Topic       : Pure Assembler Documentation
Author      : John Kormylo
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Subject     : Documentation/Pure Assembler
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View Ref-File[ Interrupt Stack Pointer ]

... is used by I/O interrupt drivers in Supervisor Mode. 

In User (normal) Mode, the USP is used as the stack pointer (A7).  One
cannot access the ISP in User Mode.

Exceptions will cause the the ISP to be used as the stack pointer
and the exception stack frame will be stored on this stack.

One can access the ISP when the MSP is active using the MOVEC
instruction.

See also Stack Pointer, User Stack Pointer,
         Supervisor Stack Pointer and Master Stack Pointer.