You said 'need to say "I´m not using the memory from XXXX to XXXX and it is inside the chip so please use it as DATA since its very fast"'. The RAM that has the fastest addressing means is the so-called "scratch pad" 'data'/'idata' memory. This has a fixed size and cannot be extended. If I'm understanding you correctly, this internal SRAM you're talking about is the MAC/MOVX and STACK/MOVX memory blocks. These either require the MOVX instructions (i.e., via DPTRs and therefore slow, although I assume not as slow as truly external memory) or using the extended stack pointer, in which case, I don't think that with C51's data overlay "stacking", it would be beneficial. If you want to use leftover MAC/MOVX memory, I don't know why you couldn't just qualify some of your data objects with 'xdata', assuming you've specified XDATA to be located in that region.