General :: Process Control From Boot Loader To User Space

For the past few days I was putting effort on understanding the software control flow starting from "Boot loader" to "Linux User space".

I am consolidating the entire process and putting forth in this forum...It would be great if someone can validate this..It might be useful to other new bees too.

Step 1 : Power up the board

Step 2 : The CPU control goes to EEPROM/storage memory where BIOS resides

Step 3: BIOS gets loaded in RAM and gets executed

Step 4: During execution, the selection of Boot device has to be done with the help of BIOS Menu [Blue screen appearance during start up in normal PC's]

Step 5: BIOS shall access the Bootloader stored in boot device [for eg.,Hard disk]. Boot loader is stored in MBR area.

for explanation purpose I take the following configurations

Bootloader = GRUB
Boot Device = Hard Disk

Step 6: GRUB shall be loaded in RAM and gets executed

Step 7: GRUB shall load the KERNEL image to RAM. Kernel image is stored in Hard Disk.

The question of "How the GRUB knows where the Kernel image is stored".

The answer is
1. In the "Grub.config" file, the location of "Kernel Image" and " Ramdisk Image" [which will be discussed later in the section] is being given.

Step 8: Kernel Image followed by Ramdisk Image is loaded in RAM by GRUB bootloader

Step 9: Kernel Image gets executed...During execution, top portion of the code shall make initial hardware initialization and latter part
of the code shall just decompress the Kernel Image

Step 10 : After decompressing the Kernel Image, it shall decompress
the already loaded Ramdisk Image

Ram disk is just creating a temporary hard disk in RAM. The main responsibility includes it consists of minimal driver files, executables, directory structures to created a TEMPORARY ROOT FILE SYSTEM.

This Temporary Root File system shall be used by Kernel Image

1. Execute the executables to access the Hard disk
2. For creating Permanent Root File System in HARD DISK

Step 11 : Kernel Shall look for the file /Linuxrc in Ramdisk. Linuxrc
is a USER script file [not sure]

Step 12: At the end of script file Linuxrc, the Ramdisk shall give the
control to "USER SPACE" [path for writing the script not known]in Linux kernel

Step 13: USER SPACE is the normal shell