The 68000 provides a nonmaskable interrupt (NMI) of CPU priority 7.
Although this interrupt cannot be generated by the Amiga hardware itself,
it can be generated on the expansion bus by external hardware.  Because
this interrupt does not pass through the 4703 interrupt controller
circuitry, it is capable of violating system code critical sections.  In
particular, it short-circuits the DISABLE mutual-exclusion mechanism.
Code that uses NMI must not assume that it can access system data
structures.

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