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

This article was previously published under Q117567
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In Windows 95, all 32-bit applications are scheduled preemptively.Preemptive multitasking allows Windows 95 to switch between 32-bitapplications whether those applications are prepared to lose control of theCPU or not. No cooperation between the application and the operating systemis required for 32-bit multitasking.

32-Bit Applications

32-bit application program interface (API) functions and the applicationsthat call those functions are designed to handle the concurrencerequirements that come with preemptive multitasking. All internal datastructures are either allocated on a per-thread basis, or protected againstcorruption by semaphores (or critical code sections). This ensures that one32-bit application does not destroy the data required by another 32-bitapplication by calling the same APIs at the same time. 32-bit Windows andits APIs allow this because they are reentrant (that is, code can be sharedby several programs at the same time).

Windows 95 further protects 32-bit applications against unintentional dataloss by running each program in its own address space. This means the datafor one application is not accessible to another application. Thedisadvantage of this protection is that applications cannot share dataamongst themselves as freely as if they were running in the same addressspace 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 applicationsmultitask cooperatively by frequently yielding control of the CPU to otherprograms. This kind of cooperative multitasking means that an applicationhas control of the CPU until it relinquishes it. Corruption of datastructures is prevented by the cooperative task switching.

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

Some 16-bit applications depend on the features offered by cooperativemultitasking. 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.
Because of this dependency on cooperation to switch between applications,if a 16-bit application stops responding (hangs) before it has yielded toanother application, it may cause Windows to hang as well. The ApplicationLocal Reboot feature introduced in Windows 3.1 helps alleviate this problemby closing the unresponsive application and allowing Windows and the otherrunning programs to continue. It is important to note that the Local Rebootfeature does not always work; its success depends on the severity of theproblem that caused the particular application to hang.

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

Windows 95 balances the requirements for backward compatibility, running ona 4-megabyte (MB) computer, and providing preemptive multitasking byretaining much of its 16-bit code base for previous functionality and byadding 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 apoorly 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.
To work around this problem, use the Local Reboot feature to close the 16-bit application that has stopped responding. Improvements have been made tothe Local Reboot feature in Windows 95 to make 16-bit applications morerobust.
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Article ID: 117567 - Last Review: 12/04/2015 10:19:17 - Revision: 1.1

Microsoft Windows 95

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