Outline of VM Data Space Support

An application that runs on an XC virtual machine can leverage data space support to operate concurrently and efficiently on the following categories of data:
  • Data spaces that are external to the application's primary address space
  • Data spaces that are external to the primary address space of the application's virtual machine
The following list summarizes VM data space support:
  • A data space is a special type of address space (in z/VM publications, data-only address space = data space):
    • It is for data only.
    • The entire space is addressable.
  • In XC mode, a data space always resides outside its owner's virtual machine primary address space.
  • Data spaces can be shared.
  • Any permitted virtual machine within the z/VM system that contains a shared data space can access that shared data space by using CPU instructions.
  • Data spaces provide integrity and isolation for the data they contain.
  • Only applications that execute in XC virtual machines can reference data spaces directly. However, an application that runs on an ESA, XA, or Z virtual machine can copy data from another user's data space by using z/VM services.
  • Access registers (one associated with each general register) allow manipulation of data in a data space.
  • The size of a data space can range from 256 pages or blocks (1 MB) to 524,288 pages (2 GB) and is fixed upon creation. If the size specified (in number of pages) is not a multiple of 256, it is rounded up to a multiple of 256.
  • CMS provides callable services library (CSL) routines for creating and managing one or more data spaces. (These routines in turn use CP macros to complete the operations.)
  • Data in a data space can be manipulated by using most of the ESA/390 assembler language instructions that have a storage-related operand. For more information, see z/VM: ESA/XC Principles of Operation and z/VM: z/Architecture Extended Configuration (z/XC) Principles of Operation.
  • Data spaces can be defined with the following attributes:
    • Scope of program usage
    • An association with CMS events that cause automatic data space-related resource clean up.
    Such a definition contributes to overall VM resource management by assuring the recovery of the data-space related resource.
  • XCONFIG directory statements define the scope of a virtual machine's use of data spaces.