RTEMS C User's Guide
Some processors support a separate stack for interrupts. When an interrupt is vectored and the interrupt is not nested, the processor will automatically switch from the current stack to the interrupt stack. The size of this stack is based solely on the worst-case stack usage by interrupt service routines.
The dedicated interrupt stack for the entire application is supplied and initialized by the reset and initialization code of the user's board support package. Since all ISRs use this stack, the stack size must take into account the worst case stack usage by any combination of nested ISRs.
RTEMS C User's Guide
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