I hope I'm on the right track in my understanding of your questions.

I assume you are talking about using a serial EEPROM or serial flash device, and that the serial protocol is something like I2C or SPI. If that is the case, C51 does not have support for accessing these types of devices as a native memory space like it does for data, idata, xdata, etc. You have to write routines to read data from, and write data to, these devices. You have to manage where all the pieces of data are located yourself.

I find it convenient to create a structure definition with the field list having component declarations for each data object I want to address within the device. The structure then becomes the device's data "map" and I use offsetof() macro to yield a data object's address within the device. This makes it an easy matter to adjust data object ordering and sizes, and to add or delete data objects, all without having recalculate addresses by hand.