When do I use xdata?

Question

I am new at embedded system programming. I am working on a device that uses an 8051 chipset. I have noticed in the sample programs that when defining variables, sometimes they u

Answer 1

The 8051 has a 128 byte range of scratch pad "pseudo-registers" that (most) compilers use as the default for declared variables. But obviously this area is very small, and you want to be able to put variables in the 16 bit memory address space too. That's what the xdata (i.e. "external data") specifier is for. What to put where depends, obviously, on what the data is and how you plan on using it.

Basically, I think this is the wrong question. You need to understand your CPU architecture first before learning how to use the C compiler's 8051-specific features.