RTEMS Ada User's Guide
Some processors do not support a separate stack for interrupts. In this case, without special assistance every task's stack must include enough space to handle the task's worst-case stack usage as well as the worst-case interrupt stack usage. This is necessary because the worst-case interrupt nesting could occur while any task is executing.
On many processors without dedicated hardware managed interrupt stacks, RTEMS manages a dedicated interrupt stack in software. If this capability is supported on a CPU, then it is logically equivalent to the processor supporting a separate interrupt stack in hardware.
RTEMS Ada User's Guide
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