I want to use A5 for extened instructions,such as dec dptr,how to use, thank you!
for using 0xa5 the logical way to do so would be to build a '251 (which I would never do, the XA is a much more intelligent 'expansion' of the '51). The '251 actually uses the 0xa5 to execute '51 instructions in '251 mode and vv.
anyhow there are ever so many that (try to) use a '51 beyond its intended use and when getting in trouble for that reason start "I need to expand the '51" or "how do I make the '51 do this" instead of "OK I chose the wrong processor" or "I will change my app to fit the '51", not to mention "Instead of trying to blame the '51 I will blame myself".
If you implement the processor core in an ASIC, you are of course free to define this opcode yourself, but you will still not get any existing compilers to know about it and to use it. I recall a post: "how do I make the Keil compiler recognice my new and exciting '51 with new and exciting instructions". Oh how many take the first step without thinking of step2.
Erik