PC bootup Presented by: Rahul Garg (2003CS10183) Rajat Sahni (2003CS10184) Varun Gulshan(2003CS10191)
PC bootup
Presented by:Rahul Garg (2003CS10183)Rajat Sahni (2003CS10184)Varun Gulshan(2003CS10191)
From power on to Kernel load
PC firmware or BIOS.
bootSect.S / Bootloader.
Boot loader to kernel.
Memory Map at Power up
MAPPED TO ROM
FREE SPACE
FREE SPACE
0xA0000
0x100000
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.
Start up Sequence
CS:IP = 0xf000:fff0. Starts in real mode.
16 bit mode. IP = 16 bit offset CS = Segment offset in 16 byte units. i.e PC = 16*CS+IP Max: 1MB of address space. A20 pain from the past.
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.
BIOS contd…
Looks for bootable media. Loads Boot sector(512 bytes) of the
media at 0x7c00 . Jumps to CS:IP = 0000:7c00 with
DL=drive id of bootable drive.
BIOS data area from 0x0000 to 0x7c00. (Contains IDT,ISR’s and data).
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.
Memory Map at this stage
BIOS DATA
BOOT SECTOR
BIOS ROMVGA
0x0000
0x00100000 (1MB)
0xA0000 (640KB)
0x7C00
FREE SPACE
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
Flow Diagram
BIOS
Bootsect.S
Stage 1
Stage 2
Setup.S
Head.S
Jumps to init
Booting with bootloader
Part of Kernelimage
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
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)
How head.S calls init?
Gunzips kernel image Enables paging, check CPU type. Get kernel params from memory Start_kernel() finally which calls init !
Memory Map during booting procedure
Bios Data
Bootsect.S0x7c00
Bootsect.S0x90000
setup.S
1 MB
Compressed kernel image
Head.S
UncompressedKernel image
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)
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…..
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.
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.
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
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
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
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!)
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
EFI : Concept
EFI Abstracts out hardware for OS loaders
EFI : Overview
EFI specification defines a set of API’s and data structures EFI device drivers EFI diagnostics and system utilities EFI shell OS loaders OS
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.
Loading Linux Kernel using ELILO
EFI initializes system directly in protected mode
Kernel loaded using EFI calls
ExitBootServices() to transfer control to kernel
EFI
EFI Boot Mgr
ELILO Load Kernel
Collect BootParams
Start_kernelJump to kernel
init
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