I have several questions regarding the use of some bios routines,i want to experiment with the bios routines.I just can't understand the use of some of those.

From the site http://map.tni.nl i found several intersting articles.

Now what is the use of bios routine #0138
RSLREG
Address : #0138
Function : Reads the primary slot register
Output : A - for the value which was read
33221100
||||||- Pagina 0 (#0000-#3FFF)
||||--- Pagina 1 (#4000-#7FFF)
||----- Pagina 2 (#8000-#BFFF)
------- Pagina 3 (#C000-#FFFF)
Registers: A

I have seen that in some konami megaroms when the game begins it has something like

Startaddress of rom: DI
call #0138
RRCA
RRCA
and #03
LD C,A
LD B,#00
etc etc (and thing is that most of those games have the same code there).

Why is it that after the call of the 138 there is that RRCA instrucion that rotates the result?

This piece of code filters out bits 2 and 3 of the A register, previously filled with the slot selection. The code probably wants to know what is mapped in page 1. The RRCA is used twice to shift/rotate the contents of the A register twice, such that bits 2-3 come on positions 0-1. The AND 3 filters this to a clean result, namely the contents of bits 0-1.

So eventually, the C register contains the slot number mapped to page 1