Linux Booting

Linux Booting • The booting procedure. • Logging in to the machine. • Control flow in the machine.

Linux Booting • The booting procedure. • Logging in to the machine. • Control flow in the machine.

The booting procedure. • Every computer has sequence of instructions that are executed when computer is powered up. – determined by the hardware designers

• Every OS has a sequence of instructions that it executes prior to normal operation.

The booting procedure. • i386 machines incorporate the Basic Input/Output System (BIOS) – – – –

stored at a prespecified location in system ROM. when computer started, begins executing BIOS code BIOS code executes the Power-On Self Test (POST) POST performs various diagnostic tests to check memory and devices for their presence and correct operation. – when POST is completed, machine ready to load and start OS.

The booting procedure. • The boot sector. – OS loaded from the system’s boot disk. – i386 treats A: as default disk if it contains a diskette. – Otherwise depends on how you’ve set the BIOS.

The booting procedure. • The boot disk – must contain a boot record at the disk’s first logical sector. – The boot record fits onto a single 512-byte disk sector (called the boot sector): 0x000 0x003 0x0XX 0x1fe

0x002 …. 0x1fd 0x1ff

Disk parameters (used by BIOS) program Bootstrap program 0xaa55 (the magic number for BIOS)

The booting procedure. • After POST BIOS copies the boot record from disk into memory • Bios then reads boot record parameters: – Boot record contains parameters that describe disk’s physical layout – Parameters are stored in well-know locations in boot record – Parameters include number of tracks and number of sectors per track.

The booting procedure. • After parameters read BIOS branches to the first location in program – then immediately branches to location 0x0XX – the value XX for Linux is 3e – the value XX for MS-DOS is 1e

• The small program at address 0x0XX loads a more sophisticated loader from other sectors on the disk • The sophisticated loader loads the OS

The booting procedure. • Disk partitioning – can have up to 4 primary partitions on one physical disk • each primary disk can have logical sub-partitions

– each partition treated like a physical disk below the abstract machine that accesses the physical disk (eg, BIOS in MS-DOS) – if partition is bootable then its logical sector number 0 will be a boot sector.

The booting procedure. • Disk partitioning – when hardware is powered up goes to head 0, track 0, sector 1 and begins executing code. – an unpartitioned disk has a jump instruction at head 0, track 0, sector 1 which jumps to location 0x0XX and starts the execution of the boot program – in a partitioned disk, physical head 0, track 0, sector 1 (first logical disk sector) describes how hard disk is partitioned into logical disks.

The booting procedure. – A partitioned disk has a 446-byte bootstrap program starting at first byte in the sector. – next is a 65-byte partition table for the disk – Table contains the 4 partition entries, each of which describes the portion of the physical disk that is used for its partition (starting sector of the partition, ending sector, number of sectors, etc.) – last 2 bytes of partition sector contain a magic number, 0xaa55 to identify the partition sector.

Partitioned Disk head 0 track 0 sector 1

446-byte bootstrap program

65-byte partition table 2 bytes: magic number, 0xaa55 for MS-DOS

4 partition entries, each describes the portion of the physical disk used for its partition

The booting procedure. • Partitioning with different OS’s – i386 can be booted with Linux from boot floppy. – contains a Linux boot program. – Can have different OS in different partitions – Windows handles by designating one partition as an active partition. System always boots from this partition.

The booting procedure. • Partitioning with different OS’s – can have a special program, Linux Loader (LILO) in system boot record. – program allows user to choose at boot time which partition is to be the active partition. – LILO is loaded in the boot record so that BIOS runs it after the POST.

Starting the Kernel • machine is powered up – hardware fetch-decode-execute cycle begins. – called the “hardware process” – this is not a Linux process; Linux has not yet started.

• • • •

POST is completed boot record read loader places OS into primary memory the booting procedure begins to run kernel code to initialize the computer’s hardware.

Starting the Kernel • Computer then starts kernel by setting supervisor mode and branching to main entry point in kernel. • main entry point is not a C program. • kernel initializes – – – – – –

trap table interrupt handler scheduler clock modules etc.

Starting the Kernel • near end of kernel initialization start process manager – – – – –

now ready for normal process abstraction PM starts the initial process initial process is first entry in kernel’s process descriptor table referred to as process 0, task[0], or INIT_TASK initial process then creates first useful Linux process to run: the init program – after init runs it executes an idle loop – only other duties of init is to use idle CPU time….runs when no other process wants CPU.

Starting the Kernel • init process initializes system – – – –

starts daemons starts file manager creates system console runs other init programs from /etc, /bin and/or /sbin, and runs /etc/rc

Logging in to the machine • during initialization kernel creates one process on each communication port that can be used to support user logins – uses the getty program

• getty – – – –

waits for a user to begin using the port then runs login program login expects user id on first line, passwd on next login verifies user by looking in /etc/passwd file

Logging in to the machine • getty – /etc/passwd file. Each entry separated by colon jbarr:eNcrYpt123:3348:35:John Barr:/home/barr:/bin/bash

– jbarr

account name

– – – – –

encrypted password user ID and group ID real name user’s home directory path to shell command preferred by user

eNcrYpt123 3348 and 35 John Barr /home/barr /bin/bash

Logging in to the machine • getty – once login verifies it changes current directory to user’s home directory – and executes the specified shell program – now user interacts directly with login process via shell. – user who logs in is using a process that was created when the machine was started – starting shell is complicated; data and stack segments created etc. – data includes user’s environmental variables (eg, PATH) – when user logs off, return from shell call to the login process

Logging in to the machine 26

Serial Port A

login

Serial Port B

login

Serial Port C

login 5

4

Serial Port Z

1 getty

3 login /etc/passwd

/bin/bash

Control Flow hardware process starts Idle Proc

program

hardware process

context switch

kernel

ISRs

Proc i

Proc j