Q77990: Switching Stacks at Interrupt Time

Article: Q77990
Product(s): Microsoft Windows Device Driver Kit
Version(s): 3.0
Operating System(s): 
Keyword(s): 
Last Modified: 03-NOV-1999

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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

                 <handle IRQ>

                 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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Keywords          :  
Technology        : kbAudDeveloper kbWin3xSearch kbWinDDKSearch kbWinDDK300
Version           : :3.0

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