Question: Is there any minimum number of clock cycles or accesses (# of CASs) before this can occur if a XPT starts and a SRT or REFRESH becomes pending simultaneously? For example, I would assume that the XPT would at least execute one RAS and CAS cycle before being interrupted by a SRT or REFRESH. Will it execute at least two memory accesses, three, or ? before it gets interrupted and a new row access begins?

Answer: Yes, there is a minimum- it is PTMIN. In this respect, XPTs are no different from any other PT. A minimum of PTMIN cycles must be spent servicing an XPT before it can get interrupted by a higher priority cycle.