Editing Interrupt handler (section)

== Stack space considerations==
In a low-level microcontroller, the chip might lack protection modes and have no [[memory management unit]] (MMU). In these chips, the execution context of an interrupt handler will be essentially the same as the interrupted program, which typically runs on a small stack of fixed size (memory resources have traditionally been extremely scant at the low end). Nested interrupts are often provided, which exacerbates stack usage. A primary constraint on the interrupt handler in this programming endeavour is to not exceed the available stack in the worst-case condition, requiring the programmer to reason globally about the stack space requirement of every implemented interrupt handler and application task.

When allocated stack space is exceeded (a condition known as a [[stack overflow]]), this is not normally detected in hardware by chips of this class. If the stack is exceeded into another writable memory area, the handler will typically work as expected, but the application will fail later (sometimes much later) due to the handler's side effect of memory corruption. If the stack is exceeded into a non-writable (or protected) memory area, the failure will usually occur inside the handler itself (generally the easier case to later debug).

In the writable case, one can implement a sentinel stack guard—a fixed value right beyond the end of the legal stack whose value ''can'' be overwritten, but never will be if the system operates correctly. It is common to regularly observe corruption of the stack guard with some kind of watch dog mechanism.  This will catch the majority of stack overflow conditions at a point in time close to the offending operation.

In a multitasking system, each thread of execution will typically have its own stack. If no special system stack is provided for interrupts, interrupts will consume stack space from whatever thread of execution is interrupted.  These designs usually contain an MMU, and the user stacks are usually configured such that stack overflow is trapped by the MMU, either as a system error (for debugging) or to remap memory to extend the space available. Memory resources at this level of microcontroller are typically far less constrained, so that stacks can be allocated with a generous safety margin.

In systems supporting high thread counts, it is better if the hardware interrupt mechanism switches the stack to a special system stack, so that none of the thread stacks need account for worst-case nested interrupt usage. Tiny CPUs as far back as the 8-bit [[Motorola 6809]] from 1978 have provided separate system and user stack pointers.