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Linux is a complete multitasking, multiuser operating system, as are all other versions of UNIX. This means that many users can log into and run programs on the same machine simultaneously.
The Linux system is mostly compatible with several UNIX standards (in as much as UNIX has standards) at the source level, including IEEE POSIX.1, UNIX System V, and Berkely System Distribution UNIX. Linux was developed with source code portability in mind, and it's easy to find commonly used features that are shared by more than one platform. Much of the free UNIX software available on the Internet and elsewhere compiles under Linux ``right out of the box.'' In addition, all of the source code for the Linux system, including the kernel, device drivers, libraries, user programs, and development tools, is freely distributable.
Other specific internal features of Linux include POSIX job control (used by shells like csh and bash), pseudoterminals ( pty devices), and support for dynamically loadable national or customized keyboard drivers. Linux supports virtual consoles that let you switch between login sessions on the same system console. Users of the screen program will find the Linux virtual console implementation familiar.
The kernel can emulate 387-FPU instructions, and systems without a math coprocessor can run programs that require floating-point math capability.
Linux supports various file systems for storing data, like the ext2 file system, which was developed specifically for Linux. The Xenix and UNIX System V file systems are also supported, as well as the Microsoft MS-DOS and Windows 95 VFAT file systems on a hard drive or floppy. The ISO 9660 CD-ROM file system is also supported.
Linux provides a complete implementation of TCP/IP networking software. This includes device drivers for many popular Ethernet cards, SLIP (Serial Line Internet Protocol) and PPP (Point-to-Point Protocol), which provide access to a TCP/IP network via a serial connection, PLIP (Parallel Line Internet Protocol), and NFS (Network File System). The complete range of TCP/IP clients and services is also supported, which includes FTP, telnet, NNTP, and SMTP.
The Linux kernel is developed to use protected-mode features of Intel 80386 and better processors. In particular, Linux uses the protected-mode, descriptor based, memory-management paradigm, and other advanced features. Anyone familiar with 80386 protected-mode programming knows that this chip was designed for multitasking systems like UNIX. Linux exploits this functionality.
The kernel supports demand-paged, loaded executables. Only those segments of a program which are actually in use are read into memory from disk. Also, copy-on-write pages are shared among executables. If several instances of a program are running at once, they share physical memory, which reduces overall usage.
In order to increase the amount of available memory, Linux also implements disk paging. Up to one gigabyte of swap space may be allocated on disk (upt to 8 partitions of 128 megabytes each). When the system requires more physical memory, it swaps inactive pages to disk, letting you run larger applications and support more users. However, swapping data to disk is no substitute for physical RAM, which is much faster.
The Linux kernel also implements a unified memory pool for user programs and disk cache. All free memory is used by the cache, which is reduced when running large programs.
Executables use dynamically linked, shared libraries: code from a single library on disk. This is not unlike the SunOS shared library mechanism. Executable files occupy less disk space, especially those which use many library functions. There are also statically linked libraries for object debugging and maintaining ``complete'' binary files when shared libraries are not installed. The libraries are dynamically linked at run time, and the programmer can use his or her own routines in place of the standard library routines.
To facilitate debugging, the kernel generates core dumps for post-mortem analysis. A core dump and an executable linked with debugging support allows a developer to determine what caused a program to crash.