Real Mode Not Supported by Windows 3.1

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows version 3.1

SUMMARY

Support for real mode has been removed from Windows 3.1. Many applications designed for Windows 3.0 do not support real mode. The reasons behind this trend toward protected mode include superior memory management and smaller, faster application code.

MORE INFORMATION

The remainder of this article lists the advantages and disadvantages of removing support for real mode from applications.

Advantages of Removing Real Mode Support

1. Protected mode code is smaller, cleaner, and more maintainable. These factors lead to a faster, more responsive, more reliable, system. Code is smaller and cleaner for the following reasons:

   a. The 286 and higher processors can track memory locations in

         hardware, which makes locking and unlocking memory objects
         unnecessary. An object can be locked once when it is allocated
         and unlocked just prior to being freed. Because the object can
         move in memory even when it is locked, it is not necessary to
         bracket each access to an object with lock and unlock calls.
   

   b. Far functions can use simplified function prolog and epilog

         code. For more information on this aspect of protected mode,
         query on the following words:
   

            prod(winsdk) and protected and streamlined
   
      c. Because protected mode code is restricted to running on 286 and
         higher processors, the Microsoft C Compiler -G2 switch can be
         used to generate smaller and faster application code.
   
   

2. Protected mode (both standard and enhanced mode) breaks the "640K barrier." Furthermore, under enhanced mode, Windows uses paged virtual memory to expand available memory by using the system hard disk as a swapping device. The large address space allows applications to have more code and data and allows users to run more applications.

3. Testing is easier because there are fewer Windows modes to test. To fully test a product that runs under real mode, five separate modes must be tested. Real mode itself contributes three of those modes: real mode with no expanded memory, real mode using the small-frame Expanded Memory Specification (EMS) and real mode using the large- frame EMS. The other two modes are standard mode and enhanced mode. Because support for real mode has been eliminated, the same amount of testing effort can concentrate on producing a better product. It also provides an opportunity to develop and test additional enhancements. For more information on EMS, query on the following words:

         prod(winsdk) and ems and developers
   

4. Based on a survey of Windows developers, most developers are targeting only protected mode because Windows performance on 8086- based and 8088-based machines is not satisfactory. Furthermore, these machines cannot address more than 640K of RAM.

5. "Wild writes," write-accesses to memory that incorrectly modify a memory location, can frequently be detected in protected mode through the mechanism of a GP-fault (an unrecoverable application error). It is not possible to detect these errors under real mode. These GP-faults provide information about application bugs before the application is released.

Removing support for real mode also benefits the end user because applications run faster and are more reliable. While small applications run quickly in real mode, larger applications run slowly. However, in protected mode, large applications also run quickly. A collection of large and small applications can be run simultaneously without any loss in speed. For example, when more applications are running simultaneously than can fit in the physical memory installed in the system, the paging mechanism (only available in enhanced mode) intelligently manages virtual memory to keep the most-frequently used memory pages in physical memory. This management speeds up the system.

Disadvantages of Removing Real Mode Support

1. The installed base of 8088-based and 8086-based machines cannot use the software.

2. Code that performs segment arithmetic cannot be used in protected mode. Therefore, some drivers and MS-DOS programs that run in real mode must be rewritten for protected mode, or they cannot be run under Windows 3.1.