a) The address passed in CX must be a real mode segment address, not a selector. Consequently, the interrupt handler must either reside in DOS memory (ie below the 1 MB boundary) or the client must allocate a real mode callback address. See DPMI functions 0100h and 0303h.
b) If the interrupt being hooked is a hardware interrupt, the memory that the interrupt handler uses should be locked.
c) DOS/32 Advanced DPMI will temporarily disable the Null-Pointer Protection (if it was enabled) in order to modify the first four interrupt vectors, INTs 00-03h.