[PIC programming]

I suggest you go to the source:

under the column labeled Support, there is a link to Online Discussion Groups. You might sign up as a member and ask your PIC questions in the appropriate group.

But to respond to this question: The PIC processors each have only one working register (W) that that can take part in most of the instructions with an implied address (a bit in the opcode). However, each model also has some number of dedicated (they operate the hardware features) and general purpose storage registers that can provide an input value to an instruction or receive the result of an instruction, but these need to be addressed in the instruction. Since there are only a few bits available for the address, the actual address also includes some bank select bits that have to be set before the addressed instruction operates, to select the correct page of register memory. Some models also include EEPROM registers that hold data even when the processor is powered down, but it takes a small program of specific instructions to store data in these locations. Fetching data from them is simpler.

The data sheet for each model goes through all this in considerable detail.