Linux os concepts internals applications in a nutshell


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Linux Operating System

Why Linux operating system?

Linux is the most commonly used open source operating system today. As an operating system, it sits underneath all of the other software, receiving requests from other software programs and relaying these requests to the computer's hardware.

In this article we will go through linux operating systems concepts and applications. You will get guidance with relevant theory and practical source code programs, illustrations to understand how it is practically used in the system to implement certain requirements.

Hardware

Hardware basics.
    An operating system works closely with the hardware system. The operating system acts as the interface between user and hardware. So in order to fully understand the Linux operating system, you need to understand the basics of the underlying hardware. At minimum a computer system consists of a CPU or microprocessor, the memory and a number of slots for the peripheral controllers.
    CPU calculates, performs logical operations and manages data flows by reading instructions from memory and then executing them. CPU operates on binary data where each digital represents a power of 2. Generally numbers are represented by hexadecimal where each digital represents a power of 16. So 8 is written in binary as 0b1000 and in hex as 0x08. Similary 14 is written in binary as 0b1110 and in hex as 0x0E.
    The CPU execution is driven by the system clock(e.g. 100MHz) which generates regular clock pulses to the processor and at each clock pulse, the cpu does some work. For example an instruction can be to read the contents of memory at location A into register B. So CPU executes one or more instruction every clock pulse. Registers are the CPU's internal storage, used for storing data and performing operations on it and can be 32 bit/62 bit wide.
    Most CPUs have special purpose registers such as Program Counter which points to the next instruction to be executed, Stack pointer which helps in accessing, saving and restoring large amount of external read/write RAM for facilitating temporary storage of data, processor status to hold information like the current state of the processor(e.g. processor mode kernel mode and user mode).
    Instructions have to be fetched from memory as they are executed. Read only memory(ROM) are used to store permanent data and RAM is used for temporary storage of data. System bus is divided into three logical functions, the address bus which specifies the memory locations (addresses) for the data transfers, the data bus((bidirectional and allows data to be read into the CPU and written from the CPU) which holds the data transfered and the control bus contains various lines used to route timing and control signals throughout the system. Peripherals are real devices, such as graphics cards or disks controlled by controller chips on the system board or on cards plugged into it.

User space vs Kernel space

User space applications.
    The system bus connects the CPU with the main memory and is separate from the buses connecting the CPU with the system's hardware peripherals. Collectively the memory space that the hardware peripherals exist in is known as I/O space. The CPU can access both the system space memory and the I/O space memory, whereas the controllers themselves can only access system memory indirectly and then only with the help of the CPU. For example floppy disk controllers will see the address space of its own registers, not the system memory. CPU will have separate instructions for accessing the memory and I/O space.
    A Linux system is basically divided in three major components: Linux File System (LFS), Shell and Kernel. Kernel is the core program which manages system hardware devices. Shell provides user interface to run the commands. File system organizes the data in systematic way. User space programs don't have direct access to the system main memory. They get access to the main memory indirectly using system calls. We have written a user space program to illustrate these concepts.
    There are times when controllers need to read or write large amounts of data directly to or from system memory. For example when user data is being written to the hard disk. In this case, Direct Memory Access (DMA) controllers are used to allow hardware peripherals to directly access system memory but this access is under strict control and supervision of the CPU.

Linux command line interface

Linux commands usage illustrations.
    Linux is a unix-like operating system. In linux all commands are run using a terminal provided by the linux system. The linux terminal can be used to do all administrative tasks. The linux commands are broadly divided into File System Commands, Process Management Commands, File Permission Commands, Searching commands, System Info Commands, File Compression Commands, Networking Commands, Process Management management commands. All these category of commands commands are illustrated by examples.

Process, thread in linux

A process in linux.
    A process is nothing but a running program. Let's say we have a problem to be solved or certain requirements to be implemented and we are writing source code for that, compiling it and creating an executable which we can run. So when we are ready with an executable and we are executing it, it is nothing but a process. Processes are separate tasks each with their own rights and responsibilities. If one process crashes it will not cause another process in the system to crash. All the practical aspects of how to create a process, running the process and observing the process in the linux process list are illustrated .
 
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