本质是movx指令，可用_AT_，XBYTE （absacc.h）控制读写。
//外部xram写
void xram_write(unsigned int addr, char byte)
{
char xdata * data pwrite; // write pointer
pwrite = (char xdata *) addr; // initialize write pointer
*pwrite = byte; // write the byte
}
//外部xram读
char xram_read(unsigned int addr)
{
char xdata * data pread; // read pointer
pread = (char xdata *) addr; // initialize read pointer
return *pread; // read the byte
}