Microsoft KB Archive/77990

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Switching Stacks at Interrupt Time

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Q77990

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The information in this article applies to:


 * Microsoft Windows Device Development Kit (DDK) for Windows, version 3.0

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SUMMARY
The Microsoft Windows Multimedia Extensions contain two functions to ease the work of device driver developers. An interrupt service routine (ISR) can use the StackEnter and StackLeave functions to switch stacks at interrupt time and avoid the problems that can occur when an ISR makes even moderate use of the stack. StackEnter and StackLeave are not documented in version 1.0 of the Microsoft Multimedia Device Development Kit (MDDK).

MORE INFORMATION
The size of the stack available to an ISR in a device driver running under Multimedia Windows cannot be guaranteed. For example, the Microsoft Windows kernel maintains a very small stack to use in its housekeeping chores. When a hardware interrupt requiring some stack storage is serviced on this kernel stack, problems can easily result.

To make this situation more realistic, assume that several lower-priority interrupts that also use the kernel stack are pending. In this worst-case scenario, the maximum size of the stack could be exceeded with potentially catastrophic results.

To prevent these catastrophic results, device driver developers should use the StackEnter and StackLeave functions exported by the MMSYSTEM dynamic-link library (DLL). StackEnter saves the current stack selector and stack pointer registers before switching to one of many stacks MMSYSTEM maintains internally for this purpose. StackLeave restores the previously saved SS:SP from the last call to StackEnter.

Calls to these functions may be nested; however, each call to StackEnter must be matched with a call to StackLeave. This is true even if nothing is pushed onto the new stack and even if StackEnter fails.

StackEnter fails if there are no more stacks available from MMSYSTEM. Failure is indicated by the carry flag being set and SS:SP not being modified on return. StackEnter should be the first call performed in an ISR.

The following assembly language code demonstrates the typical uses of StackEnter and StackLeave:

    MyISR proc far

call   StackEnter          ; Switch to a safe stack pusha                      ; Save registers



popa                       ; Restore registers call   StackLeave          ; Return to interrupt stack iret                       ; Done with ISR

MyISR endp The user can specify the size and number of stacks maintained by MMSYSTEM in the [mmsystem] section of the SYSTEM.INI file, as follows:

[mmsystem]

StackSize = 1536

StackFrames = 3 The decimal values shown above are the default values for these parameters. The minimum values allowed are as follows:

StackSize = 64

StackFrames = 1 The maximum allowed values are any product (StackSize * StackFrames) that equals 65,535 bytes (64K - 1).

Additional query words: 3.00

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