Memory mapped area of GPIO in raspberry pi - linux-kernel

Question

I have real struggles with respective to understanding the data sheet part[BCM2835-ARM-Peripherals.pdf] where in which it talks about following:
Q1: Every gpio has 40 registers[refere: Table 6-1 GPIO Register Assignment] which are common for all gpios can be used to SET/CLEAR/GPFSEL[0-5] etc,but what is the purpose of GPIO function select register which has 10 FSEL (0-9) registers [refer: Table 6-2 – GPIO Alternate function select register 0].
Q2: How can I reach to a given GPIO through memorymapped region? , is something like below
Case#1 :GPIO4 with alternate function 5 as Input:
as ( 4 < 10) it will use "0x7E200000" with FSEL4 register[refer: Table 6-2 – GPIO Alternate function select register 0] having 000 and 010 = GPIO Pin 9 takes alternate function 5.
case#2 :GPIO27 with alternate function 3 as Output:
as ( 27 < 30) it will use "0x7E20000C" with FSEL3 register[refer: Table 6-2 – GPIO Alternate function select register 0] having 001 and 111 = GPIO Pin 9 takes alternate function 5.
Please let me know whether my understanding is correct or not?
Q3: what is the purpose of following tables
Table 6-3 – GPIO Alternate function select register 1
Table 6-4 – GPIO Alternate function select register 2
Table 6-5 – GPIO Alternate function select register 3
Table 6-6 – GPIO Alternate function select register 4
Table 6-7 – GPIO Alternate function select register 5
BR,
&Sanumala

Answer 1

http://elinux.org/RPi_GPIO_Code_Samples
//
// Set up a memory regions to access GPIO
//
void setup_io()
{
/* open /dev/mem */
if ((mem_fd = open("/dev/mem", O_RDWR|O_SYNC) ) < 0) {
printf("can't open /dev/mem \n");
exit(-1);
}
/* mmap GPIO */
gpio_map = mmap(
NULL, //Any adddress in our space will do
BLOCK_SIZE, //Map length
PROT_READ|PROT_WRITE,// Enable reading & writting to mapped memory
MAP_SHARED, //Shared with other processes
mem_fd, //File to map
GPIO_BASE //Offset to GPIO peripheral
);
close(mem_fd); //No need to keep mem_fd open after mmap
if (gpio_map == MAP_FAILED) {
printf("mmap error %d\n", (int)gpio_map);//errno also set!
exit(-1);
}
// Always use volatile pointer!
gpio = (volatile unsigned *)gpio_map;
} // setup_io