The Data,idata,xdata,pdata difference of 51

 DATA: fixed refers to the front 0x00-0x7f 128 RAM, can use the ACC direct read and write, the fastest, the generated code is also minimal. idata: fixed refers to the previous 0X00-0XFF 256 RAM, where the top 128 and data 128 exactly the same, only because the way the access is different. The idata is accessed using a pointer similar to C. The statements in the assembly are: MOX ACC, @Rx.

XDATA: external expansion ram, generally refers to the external 0x0000-0xffff space, with dptr access.

PDATA: External expansion Ram low 256 bytes, address appears in A0-a7 read and write, with Movx ACC, @Rx Read and write. This is very special, and C51 as if there is a bug, it is recommended to use less. But also has his merit, the concrete usage belongs to the intermediate question.