How 16-Bit and 32-Bit Programs Multitask in Windows 95

• Microsoft Windows 95

SUMMARY

In Windows 95, all 32-bit applications are scheduled preemptively. Preemptive multitasking allows Windows 95 to switch between 32-bit applications whether those applications are prepared to lose control of the CPU or not. No cooperation between the application and the operating system is required for 32-bit multitasking.

MORE INFORMATION

32-Bit Applications

32-bit application program interface (API) functions and the applications that call those functions are designed to handle the concurrence requirements that come with preemptive multitasking. All internal data structures are either allocated on a per-thread basis, or protected against corruption by semaphores (or critical code sections). This ensures that one 32-bit application does not destroy the data required by another 32-bit application by calling the same APIs at the same time. 32-bit Windows and its APIs allow this because they are reentrant (that is, code can be shared by several programs at the same time).

Windows 95 further protects 32-bit applications against unintentional data loss by running each program in its own address space. This means the data for one application is not accessible to another application. The disadvantage of this protection is that applications cannot share data amongst themselves as freely as if they were running in the same address space and had full access to each other's code and data.

16-Bit Applications

Under Windows 3.x and Windows for Workgroups 3.x, 16-bit applications multitask cooperatively by frequently yielding control of the CPU to other programs. This kind of cooperative multitasking means that an application has control of the CPU until it relinquishes it. Corruption of data structures is prevented by the cooperative task switching.

Under Windows 3.x, Windows for Workgroups 3.x, and Windows 95, all 16-bit applications run in the same address space. By running in the same address space, applications can share resources among themselves, and are, unfortunately, at a greater risk of overwriting data from another application.

Some 16-bit applications depend on the features offered by cooperative multitasking. These applications may make the following assumptions:

• Control of the CPU, display, and other resources is not relinquished until the application itself yields control.
• Resources such as pens and brushes can be created by one application and passed on to another.
• Sharing data between applications can be done by writing to the data area of another application.

NOTE: The Local Reboot feature allows you to use CTRL+ALT+DEL to close an application that has stopped responding.

Windows 95 balances the requirements for backward compatibility, running on a 4-megabyte (MB) computer, and providing preemptive multitasking by retaining much of its 16-bit code base for previous functionality and by adding new functionality to the 32-bit code.

This means that some 32-bit calls are mapped to 16-bit code and vice versa. When a 32-bit application calls 16-bit code, there is a possibility that a poorly designed 16-bit program may cause a 32-bit application to hang.

This problem can occur if both of the following conditions are present:

• A 16-bit application hangs before yielding control.
• A 32-bit application calls an API that is redirected to 16-bit code.