This is intend 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 the Wine processes memory
and assigns a selector to that segment.  To make address conversion
simpler, Wine loads the segments in such a way that the segmented
address (16:16) is stored in memory the same way as the 32-bit linear
address.  For example, the segmented address 1237:89AB can be at the
address 0x123789AB in the Wine process space.

This also implies that a Win16 program cannot access any arbitrary
memory location.  If a pointer needs to be returned to a Win16 program,
then the memory block must be allocated using either GlobalAlloc()
or HEAP_Alloc().  The HEAP_* functions are faster than the Global*
functions but are only capable of managing a 64k memory block.  The
HEAP_* functions are used to implement local heaps.  Wine should
never call Local* functions.  These functions are reserved for use
by Win16 programs only!

The following code fragment should be used to establish a new Wine
local heap:

	#include "heap.h"

	#define MY_HEAP_SIZE	0x10000		/* Must be <= 64k */

	int MyHeapHandle;
	void *MyHeapBase;
	MDESC *MyHeap;

		...

	int InitMyHeap()
	{
	    MyHeapHandle = GlobalAlloc(GMEM_FIXED, MY_HEAP_SIZE);
	    if (MyHeapHandle == 0)
		return -1;
	    MyHeapBase = GlobalLock(MyHeapHandle);
	    HEAP_Init(&MyHeap, MyHeapBase, MY_HEAP_SIZE);
	    return 0;
	}

Memory blocks greater than 64 kilobytes in length must be allocated
using GlobalAlloc().  Because of our special memory mapping, GlobalLock()
cannot be used to obtain the address of a linearly accessible memory
block that is greater than 64kB in length.  Instead GlobalLinearLock()
should be used.  The inverse function GlobalLinearUnlock() must be 
called before the block can be freed with GlobalFree().

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.
Inorder 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.