【正文】 code to the Macintosh by using the portability library for the Macintosh and Visual C++ for piling, linking, and debugging. 5. Use the Microsoft Foundation Class Library (MFC) version to implement new functionality such as OLE containers, servers, and clients or database support using open database connectivity (ODBC). Code written using MFC is highly portable to the Macintosh. 6. Write Macintoshspecific code to take advantage of unique Macintosh features, such as Apple Events or Publish and Subscribe. The chief challenge among the families of Windows operating systems is the break from 16 bits (Windows and Windows for Workgroups operating system with integrated working) to 32 bits (Windows NT and Windows 95). In general, 16bit and 32bit code bases are somewhat inpatible, unless they are written usi ng MFC. Developers have the choice of branching their sources into two trees, or migrating everything to 32 bits. Once the Win32 choice has been made, how are legacy platforms to be run (that is, machines still running Windows )? The obvious choice is to use the Win32s174。, UNIX, and RISC (reduced instruction set puter) machines. Until recently, developers wanting to build versions of their application for more than one platform had few choices: ? Maintain separate code bases for each platform, written to the platform39。 Windows174。 version , Microsoft Windows NT174。s native application programming interface (API). ? Write to a virtual API such as those provided by crossplatform toolsets. ? Build their own multipleplatform layer and support it. Today, however, a new choice exists. Developers can use their existing code written to the Windows API and, using tools available from Microsoft and third parties, repile for all of the platforms listed above. This paper looks at the methods and some of the issues involved in doing so. Currently the most lucrative market for graphical user interface (GUI) applications, after Microsoft Windows, is the Apple Macintosh. However, vast differences separate these wholly different operating systems, requiring developers to learn new APIs, programming paradigms, and tools. Generally, Macintosh development requires a separate code base from the Windows sources, increasing the plexity of maintenance and enhancement. Because porting code from Windows to the Macintosh can be the most difficult porting case, this paper concentrates in this area. In general, if your code base is sufficiently portable to enable straightforward repiling for the Macintosh (excluding any platformspecific, or edge code, you may elect to include), you39。 API libraries, which thunk 32bit calls down to their 16bit counterparts. Developers who want their applications to be able to take advantage of the hot new RISC hardware, such as DEC Alpha AXP machines, can use the special multiple platform editions of Visual C++. These include versions for the MIPS R4000 series of processors as well as the aforementioned DEC Alpha AXP chip and the Motorola Power PC. These toolsets run under Windows NT and create highly optimized native Win32 applications for DEC Alpha and Motorola PowerPC platforms. Developers who have repiled their Win32 sources using these toolsets are amazed at how simple it is. Since the operating system is identical on all platforms, and the tools are identical, little work has to be done in order to achieve a port. The key difference in the RISC machines from Intel is the existence of a native 64bit integer, which is far more efficient than on 32bit (that is, Intel) processors. Microsoft works closely with two thirdparty UNIX tools providers, Bristol Technology and Mainsoft Corporation, to allow developers to repile their Win32based or MFCbased applications for UNIX. Developers seeking additional information should contact those panies directly. You39。t know C++. Can I still use MFC? Sure. MFC is based on C++, but you can mix C and C++ code seamlessly. ? How can I get started using MFC? Start by taking some classes and/or reading some books. Visual C++ ships with a fine tutorial on MFC (Scribble). Then, check out the MFC Migration Kit (available on CompuServe or, for a modest shipping and handling fee, from Microsoft). It will help you migrate your Cbased application code to MFC and C++. All porting will be easier if you begin today writing more portable programs. Following some basic portability guidelines will make your code less platformspecific. Never assume anything. Particularly, don39。t assume any particular byte order. contains constants that can be used to help write platformindependent macros to access individual bytes in a word. This may seem obvious, because nothing could be less portable than assembly language. Compilers, such as Microsoft Visual C++, that provide inline assemblers make it easy to slip in a little assembler code to speed things up. If you want portable code, however, avoid this temptation. It may not be necessary. Modern pilers can often generate code as good as handtuned native assembler code. Our own research at Microsoft indicates that performance problems are more often the result of poor algorithms than they are of poor code generation. Indeed, with RISC machines, handturned native assembler code may actually be worse than machinegenerated code, due to the plexity of instruction scheduling and picking