PC bootup

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PC bootup
Presented by Rahul Garg (2003CS10183) Rajat
Sahni (2003CS10184) Varun Gulshan(2003CS10191)
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From power on to Kernel load

• PC firmware or BIOS.
• bootSect.S / Bootloader.
• Boot loader to kernel.

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Memory Map at Power up
FREE SPACE
0x100000
MAPPED TO ROM
0xA0000
FREE SPACE
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ROM shadowing

• The address space occupied by BIOS Roms is not
  available as useful RAM.
• BIOS can make use of this RAM to shadow its ROM.
• Write protects this shadowed RAM, disables ROM.

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Start up Sequence

• CSIP 0xf000fff0.
• Starts in real mode.
• 16 bit mode.
• IP 16 bit offset
• CS Segment offset in 16 byte units.
• i.e PC 16CSIP
• Max 1MB of address space.
• A20 pain from the past.

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Start up Sequence

• POST(Power on self test)
• Looks for video card and executes its BIOS.
• Looks for other option ROMS e.g IDE disk.
• Does more system inventory e.g COM ports, setting
  hard disk params.
• Plug and play support.
• Sets up IDT and the interrupt service routines.

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BIOS contd

• Looks for bootable media.
• Loads Boot sector(512 bytes) of the media at
  0x7c00 .
• Jumps to CSIP 00007c00 with DLdrive id of
  bootable drive.
• BIOS data area from 0x0000 to 0x7c00. (Contains
  IDT,ISRs and data).

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BIOS system calls

• BIOS provides low level I/O routines through
  interrupts.
• Main services are
• INT 15h Get memory map.
• INT 13h Disk I/O interrupts.
• INT 19h Bootstrap loader.

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Memory Map at this stage
0x00100000 (1MB)
BIOS ROM VGA
0xA0000 (640KB)
FREE SPACE
BOOT SECTOR
0x7C00
BIOS DATA
0x0000
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Structure of MBR

• MBR 512 bytes
• 0-62 bytes BIOS Parameter Block (BPB)
• 63-445 bytes Bootsect.S
• 446-510 bytes Partition table
• 511-512 bytes Boot signature 0xaa55

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Flow Diagram
BIOS
Booting with bootloader
Stage 1
Bootsect.S
Stage 2
Setup.S
Part of Kernel image
Head.S
Jumps to init
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Role of Bootsect.S

• Moves itself to 0x90000
• Get disk parameters (passed by BIOS) and sets up
  stack
• Loads setup.S right after itself (0x90200)
• Loads compressed kernel image at 0x100000 (1 MB)
• Jumps to setup.S

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How kernel starts?

• Control starts in setup.S in real mode
• Copies system data (Memory maps, drive
  information, hardware support, APM support) into
  appropriate memory locations through BIOS calls
• Switches to protected mode
• Jumps to head.S at 0x100000 (1 MB)

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How head.S calls init?

• Gunzips kernel image
• Enables paging, check CPU type.
• Get kernel params from memory
• Start_kernel() finally which calls init !

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Memory Map during booting procedure
Uncompressed Kernel image
Compressed kernel image
Head.S
1 MB
setup.S
Bootsect.S
0x90000
Bootsect.S
0x7c00
Bios Data
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Booting linux kernel using bootloader

• Bootloader resides in the MBR instead of
  bootsect.s
• Arranges memory layout same as bootsect.S (in
  case of linux kernel)
• Loads kernel from filesystem
• Pass appropiate parameters (e.g. ramdisk_image,
  cmd_line_ptr) to the kernel
• Jumps to setup.S (with state same as achieved via
  bootsect.S)

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Biblical version Grub

• In the beginning, there was GRUB
• And GRUB loaded the kernel
• And kernel begat init
• And init begat rc
• And rc begat network and httpd and getty, And
  getty begat login, and login begat shell and then
  there was light..

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GRUB

• GRUB multistage bootloader due to large size
• Stage 1 (Loaded from MBR)
• Stage 1.5 (Optional, gets stage 2 from file
  system)
• Stage 2
• BIOS passes the drive ID to bootloader through DL
  register.
• Stage 1 loads and jumps to start.S of stage 1.5/2
  at address 0x8000.

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GRUB stage 2

• Start.S loads stage2 at address 0x8200
• Start.S then jumps to asm.S
• asm.S jumps to the C file stage2.c and also
  provides helper functions to get system
  information from bios.
• Many switches from real mode to protected mode
  and back are required to accomplish all this.

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Multiboot

• GRUB can also boot multiboot conforming kernels
  (e.g. GNU Mach, oskit) .
• Multiboot Specification Interface between
  bootloader and kernel image so that any compliant
  loader can load any compliant kernel.
• Three main aspects
• OS image format
• Machine state
• Boot Information Format

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OS Image format

• Ordinary 32 bit executable, but may be loaded at
  non default load address.
• Contains multiboot header
• load address
• Entry point
• Parameters requested
• Preferred graphics mode

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Machine State

• Particular values are required to be loaded into
  the registers like
• Pointer to boot info structure (EBX)
• Other register values
• EAX contains multiboot Magic Value
• Enable A20 Gate
• Set EFLAGS, segment register
• OS image must define the undefined registers
• ESP, GDTR, IDTR

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Boot Information Structure

• Contains the parameters that kernel requested
  through the header.
• Available Memory and memory map
• Boot Device
• Cmdline Parameters passed to kernel
• Other important info (apparently not so important
  that we list them out!)

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Booting A messy procedure !

• Booting in real mode to maintain backward
  compatibility.
• Frequent switches between real mode and protected
  mode
• Bootloader code becomes messy and involves a lot
  of hackwork
• BIOS 25 yrs old
• Solution Extensible Firmware Interface

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EFI Concept
EFI Abstracts out hardware for OS loaders
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EFI Overview

• EFI specification defines a set of APIs and
  data structures
• EFI device drivers
• EFI diagnostics and system utilities
• EFI shell
• OS loaders
• OS

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Booting in EFI Environment
OS Loader is a special ELF binary which calls
ExitBootServices() which transfers control to
the OS. Elilo is an example of an EFI Compliant
boot loader which can load linux kernel.
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Loading Linux Kernel using ELILO
EFI

• EFI initializes system directly in protected mode
• Kernel loaded using EFI calls
• ExitBootServices() to transfer control to kernel

EFI Boot Mgr
ELILO
Load Kernel
Collect Boot Params
Jump to kernel
Start_kernel
init
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References

• Linux HOWTOs (www.tldp.org)
• Grub documentation and source code
• www.bioscentral.com
• www.linuxjournal.com
• www.kernelthread.com
• www.intel.com/technology/efi
• EFI Specification

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