hi,
since I overgrew my old 89s53 I want to switch to 89C51RC
My congratulations! By the way, have you not enough program or data memory?
I have a question about accessing extra 256 bytes of RAM. Is this PDATA? Do I declare variables to PDATA.
Sure, you may.
Where is then stack starting? On 256 or 512?
I have not seen Atmel`s parts which can access either extra internal or external memory for stack area. There are some Dallas/Maxim chips which can do it - such devices use more than 8-bit stack pointer register.
So, stack pointer may be set only inside 0x00...0xFF of data memory; it is the limit you should live with.
I think it starts on 256 (IDATA),
Stack area is started from where you indicate it yourself with MOV SP,#stack_value. And as long as SP is 8-bit register so you are not able to load more than 0xFF value to there.
so I want to move my variables to PDATA.
Do it then. Just note that it requires some more time than for variables located in "main" memory because different instruction is used (MOVX) and there are some additional commands are involved (here: loading R0/R1/DPTR and swithching the internal/external memory access flag).
I know I must setup some bits first to access to this segment.
Yes, I have already said it above. You need to manipulate with AUXR/EXTRAM bit for grant access for ERAM.

Good days!