behavior. But this support is
                  limited. Portable programs to Windows NT shouldn't
                  need them.
                </para>
                <para>
                  Wine will not support native VxD.
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </sect3>
      </sect2>

      <sect2>
        <title>KERNEL Module</title>

        <para>
          FIXME: Needs some content...
        </para>
      </sect2>

      <sect2>
        <title>GDI Module</title>

        <sect3>
          <title>X Windows System interface</title>

          <para>
            The X libraries used to implement X clients (such as Wine)
            do not work properly if multiple threads access the same
            display concurrently. It is possible to compile the X
            libraries to perform their own synchronization (initiated
            by calling <function>XInitThreads()</function>). However,
            Wine does not use this approach. Instead Wine performs its
            own synchronization using the
            <function>wine_tsx11_lock()</function> / <function>wine_tsx11_unlock()</function>
            functions.  This locking protects library access
            with a critical section, and also arranges things so that
            X libraries compiled without <option>-D_REENTRANT</option>
            (eg. with global <varname>errno</varname> variable) will
            work with Wine.
          </para>
          <para>
            In the past, all calls to X used to go through a wrapper called
             <function>TSX...()</function> (for "Thread Safe X ...").
            While it is still being used in the code, it's inefficient
            as the lock is potentially aquired and released unnecessarily.
            New code should explicitly aquire the lock.
          </para>
        </sect3>
      </sect2>

      <sect2>
        <title>USER Module</title>

        <para>
          USER implements windowing and messaging subsystems. It also
          contains code for common controls and for other
          miscellaneous  stuff (rectangles, clipboard, WNet, etc).
          Wine USER code is  located in <filename>windows/</filename>,
          <filename>controls/</filename>, and
          <filename>misc/</filename> directories.
        </para>

        <sect3>
          <title>Windowing subsystem</title>

          <para><filename>windows/win.c</filename></para>
          <para><filename>windows/winpos.c</filename></para>
          <para>
            Windows are arranged into parent/child hierarchy with one
            common ancestor for all windows (desktop window). Each
            window structure contains a pointer to the immediate
            ancestor (parent window if <constant>WS_CHILD</constant>
            style bit is set), a pointer to the sibling (returned by
            <function>GetWindow(..., GW_NEXT)</function>), a pointer
            to the owner  window (set only for popup window if it was
            created with valid  <varname>hwndParent</varname>
            parameter), and a pointer to the first child window
            (<function>GetWindow(.., GW_CHILD)</function>). All popup
            and non-child windows are therefore placed in the first
            level of this hierarchy and their ancestor link
            (<varname>wnd-&gt;parent</varname>) points to the desktop
            window.
          </para>
          <screen>
   Desktop window                       - root window
    |     \      `-.
    |      \        `-.
   popup -&gt; wnd1  -&gt;  wnd2                - top level windows    
    |       \   `-.      `-.
    |        \     `-.      `-.
   child1  child2 -&gt; child3  child4     - child windows
          </screen>
          <para>
            Horizontal arrows denote sibling relationship, vertical
            lines - ancestor/child. To summarize, all windows with the
            same immediate ancestor are sibling windows, all windows
            which do not have desktop as their immediate ancestor are
            child windows. Popup windows behave as topmost top-level
            windows unless they are owned. In this case the only
            requirement is that they must precede their owners in the
            top-level sibling list (they are not topmost). Child
            windows are confined to the client area of their parent
            windows (client area is where window gets to do its own
            drawing, non-client area consists of caption, menu,
            borders, intrinsic scrollbars, and
            minimize/maximize/close/help buttons). 
          </para>
          <para>
            Another fairly important concept is
            <firstterm>z-order</firstterm>. It is derived from the
            ancestor/child hierarchy and is used to determine
            "above/below" relationship. For instance, in the example
            above, z-order is
          </para>
          <screen>
child1-&gt;popup-&gt;child2-&gt;child3-&gt;wnd1-&gt;child4-&gt;wnd2-&gt;desktop.
          </screen>
          <para>
            Current  active window ("foreground window" in Win32) is
            moved to the front of z-order unless its top-level
            ancestor owns popup windows.
          </para>
          <para>
            All these issues are dealt with (or supposed to be) in
            <filename>windows/winpos.c</filename> with
            <function>SetWindowPos()</function> being the primary
            interface to the window manager.
          </para>
          <para>
            Wine specifics: in default and managed mode each top-level
            window gets its own X counterpart with desktop window
            being basically a fake stub. In desktop mode, however,
            only desktop window has an X window associated with it.
            Also, <function>SetWindowPos()</function> should
            eventually be implemented via
            <function>Begin/End/DeferWindowPos()</function> calls and
            not the other way around.
          </para>

