ID Number: Q77990
1.00
WINDOWS
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 reference words: 1.00