Linux Boot Notes

The following are my notes as to how I think things work, they may be wrong.

For someone with a familiarity with MS-DOS but not (yet) with Linux see DOStoLinux-HOWTO

Sequence of events

• Power ON
  • Power on Reset causes CPU to jump to reset vector in BIOS
  • BIOS starts executing to brings up system hardware
    • Sets timer and starts DRAM refresh
    • looks for keyboard, display(initializes)
    • looks for and tests RAM
    • finds com/printer ports and builds tables
    • catalogs mass storage, floppies, hard drives
  • BIOS rund Plug&Play (if present) to configure system P&P resources
  • BIOS loads sector 0, cylinder 0 of  primary drive  (Master Boot Record) and transfers execution to that code

• MBR either simply loads rest of OS or else loads a secondary loader (called a boot loader) like LILO. Note that this implies a rudimentary file structure/system. At least enough to know what sector/cylinders to load to continue.

• If LILO was loaded, it looks at /etc/lilo.config to decide what to do. It either loads another OS boot (DOS for example) or else loads a kernel and transfers execution to it.
  
  Assume we are loading Linux from this point on.

  
  Boot Disk and Kernel
  The kernel might come from any mass storage source, floppy or hard drive (or ROM?). We call this source the boot disk. The boot disk may load the kernel via another boot loader(like LILO). But wherever it comes from, this boot source has a kernel on it. The kernel is one of three essential Linux pieces, the other two being the file system and the shell.See Bootdisk-HOWTO
  The kernel uses a pointer to / (root). to know where the file system is. This pointer could be compiled in or else it might have been set by rdev or it might have been passed in to the kernel by LILO.
  LILO could pass in a  command line: linux ah152x=0x340,11,7,1 root=/dev/sda1 (note the spaces between arguments and the commas between parameters. This example would both set the root pointer as well as pass some h/w specific information to the kernel. See BootPrompt-HOWTO  and  /usr/doc/HOWTO/mini/LILO).
  
  "rdev zImage /dev/sda1"  would set the file syem pointer to the first partition of a SCSI disk,  ("rdev <filename>" will return that pointer). rdev is able to set  the five special values which are stored in the kernel itself. These are:

  • root device
  • swap device
  • RAM disk parameters
  • default video mode
  • readonly/readwrite setting of root device

• looks for special  'rdev' values
• sorts special h/w setup functions (like ah152x=0x340,11,7,1)
• interprets environment variables
• remaining arguments  (like 'single' for single user) passed into process #1

• 
  Root Disk
  

• The device containing /  (the root) is known as the Root Disk and provides
  • file system  (FS) support for the kernel: /dev, /proc, /bin, /etc, /lib, /usr, /tmp
  • Basic Utilites, bash, ls, cp etc
  • Runtime Lib which has the basic funtions used by utilities