          <sect4>
            <title>Visible region, clipping region and update region</title>

            <para><filename>windows/dce.c</filename></para>
            <para><filename>windows/winpos.c</filename></para>
            <para><filename>windows/painting.c</filename></para>

            <screen>
    ________________________
   |_________               |  A and B are child windows of C
   |    A    |______        | 
   |         |      |       |
   |---------'      |       |
   |   |      B     |       |
   |   |            |       |
   |   `------------'       |
   |                   C    |
   `------------------------'
            </screen>
            <para>
              Visible region determines which part of the window is
              not obscured by other windows. If a window has the
              <constant>WS_CLIPCHILDREN</constant> style then all
              areas below its children are considered invisible.
              Similarly, if the <constant>WS_CLIPSIBLINGS</constant>
              bit is in effect then all areas obscured by its siblings
              are invisible. Child windows are always clipped by the
              boundaries of their parent windows.
            </para>
            <para>
              B has a <constant>WS_CLIPSIBLINGS</constant> style:
            </para>
            <screen>
   .          ______ 
   :         |      |
   |   ,-----'      |
   |   |      B     | - visible region of B
   |   |            |
   :   `------------'
            </screen>
            <para>
              When the program requests a <firstterm>display
                context</firstterm> (DC) for a window it  can specify
              an optional clipping region that further restricts the
              area where the graphics output can appear. This area is
              calculated as an intersection of the visible region and
              a clipping region. 
            </para>
            <para>
              Program asked for a DC with a clipping region:
            </para>
            <screen>
          ______
      ,--|--.   |     .    ,--. 
   ,--+--'  |   |     :   _:  |
   |  |   B |   |  =&gt; |  |    | - DC region where the painting will
   |  |     |   |     |  |    |   be visible
   `--|-----|---'     :  `----'
      `-----'
            </screen>
            <para>
              When the window manager detects that some part of the window
              became visible it adds this area to the update region of this
              window and then generates <constant>WM_ERASEBKGND</constant> and
              <constant>WM_PAINT</constant> messages.  In addition,
              <constant>WM_NCPAINT</constant> message is sent when the
              uncovered area  intersects a nonclient part of the window.
              Application must reply to the <constant>WM_PAINT</constant>
              message by calling the
              <function>BeginPaint()</function>/<function>EndPaint()</function>
              pair of functions. <function>BeginPaint()</function> returns a DC
              that uses accumulated update region as a clipping region. This
              operation cleans up invalidated area and the window will not
              receive another <constant>WM_PAINT</constant> until the window
              manager creates a new update region.
            </para>
            <para>
              A was moved to the left:
            </para>
            <screen>
    ________________________       ...          / C update region
   |______                  |     :      .___ /
   | A    |_________        |  =&gt; |   ...|___|..
   |      |         |       |     |   :  |   |
   |------'         |       |     |   :  '---' 
   |   |      B     |       |     |   :      \
   |   |            |       |     :            \
   |   `------------'       |                    B update region
   |                   C    |
   `------------------------'
            </screen>
            <para>
              Windows maintains a display context cache consisting of
              entries that include the DC itself, the window to which
              it belongs, and an optional clipping region (visible
              region is stored in the DC itself). When an API call
              changes the state of the window tree, window manager has
              to go through the DC cache to recalculate visible
              regions for entries whose windows were involved in the
              operation. DC entries (DCE) can be either private to the
              window, or private to the window class, or shared
              between all windows (Windows 3.1 limits the number of
              shared DCEs to 5).
            </para>
          </sect4>
        </sect3>

        <sect3>
          <title>Messaging subsystem</title>

          <para><filename>windows/queue.c</filename></para>
          <para><filename>windows/message.c</filename></para>

