1. COMP075-OS2

2. COMP075 OS2 My name: Professor Baker
Or:
Course web page:
Lecture notes and lab assignments will be posted on the course web page
Students are expected to follow the lab report guidelines posted here:

3. Course Outline Assessments Two written test @ 10% 20%
Weeks 7 and 15
Two lab 10% 20%
Ten labs at 5% 50%
Professionalism and comprehension
10%

4. Course Content More operating system concepts, following on OS1
Windows and linux platforms, emphasis on linux
Focus on command line utilities
Scripting with bash and power shell

5. Boot Process

6. Booting
Booting is the process that loads an operating system into memory and activates it
Every device that has an OS has a boot process
Servers, laptops, phones, tablets, routers, appliances
Some are very simple and rigid, others are complex and flexible – even scriptable
All seem like they start from nothing

7. Boot Strap
The term “Booting” derives from an old admonition to “Pull yourself up by your boot straps”
Physically impossible, but illustrates the virtue of self reliance
Programs can't load themselves, because they would have to be loaded to do that
So how do we get started?

8. Pre-Boot Loader Days
In the days before boot loaders, switches on the front of computers allowed programmers to load binary values at arbitrary selected memory locations
In this tedious manner software could be manually loaded and then run via a run button
This was flexible, but error prone and tedious
More desirable to have the computer do this using a boot loader program

9. The Boot Loader Problem
The fundamental problem of using boot loader software to load the OS is that the boot loader itself has to get loaded
Typically this is accomplished by pre-loading the bootloader into some kind of non-volatile memory
This non-volatile memory may be executable, like the ROM BIOS in IBM PC computers
Or may be something that can be transferred to main memory using simple circuitry

10. Examples Atari computers ran the OS from ROM
ROM is slower than RAM, so not a good option for modern OS
But embedded devices do this to avoid boot delay
IBM PC has BIOS in ROM. BIOS can load OS to RAM from other sources
Some early mainframes copied drum based OS to RAM using hard wired circuitry
PDP11 booted from a matrix of diodes. They were all 1's when shipped but could be programed by cutting off diodes for zeroes

11. Examples PDP8 used front panel switches to program the boot loader
Very simple loader obviously

12. Boot Loader Functions Basic function is to load an OS and start it
Most also perform a hardware test to ensure that the system is functioning properly
Power On Self Test or POST
Usually involves memory test
Some load a secondary loader
It may be larger and more sophisticated
Able to offer boot options to users such as multi-boot
Results in multi-stage boot process
Boot loader may load another primary boot loader
Chain loading

13. Current Era, Cisco IOS Cisco IOS boot loader is called ROMmon
ROM monitor
Remember early OS were sometimes called monitors
When powered on runs POST, looks for OS in Flash, ROM or network, then looks for config in NVRAM

14. Booting IOS

15. Booting Apple iOS Multi-stage boot process
Starts with small loader in ROM (BootROM)
Subsequent stages loaded from flash
Boot ROM verifies RSA signature of first level boot loader LLB, and loads it
LLB verifies RSA signature of second level boot loader iBoot, and loads it
iBoot verifies signature of kernel and NAND flash memory and loads them

16. iOS Boot Modes
Normal mode is used if no problems are encountered during boot
Recovery mode is invoked if problems occur
DFU (device firmware update) mode can be invoked by holding certain buttons during boot process
Used for obvious purpose but also to jailbreak the phone
And for forensic analysis

17. Apple iOS Boot Modes
Normal Boot Sequence
DFU Boot Sequence

18. IBM PC Boot History IBM PC introduced in August 1981
Basically the same boot process was retained until very recently
Newer PCs follow the UEFI specification
But that can emulate the original BIOS based process
BIOS based process has many limitations because of the antiquity of the design
But it survived a long time because of flexibility that allowed limitations to be mitigated

19. PC BIOS Boot Sequence On power up control transfers to the ROM BIOS
BIOS conducts POST then looks for source of operating system
BIOS includes portions of the OS that conduct basic IO, so it is capable of looking at devices to locate one that is bootable
These instructions run in limited memory and with limited instruction set
As if on the original PC hardware

20. Bootable Devices System setup stores a boot sequence in BIOS
ie CD, USB, Hard drive
First bootable device from sequence is attempted
Readable and with AA55h at end of first sector
If none found control returns to BIOS
BIOS may then attempt network boot or ROM BASIC
Original IBM PC with no disk drives could boot into a basic interpreter on ROM

