Where as all defined SFR's (IE ones that aren't reserved) fit within the range of S:080-S:0FF. So (considering the available IP variants) how can one address or even access S:000-S:07F or S:100-S:1FF? There doesn't appear to be a mechanism listed in the manuals I've looked at (IE the only instructions that can access the SFR's are direct 8bit address byte instructions and they have the range of S:080-S:0FF). Am I missing something or does that sum up the fact although said to exist no SFR's can be really addressed beyond the MCS51 range in the MCS251 architecture?
Stephen
Your assumption is correct: the compiler/assembler (for example CX51) decides when to use a prefixed instruction based on the sfr address. The same is true for bits of extended sfrs.