          <para>
            Each Windows task/thread has its own message queue - this
            is where it gets messages from. Messages can be:
            <orderedlist>
              <listitem>
                <para>
                  generated on the fly (<constant>WM_PAINT</constant>,
                  <constant>WM_NCPAINT</constant>,
                  <constant>WM_TIMER</constant>)
                </para>
              </listitem>
              <listitem>
                <para>
                  created by the system (hardware messages)
                </para>
              </listitem>
              <listitem>
                <para>
                  posted by other tasks/threads (<function>PostMessage</function>)
                </para>
              </listitem>
              <listitem>
                <para>
                  sent by other tasks/threads (<function>SendMessage</function>)
                </para>
              </listitem>
            </orderedlist>
          </para>
          <para>
            Message priority:
          </para>
          <para>
            First the system looks for sent messages, then for posted
            messages, then for hardware messages, then it checks if
            the queue has the "dirty window" bit set, and, finally, it
            checks for expired timers. See
            <filename>windows/message.c</filename>.
          </para>
          <para>
            From all these different types of messages, only posted
            messages go directly into the private message queue.
            System messages (even in Win95) are first collected in the
            system message queue and then they either sit there until
            <function>Get/PeekMessage</function> gets to process them
            or, as in Win95, if system queue is getting clobbered, a
            special thread ("raw input thread") assigns them to the
            private queues. Sent messages are queued separately and
            the sender sleeps until it gets a reply. Special messages
            are generated on the fly depending on the window/queue
            state. If the window update region is not empty, the
            system sets the <constant>QS_PAINT</constant> bit in the
            owning queue and eventually this window receives a
            <constant>WM_PAINT</constant> message
            (<constant>WM_NCPAINT</constant> too if the update region
            intersects with the non-client area). A timer event is
            raised when one of the queue timers expire. Depending on
            the timer parameters <function>DispatchMessage</function>
            either calls the callback function or the window
            procedure. If there are no messages pending the
            task/thread sleeps until messages appear.
          </para>
          <para>
            There are several tricky moments (open for discussion) - 
          </para>

