The stack storage required for a ROM microcode API function differs from call to call. In addition, several APIs themselves make ROM microcode API calls. However, a reserve of 256 bytes should be sufficient for any call.
The stack storage space required for ROM microcode APIs and servicing hardware interrupts also differs from call to call and handler to handler. Hardware interrupts can occur at any time, including while API functions are executing. Any storage space set aside for this purpose is in addition to the storage required for a API function. An additional consideration is that hardware interrupts can be nested. A reserve of 512 bytes, plus the total requirement for local variables within application-supplied extensions, should be sufficient for the microcode APIs and the hardware interrupts.
All local variables are allocated storage from the stack. To determine the stack storage requirement, find the path through the application that allocates the most space for local variables, taking into consideration function-call nesting. (This may not be a trivial process to accomplish.)
The total stack requirement for a 7526 CFR application will be the sum of all the preceding quantities.