I want to use A5 for extened instructions,such as dec dptr,how to use, thank you!
"I want to have similar extened instructions"
Then you will have to design your own processor, and design-in those instructions.
This forum is for Keil tools; not processor or instruction set design - so you really need to find a more appropriate forum.
Of course, once you've redesigned the instruction set, it will no longer be supported by the Keil tools - or, in fact, any standard 8051 tools. So, again, you will have to find another forum to discuss tools & programming for your new processor.
...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!
;-)