This is intended to be a document to help new developers get started.
Existing developers should feel free to add their comments.

MEMORY AND SEGMENTS:

NE (Win16) executables consist of multiple segments.  The Wine loader
loads each segment into a unique location in the Wine processes memory
and assigns a selector to that segment.  Because of this, it's not
possible to exchange addresses freely between 16-bit and 32-bit code.
Addresses used by 16-bit code are segmented addresses (16:16), formed
by a 16-bit selector and a 16-bit offset.  Those used by the Wine code
are regular 32-bit linear addresses.

There's three ways to obtain a segmented pointer:
  - Allocate a block of memory from the global heap and use
    WIN16_GlobalLock to get its segmented address.
  - Allocate a block of memory from a local heap, and build the
    segmented address from the local heap selector (see the
    USER_HEAP_* macros for an example of this).
  - Declare the argument as 'segptr' instead of 'ptr' in the spec file
    for a given API function.

Once you have a segmented pointer, it must be converted to a linear
pointer before you can use it from 32-bit code.  This can be done with
the PTR_SEG_TO_LIN() and PTR_SEG_OFF_TO_LIN() macros.  The linear
pointer can then be used freely with standard Unix functions like
memcpy() etc. without worrying about 64k boundaries.  Note: there's no
easy way to convert back from a linear to a segmented address.

In most cases, you don't need to worry about segmented address, as the
conversion is made automatically by the callback code and the API
functions only see linear addresses. However, in some cases it is
necessary to manipulate segmented addresses; the most frequent cases
are:
  - API functions that return a pointer
  - lParam of Windows messages that point to a structure
  - Pointers contained inside structures accessed by 16-bit code.

It is usually a good practice to used the type 'SEGPTR' for segmented
pointers, instead of something like 'LPSTR' or 'char *'.  As SEGPTR is
defined as a DWORD, you'll get a compilation warning if you mistakenly
use it as a regular 32-bit pointer.

API ENTRY POINTS:

Because Win16 programs use a 16-bit stack and because they can only
call 16:16 addressed functions, all API entry points must be at low
address offsets and must have the arguments translated and moved to
Wines 32-bit stack.  This task is handled by the code in the "if1632"
directory.  To define a new API entry point handler you must place a
new entry in the appropriate API specification file.  These files are
named *.spec.  For example, the API specification file for the USER DLL
is contained in the file user.spec.  These entries are processed by
the "build" program to create dll_*.s and dll_tab_*.c.  The dll_*.s
files contain the entry point code for each API call, and the dll_tab_*.s
files contain tables used by relay.c to translate arguments and transfer
control to the proper handler.  The format of the *.spec files is
documented in the file "tools/build-spec.txt".

REGISTER FUNCTIONS:

Some functions are defined as type "register" in the DLL specification files.
In order to return values in the registers to the WIN16 program, the handler
function must exit by calling ReturnFromRegisterFunc().  Look at the function
DOS3Call() for an example of how this works.

DEBUG MESSAGES:

To display a message only during debugging, you normally write something
like this:

#ifdef DEBUG_WIN
	printf("abc...");
#endif

You can write this shorter (and better) in this way:

	dprintf_win(stddeb,"abc...");

All symbols of the form dprintf_xxxx are macros defined in include/debug.h .
The macro-definitions are generated by the shell-script tools/make_debug. It
scans the source code for symbols of this forms and puts the necessary
macro definitions in include/debug.h and include/stddebug.h . These macros
test for the symbol DEBUG_XXXX (e.g. dprintf_win refers to DEBUG_WIN) being 
defined and thus decided whether to actually display the text. If you want
to enable specific types of messages, simply put the corresponding
#define DEBUG_XXXX in include/stddebug.h . If you want to enable or disable
a specific type of message in just one c-source-file, put the corresponding 
#define DEBUG_XXXX or #undefine DEBUG_XXXX between #include<stddebug.h> and
#include <debug.h> in that specific file. In addition you can change the 
types of displayed messages by supplying the "-debugmsg" option to Wine. 
If your debugging code is more complex than just printf, you can use the
symbols debugging_XXX as well. These are true when XXX is enabled, either
permanent or in the command line. So instead of writing

#ifdef DEBUG_WIN
	DumpSomeStructure(&str);
#endif

write
	if(debugging_win)DumpSomeStructure(&str);
Don't worry about the inefficiency of the test. If it is permanently 
disabled (thus debugging_win is 0 at compile time), the compiler will 
eliminate the dead code.

The file handle "stddeb" is intended for displaying standard informational
messages, whereas "stdnimp" is intended for displaying messages concerning
not yet implemented functions.

You have to start tools/make_debug only if you introduced a new macro,
e.g.  dprintf_win32s - not if you just changed one of the #define
DEBUG_XXX's in include/stddebug.h or in a specific file.