Main Algorithms of a Linux Kernel

Signals

int send_sig_info(int sig, struct siginfo *info, struct task_struct *);

sig – refers the signal number

info – refers the sender

t – refers to the tasks (the kernel may send signals to many processes)

Booting the System

There are many bootloaders available for linux, the common ones being the LILO and the GRUB loader

LILO – LInux LOader

GRUB – GRand unified Bootloader

The steps while booting the kernel (only relevant steps are given)

Entry point at start which is available at arch/x86/boot/setup.S (This is responsible for initializing the hardware (assembler code)
Once the hardware is initialized, the process is switched to protected mode by setting a bit word in the machine status word.
Next the assembler instruction, jmpi 0×100000 _KERNEL_CS, jumps to the start address of the 32 bit code of the actual operating system kernel and continues from startup_32 and in the file arch/x86/kernel/head.S . More sections of the hardwares are initialized here like Memory Management unit (Page tables), the Co processor, and the environment (stack, environment,etc)
The first C function start_kernel() from init/main.c is called
the following list the assembly linkage of the start_kernel function

asmlinkage void __init start_kernel(void)

{

char * command_line;

printk(linux_banner); //print kernel, the banner

setup_arch(&command_line);//architecture dependent codes relevant to x86

trap_init();

init_IRQ(); //hardware interrupt initialization

sched_init(); //initialize the schedules

time_init();

softirq_init(); //soft interrupts

console_init();//initialize the console

init_modules();//initialize the modules (device drivers)

…..}

the init is called (will be searched in /sbin/init or /etc/init or /bin/init). if the init is not available, then a shell (/bin/bash) will be opened for debugging

Hardware interrupts (IRQ)

Interrupts are used to allow the hardware to communicate with the operating system, there are two problems while writing interrupt routine,

firstly, The interrupt routines should serve the hardware as quickly as possible

This can be solved by the following mechanisms

disabling all the software interrupts while servicing the hardware interrupts.
the processing of data is carried out asynchronously by the software interrupts through “tasklets ” or “bottom halves”

Software Interrupts

It is like a hardware interrupt but can be started only at certain times
The number of interrupts is limited
enum {HI_SOFTIRQ, NET_TX_SOFTIRQ, NET_RX_SOFTIRQ,TASKLET_SOFTIRQ}; the date types tells the software interrupts for hi priority software interrupts, Network Tranmssion and Receiving Interrupt and tasklet interrupt). upon interrupt is generated, the Interrupt routine will be executed

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