Since the construct pre-dated long-displacement, it's quite possible that it means very little with respect to long displacement. Or perhaps, for long displacement, the "end" is to some extent ignored (although it might be used to select from among multiple choices).
I think that the main reason the construct was introduced was so that the coder could make it clear which register to use when there might be overlap if the entire 4K range was in play. For example, USING *,RegA USING *+1000,RegB Produces ambiguity when referencing anything in the offset range 1000 through 4095, which can result in ** ASMA303W Multiple address resolutions may result from this USING and the USING on statement number ... As opposed to USING (*,*+1000),RegA USING *+1000,RegB Has anyone not been burned (in particular, prior to that warning message) by having coded USING *,RegA USING *+4096,RegB ... * Failed to "DROP" * Some place where RegA,RegB does not have the right value BASR RegC,0 USING *,RegC ... Some reference that happened to be closer to RegA+x'1000' (i.e, RegB) than to RegC so that the assembler used RegB as the base reg Peter Relson z/OS Core Technology Design
