What is Linux?
Linux is a clone of the operating system Unix, written from scratch by Linus Torvalds with assistance from a loosely-knit team of hackers across the Net. It aims towards POSIX and Single UNIX Specification compliance.
It has all the features you would expect in a modern fully-fledged Unix, including true multitasking, virtual memory, shared libraries, demand loading, shared copy-on-write executables, proper memory management, and multistack networking including IPv4 and IPv6.
Although originally developed first for 32-bit x86-based PCs (386 or higher), today Linux also runs on (at least) the Compaq Alpha AXP, Sun SPARC and UltraSPARC, Motorola 68000, PowerPC, PowerPC64, ARM, Hitachi SuperH, IBM S/390, MIPS, HP PA-RISC, Intel IA-64, DEC VAX, AMD x86-64, AXIS CRIS, and Renesas M32R architectures.
Linux is easily portable to most general-purpose 32- or 64-bit architectures as long as they have a paged memory management unit (PMMU) and a port of the GNU C compiler (gcc) (part of The GNU Compiler Collection, GCC). Linux has also been ported to a number of architectures without a PMMU, although functionality is then obviously somewhat limited. See the µClinux project for more info.
What Is a Kernel?
Let's start by providing a definition for the term kernel. The UNIX kernel is the software that manages the user program's access to the systems hardware and software resources. These resources range from being granted CPU time, accessing memory, reading and writing to the disk drives, connecting to the network, and interacting with the terminal or GUI interface. The kernel makes this all possible by controlling and providing access to memory, processor, input/output devices, disk files, and special services to user programs.
Kernel Services
The basic UNIX kernel can be broken into four main subsystems:
Process Management
Memory Management
I/O Management
File Management
These subsystems should be viewed as separate entities that work in concert to provide services to a program that enable it to do meaningful work. These management subsystems make it possible for a user to access a database via a Web interface, print a report, or do something as complex as managing a 911 emergency system. At any moment in the system, numerous programs may request services from these subsystems. It is the kernel's responsibility to schedule work and, if the process is authorized, grant access to utilize these subsystems. In short, programs interact with the subsystems via software libraries and the systems call interface. Refer to your UNIX
reference manuals for descriptions of the systems calls and libraries supported by your system. Because each of the subsystems is key to enabling a process to perform a useful function, we will cover the basics of each subsystem. We'll start by looking at how the UNIX kernel comes to life by way of the system initialization process.
System Initialization
System initialization (booting) is the first step toward bringing your system into an operational state. A number of machine-dependent and machine-independent steps are gone through before your system is ready to begin servicing users. At system startup, there is nothing running on the Central Processing Unit (CPU). The kernel is a complex program that must have its binary image loaded at a specific address from some type of storage device, usually a disk drive. The boot disk maintains a small restricted area called the
boot sector that contains a boot program that loads and initializes the kernel. You'll find that this is a vendor specific procedure that reflects the architectural hardware differences between the various UNIX vendor platforms. When this step is completed, the CPU must jump to a specific memory address and start executing the code at that location. Once the kernel is loaded, it goes through its own hardware and software initialization.
Kernel Mode
The operating system, or kernel, runs in a privileged manner known as kernel mode. This mode of operation allows the kernel to run without being interfered with by other programs currently in the system. The microprocessor enforces this line of demarcation between user and kernel level mode. With the kernel operating in its own protected address space, it is guaranteed to maintain the integrity of its own data structures
and that of other processes. (That's not to say that a privileged process could not inadvertently cause corruption within the kernel.) These data structures are used by the kernel to manage and control itself and any other programs that may be running in the system. If any of these data structures were allowed to be accidentally or intentionally altered, the system could quickly crash. Now that we have learned what a UNIX kernel is and how it is loaded into the system, we are ready to take a look at the four UNIX subsystems Process Management, Memory Management, Filesystem Management and I/O Management.
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