assembler or C?
It doesn't matter. What matters is that if you approach the programming with a C attitude (the compiler will take care of it for me) you will fail.
If you choose C and decide to forget about DATA, IDATA and XDATA you will fail for sure.
absolutely

In PC C
PC C and microcontroller C are completely different things why do people keep confusing the two, a PC C programmer will have no clue when it comes to microcontroller programming because it is highly hardware dependent whil PC C you don't need to understand the hardware. However in both situations you need to understand how the compiler thinks to be able to write efficient programs.

Mahmood