          <itemizedlist>
            <listitem>
              <para>
                System message order has to be honored and messages
                should be processed within correct task/thread
                context. Therefore when <function>Get/PeekMessage</function> encounters
                unassigned system message and this message appears not
                to be for the current task/thread it should either
                skip it (or get rid of it by moving it into the
                private message queue of the target task/thread -
                Win95, AFAIK) and look further or roll back and then
                yield until this message gets processed when system
                switches to the correct context (Win16). In the first
                case we lose correct message ordering, in the second
                case we have the infamous synchronous system message
                queue. Here is a post to one of the OS/2 newsgroup I
                found to be relevant:
              </para>
              <blockquote>
                <attribution>by David Charlap</attribution>
                <para>
                  " Here's the problem in a nutshell, and there is no
                  good solution. Every possible solution creates a
                  different problem.
                </para>
                <para>
                  With a windowing system, events can go to many
                  different windows. Most are sent by applications or
                  by the OS when things relating to that window happen
                  (like repainting, timers, etc.)
                </para>
                <para>
                  Mouse input events go to the window you click on
                  (unless some window captures the mouse).
                </para>
                <para>
                  So far, no problem.  Whenever an event happens, you
                  put a message on the target window's message queue.
                  Every process has a message queue.  If the process
                  queue fills up, the messages back up onto the system
                  queue.
                </para>
                <para>
                  This is the first cause of apps hanging the GUI.  If
                  an app doesn't handle messages and they back up into
                  the system queue, other apps can't get any more
                  messages.  The reason is that the next message in
                  line can't go anywhere, and the system won't skip
                  over it.
                </para>
                <para>
                  This can be fixed by making apps have bigger private
                  message queues. The SIQ fix does this.  PMQSIZE does
                  this for systems without the SIQ fix.  Applications
                  can also request large queues on their own.
                </para>
                <para>
                  Another source of the problem, however, happens when
                  you include keyboard events.  When you press a key,
                  there's no easy way to know what window the
                  keystroke message should be delivered to.
                </para>
                <para>
                  Most windowing systems use a concept known as
                  "focus".  The window with focus gets all incoming
                  keyboard messages.  Focus can be changed from window
                  to window by apps or by users clicking on windows.
                </para>
                <para>
                  This is the second source of the problem.  Suppose
                  window A has focus. You click on window B and start
                  typing before the window gets focus. Where should
                  the keystrokes go?  On the one hand, they should go
                  to A until the focus actually changes to B.  On the
                  other hand, you probably want the keystrokes to go
                  to B, since you clicked there first.
                </para>
                <para>
                  OS/2's solution is that when a focus-changing event
                  happens (like clicking on a window), OS/2 holds all
                  messages in the system queue until the focus change
                  actually happens.  This way, subsequent keystrokes
                  go to the window you clicked on, even if it takes a
                  while for that window to get focus.
                </para>
                <para>
                  The downside is that if the window takes a real long
                  time to get focus (maybe it's not handling events,
                  or maybe the window losing focus isn't handling
                  events), everything backs up in the system queue and
                  the system appears hung.
                </para>
                <para>
                  There are a few solutions to this problem.
                </para>
                <para>
                  One is to make focus policy asynchronous.  That is,
                  focus changing has absolutely nothing to do with the
                  keyboard.  If you click on a window and start typing
                  before the focus actually changes, the keystrokes go
                  to the first window until focus changes, then they
                  go to the second. This is what X-windows does.
                </para>
                <para>
                  Another is what NT does.  When focus changes,
                  keyboard events are held in the system message
                  queue, but other events are allowed through. This is
                  "asynchronous" because the messages in the system
                  queue are delivered to the application queues in a
                  different order from that with which they were
                  posted.  If a bad app won't handle the "lose focus"
                  message, it's of no consequence - the app receiving
                  focus will get its "gain focus" message, and the
                  keystrokes will go to it.
                </para>
                <para>
                  The NT solution also takes care of the application
                  queue filling up problem.  Since the system delivers
                  messages asynchronously, messages waiting in the
                  system queue will just sit there and the rest of the
                  messages will be delivered to their apps.
                </para>
                <para>
                  The OS/2 SIQ solution is this:  When a
                  focus-changing event happens, in addition to
                  blocking further messages from the application
                  queues, a timer is started.  When the timer goes
                  off, if the focus change has not yet happened, the
                  bad app has its focus taken away and all messages
                  targeted at that window are skipped.  When the bad
                  app finally handles the focus change message, OS/2
                  will detect this and stop skipping its messages.
                </para>

                <para>
                  As for the pros and cons:
                </para>
                <para>
                  The X-windows solution is probably the easiest.  The
                  problem is that users generally don't like having to
                  wait for the focus to change before they start
                  typing.  On many occasions, you can type and the
                  characters end up in the wrong window because
                  something (usually heavy system load) is preventing
                  the focus change from happening in a timely manner.
                </para>
                <para>
                  The NT solution seems pretty nice, but making the
                  system message queue asynchronous can cause similar
                  problems to the X-windows problem. Since messages
                  can be delivered out of order, programs must not
                  assume that two messages posted in a particular
                  order will be delivered in that same order.  This
                  can break legacy apps, but since Win32 always had an
                  asynchronous queue, it is fair to simply tell app
                  designers "don't do that".  It's harder to tell app
                  designers something like that on OS/2 - they'll
                  complain "you changed the rules and our apps are
                  breaking."
                </para>
                <para>
                  The OS/2 solution's problem is that nothing happens
                  until you try to change window focus, and then wait
                  for the timeout.  Until then, the bad app is not
                  detected and nothing is done."
                </para>
              </blockquote>
            </listitem>

            <listitem>
              <para>
                Intertask/interthread
                <function>SendMessage</function>. The system has to
                inform the target queue about the forthcoming message,
                then it has to carry out the context switch and wait
                until the result is available.  Win16 stores necessary
                parameters in the queue structure and then calls
                <function>DirectedYield()</function> function.
                However, in Win32 there could be  several messages
                pending sent by preemptively executing threads, and in
                this case <function>SendMessage</function> has to
                build some sort of message queue for sent messages.
                Another issue is what to do with messages sent to the
                sender when it is blocked inside its own
                <function>SendMessage</function>. 
              </para>
            </listitem>
          </itemizedlist>
        </sect3>
      </sect2>
    </sect1>