21. Booting from Bootable Disk
First sector on disk called Master Boot Record
MBR contains OS independent boot code and partition table
Conventionally looks for partition marked “active”, loads its first sector (VBR)
VBR may be OS specific
Windows VBR knows about file systems and can locate next stage loader in file system
Linux systems put GRUB stage 1 in MBR

22. File System Based Booting
The GNU GRUB boot loader stores stage 1.5 in unused sectors between MBR and first partition
First partition starts in sector 63
This allows it to access file systems for configuration, menus, kernel images etc
Space available in MBR is quite limited because of partition table and other things so code contained there must be very simple
Conventional MBR loads VBR which can be more complex because space is less limited

23. Booting without File System Access
Problem then is how do you find the next boot stage, configuration files, or kernel
Early windows loaders required stage 2 to be at fixed location in partition and kernel files to be in first two directory entries
SYS command put them there
Before GRUB the LILO boot loader had a similar problem
LILO had an install command that stored the address of the kernel in the MBR or VBR

24. Booting Windows
As is usual with windows, the sequence varies depending on the windows version
All versions install a conventional OS independent loader in the MBR, which chains to an OS dependent loader in the VBR
The VBR loader has a minimal FS implementation for FAT and NTFS to allow the next stage loaders to be located

25. Booting XP Also applies to NT, S2000, and S2003
MBR has been loaded and causes VBR to be loaded and run.
VBR locates ntldr and gives it control.
Ntldr:
Changes to 32 bit mode
Looks for boot.ini in the root of the filesystem
May contain info for multi-boot menu
Runs ntdetect.com to detect hardware and profiles

26. Booting XP (cont.) ntldr:
Loads hardware abstraction layer (hal.dll and halx.dll)
Also reads HKEY_LOCAL_ MACHINE\SYSTEM registry file and others
Finally loads ntoskrnl.exe (the kernel)
Kernel creates HKEY_LOCAL_MACHINE\HARDWARE registry key based on info from ntldr
At this point system is ready to start services and allow logon

27. Booting Vista (etc.) Applies to 7, 8, S2008 and S2012
As usual BIOS has loaded MBR and it has located and started VBR
VBR starts bootmngr
Bootmngr loads BCD (boot configuration database) from /boot in active partition
BCD is a registry file that gets loaded into HKEY_LOCAL_MACHINE\HARDWARE
BCD may call for multi-boot in which case a menu is displayed

28. Booting Vista (etc.)
Once OS is selected, VBR for its partition may get loaded or ntldr for older MS OS
When ready, winload.exe is invoked to load the actual OS kernel ntoskrnl.exe and device drivers
Or winresume.exe
For windows versions capable of using bitlocker drive encryption the active partition is an unencrypted hidden system partition
Bootmnger and BCD are loaded from this partition, so decryption can be set up for the encrypted windows partition

29. GNU GRUB Grand Unified Boot Loader Linux and Solaris on x86
Free and open source
Includes a command line shell used to install grub from a running system
Commands to identify partitions and assist with installation of GRUB
Has search function to find partition containing kernel
Multi-stage boot process

30. GRUB Stages Stage 1 is installed on the MBR (or a VBR)
Stage 1.5 occupies the remaining unused sectors before the start of partition zero
First partition starts on sector 63
Stage 1.5 has file system code so that configuration and next boot stages can be obtained from a file system on any of the disk's partitions (/boot/grub directory)
Stage 2 presents a menu based on configuration file to allow user to select OS to boot

31. GRUB Actions
Normally user selection boots a linux kernel with parameters from the GRUB configuration
User can override these options at run time
for example to start in run level 1
Or user can access a GRUB command prompt
To boot a Windows system GRUB passes control to the VBR from the windows partition
Windows loader then takes over the boot process

32. Linux Startup
Windows had multi-stage loaders from the VBR, BCD files and other devices to mitigate the bootstrap problems in getting an OS started:
The OS has to have drivers to access disks and filesystems, but the drivers are in the filesystems on the disk
Linux kernels have statically compiled drivers for accessing a RAM disk
When linux is installed a custom ram disk image called initrd is created with the drivers needed to access the installed hardware and filesystems
Linux kernel mounts this image as root (/), loads the needed drivers from it and uses them to mount the real root filesystem

33. References http://www.mossywell.com/boot-sequence/