I want to use A5 for extened instructions,such as dec dptr,how to use, thank you!
...it will no longer be supported by the Keil tools - or, in fact, any standard 8051 tools.
Yes it will - it's just that the extended instructions will not be used. Hopefully, the chip would be a strict super-set.
Yes, of course - thus rendering the whole operation pointless!
;-)