For the first question ,San already give the solution about memory structure.

Another which stuck in your mind is how assembler define memory ,isn't it?
I 've investigated 51's manual ,The instruction which operate on banked register and direct memory content are not the same .I mean that CPU will look at instruction and treat them different!

For example
add a,rn
if you use register bank 0
this will be 0010 1 000
add register bank No.
1 byte instruction

add a,0h
this will be 0010 0101 00h
add direct data location
2 bytes instruction

Your assembler doesn't define out whether location of address is but make a register bank operate as a different way.

Hope this make you clear !!