OS Boot Sequence and File System (implication to “Boot Sector Viruses”) Department of Computer Science Southern Illinois University Edwardsville Spring, 2016 Dr. Hiroshi Fujinoki CS 314 Operating Systems OS-Boot/000

OS Boot Sequence Step 1: Boot ROM Mother Board Boot ROM Processor BIOS Codes (Disk I/O Subroutines) I/O Device (e.g., HDD) I/O Device (e.g., HDD) I/O Device (e.g., HDD) Are you there? POST Device polling CS 314 Operating Systems OS-Boot/001

OS Boot Sequence Step 1: Boot ROM CS 314 Operating Systems OS-Boot/003

OS Boot Sequence Step 1: Boot ROM CS 314 Operating Systems OS-Boot/004

Mother Board Boot ROM Processor BIOS Codes (Disk I/O Subroutines) Track Drive spindle hole Sector Memory Bootable Device The very first physical sector of this physical drive MBR (Master Boot Record) OS Boot Sequence Step 2: Load & Execute MBR CS 314 Operating Systems

MBR (Boot Strap Loader) Program Code Area Partition table Scan the partition table Find which partition has OS Jump to the OS (OS boot sector) Partition Information Where in this drive this partition starts The type of partition (OS bootable?) Partition Information Where in this drive this partition starts The type of partition (OS bootable?) Partition Information Where in this drive this partition starts The type of partition (OS bootable?) Partition Information Where in this drive this partition starts The type of partition (OS bootable?) OS Boot Sequence Step 2: Load & Execute MBR CS 314 Operating Systems OS-Boot/006

OS Boot Sequence Step 2: Load & Execute MBR CS 314 Operating Systems OS-Boot/007

Mother Board Boot ROM Processor BIOS Codes (Disk I/O Subroutines) Track Memory OS Boot Sector Bootable Device The very first physical sector of this physical drive MBR (Master Boot Record) OS Boot Sequence Step 3: Load & Execute OS Boot Loader CS 314 Operating Systems OS-Boot/008

OS Boot Sector “JUMP XXXX” instruction OS Loader (IPL: Initial Program Loader) Load OS to memory Initialize OS Start the OS Logical disk parameter block XXXX 0000 File system type Size of the root directory Other information Number of sectors available - Cluster size OS Boot Sequence Step 3: Load & Execute OS Boot Loader CS 314 Operating Systems OS-Boot/009

OS Boot Sequence Step 3: Load & Execute OS Boot Loader CS 314 Operating Systems OS-Boot/010

File System Implementation  Structure of a file system Disk (Disk Platter) Physical Drive Capacity CS 314 Operating Systems OS-Boot/011

File System Implementation  Structure of a file system Physical Drive Capacity MBR Code Partition Table Partition #1Partition #N    Logical Drive Size of a partition The first sector for a partition The last sector for a partition Type of file system used for this partition Information if a partition is “bootable” Jump to the first bootable partition “BOOT-strap Loader” CS 314 Operating Systems OS-Boot/012

File System Implementation  Structure of a file system Physical Drive Capacity MBR Partition Table Partition #1Partition #N    Logical Drive Boot Block Super Block Free Space Management File Allocation Information The Root Directory Other files & directories Logical Drive A.K.A. “Boot Sector” Read (load) drive parameters Load OS kernel files & execute them IPL: Initial Program Loader (“Boot Strap”) CS 314 Operating Systems OS-Boot/013

System Boot Sequence  You turn on power  CPU jumps to the beginning of BIOS ROM  CPU executes POST and initializes hardware - Memory access latency, initialize video card, etc.  CPU executes BIOS routine to load MBR  CPU jumps to the routine in MBR  MBR contains a routine (“Boot Strap Loader”) to check the partition table - Find out which logical drive is the system boot drive - Load the boot block (“Boot Strap”) of the boot drive and CPU jumps to it - The boot block contains a routine to start OS (= start loading OS system files & drives) CS 314 Operating Systems OS-Boot/014

File System Implementation  Structure of a file system Physical Drive Capacity MBR Partition Table Partition #1Partition #N    Logical Drive Boot Block Super Block Free Space Management File Allocation Information The Root Directory Other files & directories Logical Drive Power-On CS 314 Operating Systems OS-Boot/015

Boot-Sector Viruses & Its Significance Mother Board Boot ROM Processor BIOS Codes (Disk I/O Subroutines) Track Drive spindle hole Sector Memory BIOS Boot Strap OS (& drivers) OS Loader Bootable Device Applications OS Disk I/O library Boot-Sector Viruses MBR CS 314 Operating Systems

OS Boot Sector “JUMP XXXX” instruction OS Loader (IPL: Initial Program Loader) Load OS to memory Initialize OS Start the OS Logical disk parameter block XXXX 0000 File system type Size of the root directory Other information Number of sectors available - Cluster size Virus Code YYYY “JUMP YYYY” instruction “JUMP XXXX” instruction Disable firewall Modify virus-detection program Boot-Sector Viruses & Its Significance Modify OS system calls CS 314 Operating Systems OS-Boot/017

Boot-Sector Viruses & Its Significance CS 314 Operating Systems OS-Boot/018