Linux Information Sheet: Linux Features

## 2. Linux Features

- multitasking: several programs running at once.
- multiuser: several users on the same machine at once (and no two-user licenses!).
- multiplatform: runs on many different CPUs, not just Intel.
- runs in protected mode on the 386.
- has memory protection between processes, so that one program can't bring the whole system down.
- demand loads executables: Linux only reads from disk those parts of a program that are actually used.
- shared copy-on-write pages among executables. This means that multiple process can use the same memory to run in. When one tries to write to that memory, that page (4KB piece of memory) is copied somewhere else. Copy-on-write has two benefits: increasing speed and decreasing memory use.
- virtual memory using paging (not swapping whole processes) to disk: to a separate partition or a file in the filesystem, or both, with the possibility of adding more swapping areas during runtime (yes, they're still called swapping areas). A total of 16 of these 128 MB swapping areas can be used at once, for a theoretical total of 2 GB of useable swap space.
- a unified memory pool for user programs and disk cache, so that all free memory can be used for caching, and the cache can be reduced when running large programs.
- dynamically linked shared libraries (DLL's), and static libraries too, of course.
- does core dumps for post-mortem analysis, allowing the use of a debugger on a program not only while it is running but also after it has crashed.
- mostly compatible with POSIX, System V, and BSD at the source level.
- through an iBCS2-compliant emulation module, mostly compatible with SCO, SVR3, and SVR4 at the binary level.
- all source code is available, including the whole kernel and all drivers, the development tools and all user programs; also, all of it is freely distributable. There are some commercial programs being provided for Linux now without source, but everything that has been free is still free.
- POSIX job control.
- pseudoterminals (pty's).
- 387-emulation in the kernel so that programs don't need to do their own math emulation. Every computer running Linux appears to have a math coprocessor. Of course, if your computer already contains an FPU, it will be used instead of the emulation, and you can even compile your own kernel with math emulation removed, for a small memory gain.
- support for many national or customized keyboards, and it is fairly easy to add new ones dynamically.
- multiple virtual consoles: several independent login sessions through the console, you switch by pressing a hot-key combination (not dependent on video hardware). These are dynamically allocated; you can use up to 64.
- Supports several common filesystems, including minix-1, Xenix, and all the common system V filesystems, and has an advanced filesystem of its own, which offers filesystems of up to 4 TB, and names up to 255 characters long.
- transparent access to MS-DOS partitions (or OS/2 FAT partitions) via a special filesystem: you don't need any special commands to use the MS-DOS partition, it looks just like a normal Unix filesystem (except for funny restrictions on filenames, permissions, and so on). MS-DOS 6 compressed partitions do not work at this time without a patch (dmsdosfs). VFAT (WNT, Windows 95) support has been added to the development kernel, and will be in the next stable release.
- special filesystem called UMSDOS which allows Linux to be installed on a DOS filesystem.
- read-only HPFS-2 support for OS/2 2.1
- read-only HFS (Macintosh) file system support is available as a patch, with read-write support underway (no scheduled release date).
- CD-ROM filesystem which reads all standard formats of CD-ROMs.
- TCP/IP networking, including ftp, telnet, NFS, etc.
- Appletalk server
- Netware client and server
- Lan Manager (SMB) client and server