Operating Systems
Introduction
An operating system is a system software that manages computer hardware, software resources, and provides common services for computer programs.
Without an OS, only one software would run and would be in charge of everything on the computer, including the hardware.
System & application softwares
System softwares are in charge of:
managing memory;
managing programs execution;
allowing several programs to exchange data between them;
managing all computer's peripherals;
managing files, network, sound & display.
Application softwares let the user do tasks that may involve calls to some system softwares.
But depending on the hardware, an OS can't manage all devices or peripherals. That's why the computer needs drivers, which is a system software, often provided by the manufacturer of the device.
System call
Request from software in user space to OS in kernel space;
Allows software to use OS service (memory management, file management etc);
Interrupts current program execution to execute an OS service;
Interrupts
An interrupt is a request for the processor to stop currently executing program in order to execute another one. After this last program is done, it resumes the stopped program. It is stored in the processor.
When the interrupt is requested, it runs an Interrupt Service Routine (or ISR).
Since there is a lot of different devices & peripherals, there is multiple ISR. That's why we use an interrupt vector table.
Interrupt vectors
An interrupt vector is a pointer that stores the address of an interrupt handler.
First, it is initialized by the BIOS during the booting process. Then, the OS will replace this interrupt handler by its own one.
Since there is multiple interrupt vector in a system, those are stored in an interrupt vector table, that associate a list of interrupt handlers with a list of interrupt requests. It is stored in the memory.
Interrupt requests
On x86 processors, INT is the assembly language instruction (or machine instruction) that generates an interrupt.
This instruction needs some parameters in order to work, such as the number or the address of an interrupt, ... For example, an ISR for printing a letter on the screen will need the letter to display as a parameter.
Those parameters are copied in ultra-fast memory in the processor, called registers.
Kernel
A kernel is the core program of a computer's OS that has complete control over everything in the system. It is always resident in memory.
Its objectives are:
To establish communication between user level application and hardware;
To decide state of incoming processes;
To control disk management;
To control memory management;
To control task management.
Types of kernel
Megalithic
Whole OS in kernel space;
Monolithic
All OS services operate in the kernel;
Every services are called through system calls.
Pros:
Fast because processes are in the same space.
Cons:
If one service crashes, the whole OS crashes;
Hard to maintain due to the size of the kernel;
Adding a new service requires to modify the entire kernel.
Examples:
Unix;
Linux;
Open VMS;
XTS-400.
Micro
OS services are implemented into different spaces (kernel & user space for example);
Services are called through IPC (Inter Process Communication) mechanisms;
Minimalist approach.
Pros:
Stable because if a service in user space crashes, the kernel is not affected;
Easy to maintain due to the small size of the kernel.
Cons:
Slower because there are lot of context switches between kernel & user space.
Examples:
Mach;
L4;
AmigaOS;
Minix;
K42.
Hybrid
Mix of monolithic & micro kernels;
Micro kernel with some non-essential code in kernel space (like networking or file system).
Pros:
Tries to get the best of both worlds.
Cons:
More complex than micro kernels.
Examples:
Windows NT;
Netware;
BeOS.
Exo
Nano
Linux kernel
Windows kernel
Boot
Linux booting process
Power on
POST
BIOS
MBR
GRUB
Kernel
Systemd (Init)
Sources
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