CMS Virtual Machine Environments

z/VM® provides two versions of CMS:
ESA/390 CMS (generally referred to simply as CMS)
CMS runs in the following virtual machine architectures:
ESA/390 (ESA or XA virtual machine)
An ESA virtual machine simulates IBM® Enterprise Systems Architecture/390 (ESA/390), which is a superset of IBM Enterprise Systems Architecture/370 (ESA/370), which is a superset of IBM System/370 Extended Architecture (370-XA). The XA virtual machine designation is supported for compatibility; an XA virtual machine is functionally equivalent to an ESA virtual machine.
ESA/XC (XC virtual machine)
An XC virtual machine processes according to IBM Enterprise Systems Architecture/Extended Configuration (ESA/XC), which is an architecture unique to virtual machines. Although XC virtual machines run with dynamic address translation off, they can take advantage of a subset of dynamic address translation architectural features, and in particular, data spaces.
z/Architecture® CMS (z/CMS)
z/CMS runs in the following virtual machine architectures:
z/Architecture (ESA or XA virtual machine)
z/Architecture uses 31-bit addressing mode in an ESA , XA, or Z virtual machine. CMS programs can use z/Architecture instructions, including those that operate on 64-bit registers, while permitting existing ESA/390 architecture CMS programs to continue to function without change.

When z/CMS is IPLed in an ESA/390 (ESA or XA) virtual machine, z/CMS switches the virtual machine to z/Architecture mode and thereafter executes in z/Architecture mode.

z/XC (XC virtual machine)
A z/XC guest uses VM Data Spaces with z/Architecture in the same way that an ESA/XC guest uses VM Data Spaces with Enterprise Systems Architecture. CMS applications that run in z/Architecture can use multiple address spaces. z/CMS can use VM Data Spaces for accessing Shared File System (SFS) Directory Control (DIRCONTROL) directories. z/XC supports programs that employ z/Architecture instructions and registers (within the limits of z/CMS support) and programs that exploit data spaces in the same CMS session.

When z/CMS is IPLed in an XC virtual machine, z/CMS switches the virtual machine to z/XC mode and thereafter executes in z/XC mode.

Unless otherwise indicated, CMS means either version, and descriptions of CMS functions apply to both CMS and z/CMS. For information on the differences between z/CMS and CMS, see z/VM: CMS Planning and Administration.