I want to use a larger data segment(not xdata), can I set it in uVision51?
I don't think so. Since Data segment have 128 bytes and only use direct address mode, the option is to use indirect address do reach 256 bytes ( Idata ) or use an external 256 bytes area ( Pdata ). This segments definitions are defined by the hardware and instructions set. So you won't be able to change it. - Alex