    <sect1 id="arch-dlls">
      <title>Wine/Windows DLLs</title>

      <para>
        This document mainly deals with the status of current DLL
        support by Wine.  The Wine ini file currently supports
        settings to change the load order of DLLs.  The load order
        depends on several issues, which results in different settings
        for various DLLs.
      </para>

      <sect2>
        <title>Pros of Native DLLs</title>

        <para>
          Native DLLs of course guarantee 100% compatibility for
          routines they implement. For example, using the native USER
          DLL would maintain a virtually perfect and Windows 95-like
          look for window borders, dialog controls, and so on. Using
          the built-in Wine version of this library, on the other
          hand, would produce a display that does not precisely mimic
          that of Windows 95.  Such subtle differences can be
          engendered in other important DLLs, such as the common
          controls library COMMCTRL or the common dialogs library
          COMMDLG, when built-in Wine DLLs outrank other types in load
          order.
        </para>
        <para>
          More significant, less aesthetically-oriented problems can
          result if the built-in Wine version of the SHELL DLL is
          loaded before the native version of this library. SHELL
          contains routines such as those used by installer utilities
          to create desktop shortcuts. Some installers might fail when
          using Wine's built-in SHELL.
        </para>
      </sect2>

      <sect2>
        <title>Cons of Native DLLs</title>

        <para>
          Not every application performs better under native DLLs. If
          a library tries to access features of the rest of the system
          that are not fully implemented in Wine, the native DLL might
          work much worse than the corresponding built-in one, if at
          all. For example, the native Windows GDI library must be
          paired with a Windows display driver, which of course is not
          present under Intel Unix and Wine.
        </para>
        <para>
          Finally, occasionally built-in Wine DLLs implement more
          features than the corresponding native Windows DLLs.
          Probably the most important example of such behavior is the
          integration of Wine with X provided by Wine's built-in USER
          DLL. Should the native Windows USER library take load-order
          precedence, such features as the ability to use the
          clipboard or drag-and-drop between Wine windows and X
          windows will be lost.
        </para>
      </sect2>

      <sect2>
        <title>Deciding Between Native and Built-In DLLs</title>

        <para>
          Clearly, there is no one rule-of-thumb regarding which
          load-order to use. So, you must become familiar with
          what specific DLLs do and which other DLLs or features 
          a given library interacts with, and use this information 
          to make a case-by-case decision.
        </para>

      </sect2>

      <sect2>
        <title>Load Order for DLLs</title>

        <para>
          Using the DLL sections from the wine configuration file, the
          load order can be tweaked to a high degree. In general it is
          advised not to change the settings of the configuration
          file. The default configuration specifies the right load
          order for the most important DLLs.
        </para>
        <para>
          The default load order follows this algorithm: for all DLLs
          which have a fully-functional Wine implementation, or where
          the native DLL is known not to work, the built-in library
          will be loaded first. In all other cases, the native DLL
          takes load-order precedence.
        </para>
        <para>
          The <varname>DefaultLoadOrder</varname> from the
          [DllDefaults] section specifies for all DLLs which version
          to try first. See manpage for explanation of the arguments.
        </para>
        <para>
          The [DllOverrides] section deals with DLLs, which need a
          different-from-default treatment. 
        </para>
        <para>
          The [DllPairs] section is for DLLs, which must be loaded in
          pairs. In general, these are DLLs for either 16-bit or
          32-bit applications. In most cases in Windows, the 32-bit
          version cannot be used without its 16-bit counterpart. For
          Wine, it is customary that the 16-bit implementations rely
          on the 32-bit implementations and cast the results back to
          16-bit arguments. Changing anything in this section is bound
          to result in errors.
        </para>
        <para>
          For the future, the Wine implementation of Windows DLL seems
          to head towards unifying the 16 and 32 bit DLLs wherever
          possible, resulting in larger DLLs.  They are stored in the
          <filename>dlls/</filename> subdirectory using the 32-bit
          name.
        </para>
      </sect2>

      <sect2>
        <title>Understanding What DLLs Do</title>

        <para>
          The following list briefly describes each of the DLLs
          commonly found in Windows whose load order may be modified
          during the configuration and compilation of Wine.
        </para>
        <para>
          (See also <filename>./DEVELOPER-HINTS</filename> or the
          <filename>dlls/</filename> subdirectory to see which DLLs
          are currently being rewritten for Wine)
        </para>

