While booting up, the Raspberry Pi reads some configuration parameters from the SD card. These parameters are stored in a file named config.txt. This file is within the part of the SD card which is visible to Windows PCs. Therefore, you can edit this configuration file from a Windows PC, from a Mac, from a Linux PC, or from within the Raspberry Pi itself.
This configuration file contains instructions which are necessary to setup the display. If you have severe problems with the display, you will need to connect the SD card to a PC so that you can edit the configuration file. If the display is working but you wish to make minor adjustments, you may find it more convenient to edit the file from within the Raspberry Pi itself.
sudo nano /boot/config.txt
here some boot options
disable_commandline_tags Stops start.elf from filling in ATAGS (memory from 0×100) before launching kernel
cmdline (string) Command line parameters. Can be used instead of cmdline.txt file
kernel (string) Alternative name to use when loading kernel.
kernel_address address to load kernel.img file at
ramfsfile (string) ramfs file to load
device_tree_address address to load device_tree at
init_uart_baud initial uart baud rate. Default 115200
init_uart_clock initial uart clock. Default 3000000
init_emmc_clock initial emmc clock. Default 50000000 (50mhz default. increasing this can speedup your SD-card)
boot_delay Waits for given number of seconds in start.elf before loading kernel.img.