Several (non-IBM) Linux® distributions can be used on a mainframe.

There are two generic names for these distributions:

• Linux for S/390® (uses 31-bit addressing and 32-bit registers)
• Linux for System z® (uses 64-bit addressing and registers)

The phrase Linux for System z is used to refer to Linux running on an S/390 system or System z, when there is no specific need to refer explicitly to either the 31-bit version or the 64-bit version. We assume students are generally familiar with Linux and therefore we mention only those characteristics that are relevant for mainframe usage. These include the following:

• Linux uses traditional count key data (CKD) disk devices and SAN-connected SCSI-type devices. Other mainframe operating systems can recognize these drives as Linux drives, but cannot use the data formats on the drives. That is, there is no sharing of data between Linux and other mainframe operating systems.
• Linux does not use 3270 display terminals, while all other mainframe operating systems use 3270s as their basic terminal architecture. Linux uses X-Window System based terminals or X-Window System emulators on PCs; it also supports typical ASCII terminals, usually connected through the telnet protocol. The X-Window System is the standard for graphical interfaces in Linux. It is the middle layer between the hardware and the window manager.
• With the proper setup, a Linux system under z/VM® can be quickly cloned to make another, separate Linux image. The z/VM emulated LAN can be used to connect multiple Linux images and to provide an external LAN route for them. Read-only file systems, such as a typical /usr file system, can be shared by Linux images.
• Linux on a mainframe operates with the ASCII character set, not the EBCDIC form of stored data that is typically used on mainframes. Here, EBCDIC is used only when writing to such character-sensitive devices as displays and printers. The Linux drivers for these devices handle the character translation.