<!-- FIXME: Should convert this table into a VariableList element -->
        <screen>
ADVAPI32.DLL:      32-bit application advanced programming interfaces
                   like crypto, systeminfo, security and event logging
AVIFILE.DLL:       32-bit application programming interfaces for the
                   Audio Video Interleave (AVI) Windows-specific
                   Microsoft audio-video standard
COMMCTRL.DLL:      16-bit common controls
COMCTL32.DLL:      32-bit common controls
COMDLG32.DLL:      32-bit common dialogs
COMMDLG.DLL:       16-bit common dialogs
COMPOBJ.DLL:       OLE 16- and 32-bit compatibility libraries
CRTDLL.DLL:        Microsoft C runtime
DCIMAN.DLL:        16-bit
DCIMAN32.DLL:      32-bit display controls
DDEML.DLL:         DDE messaging
D3D*.DLL           DirectX/Direct3D drawing libraries
DDRAW.DLL:         DirectX drawing libraries
DINPUT.DLL:        DirectX input libraries
DISPLAY.DLL:       Display libraries
DPLAY.DLL, DPLAYX.DLL:  DirectX playback libraries
DSOUND.DLL:        DirectX audio libraries
GDI.DLL:           16-bit graphics driver interface
GDI32.DLL:         32-bit graphics driver interface
IMAGEHLP.DLL:      32-bit IMM API helper libraries (for PE-executables)
IMM32.DLL:         32-bit IMM API
IMGUTIL.DLL:       
KERNEL32.DLL       32-bit kernel DLL
KEYBOARD.DLL:      Keyboard drivers
LZ32.DLL:          32-bit Lempel-Ziv or LZ file compression
                   used by the installshield installers (???).
LZEXPAND.DLL:      LZ file expansion; needed for Windows Setup
MMSYSTEM.DLL:      Core of the Windows multimedia system
MOUSE.DLL:         Mouse drivers
MPR.DLL:           32-bit Windows network interface
MSACM.DLL:         Core of the Addressed Call Mode or ACM system
MSACM32.DLL:       Core of the 32-bit ACM system
                   Audio Compression Manager ???
MSNET32.DLL        32-bit network APIs
MSVFW32.DLL:       32-bit Windows video system
MSVIDEO.DLL:       16-bit Windows video system
OLE2.DLL:          OLE 2.0 libraries
OLE32.DLL:         32-bit OLE 2.0 components
OLE2CONV.DLL:      Import filter for graphics files
OLE2DISP.DLL, OLE2NLS.DLL: OLE 2.1 16- and 32-bit interoperability
OLE2PROX.DLL:      Proxy server for OLE 2.0
OLE2THK.DLL:       Thunking for OLE 2.0
OLEAUT32.DLL       32-bit OLE 2.0 automation
OLECLI.DLL:        16-bit OLE client
OLECLI32.DLL:      32-bit OLE client
OLEDLG.DLL:        OLE 2.0 user interface support
OLESVR.DLL:        16-bit OLE server libraries
OLESVR32.DLL:      32-bit OLE server libraries
PSAPI.DLL:         Proces Status API libraries
RASAPI16.DLL:      16-bit Remote Access Services libraries
RASAPI32.DLL:      32-bit Remote Access Services libraries
SHELL.DLL:         16-bit Windows shell used by Setup
SHELL32.DLL:       32-bit Windows shell (COM object?)
TAPI/TAPI32/TAPIADDR:  Telephone API (for Modems)
W32SKRNL:          Win32s Kernel ? (not in use for Win95 and up!)
WIN32S16.DLL:      Application compatibility for Win32s
WIN87EM.DLL:       80387 math-emulation libraries
WINASPI.DLL:       Advanced SCSI Peripheral Interface or ASPI libraries
WINDEBUG.DLL       Windows debugger
WINMM.DLL:         Libraries for multimedia thunking
WING.DLL:          Libraries required to "draw" graphics
WINSOCK.DLL:       Sockets APIs
WINSPOOL.DLL:      Print spooler libraries
WNASPI32.DLL:      32-bit ASPI libraries
WSOCK32.DLL:       32-bit sockets APIs
        </screen>
      </sect2>
    </sect1>
  </chapter>

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