The multiple-address-space facility under z/OS® allows VTAM® RPL-based macroinstructions that are issued in different address spaces to reference one ACB. The parts of the application program executing in the address spaces selected are logically grouped into a single logical unit associated with the ACB. Thus, the application program can be thought of as spanning multiple address spaces. The multiple-address-space facility can provide increased flexibility and error isolation for application programs. Improved operation can be achieved by running one session for each address space to maximize isolation or protection, or by grouping a number of sessions in one address space, based on the installation's requirements.

See Isolation of errors for further information about error isolation.

When using the multiple-address-space facility, sessions associated with an ACB can reside in any available address space in addition to the address space used for opening the ACB. When not using the multiple-address-space facility, all sessions associated with a particular ACB must reside in the address space used for opening the ACB.