Cross memory terminology

The following terms are used within this section. For definitions of other terms used in this document, see the glossary.

Address space control (ASC) mode: The mode (determined by the PSW) that tells the system where to find the data it is to reference. Two ASC modes are AR and primary. For each ASC mode, the following table defines:
- The address space from which the system fetches instructions
- The address space or data space that the system accesses when an instruction, other than an MVCP or MVCS instruction, references data
- The address space the system accesses when an MVCP or MVCS instruction references data

AR ASC mode: The ASC mode in which the system uses both the GPR (used as the base register) and the corresponding AR to resolve an address in an address/data space.

ASC Mode	Instruction Fetch	Data Access	MVCP/MVCS
Primary ASC mode	Primary address space	Primary address space	Primary or secondary address space
Secondary ASC mode	Primary address space (see Note)	Secondary address space	Primary or secondary address space
AR ASC mode	Primary address space	Address space indicated by the AR	Unavailable. Causes an abend in AR ASC mode.
Note: Prior to ESA/370 architecture, the address space from which instructions are fetched is unpredictable when a program is running in secondary ASC mode.

Basic PC: Transfers control to another program, the PC routine. The basic PC requires the service provider to save and restore the user's environment. The PC routine can be in the same address space as the program that issues the PC instruction, or in a different address space.
Cross memory local (CML) lock: The LOCAL lock of an address space other than the home address space.
Cross memory mode: Cross memory mode exists when at least one of the following conditions are true:
- The primary address space (PASN) and the home address space (HASN) are different address spaces.
- The secondary address space (SASN) and the home address space (HASN) are different address spaces.
- The ASC mode is secondary.
Home address space: The address space in which MVS™ initially dispatches a TCB or SRB (work unit). In the case of a TCB, the home address space contains the TCB. PSAAOLD points to the home address space. When MVS initially dispatches a work unit, the home address space, the primary address space, and the secondary address space are all the same. During execution of the work unit, the home address space remains the same. The primary and secondary address spaces may be changed, however, through the PC, PR, PT, or SSAR instructions.
LX reuse facility: The LX reuse facility is available with z/OS® V1R6 to provide additional LXs and improve reusability of LXs. It is enabled when running on a z890 or z990 processor at driver level 55 or above, with APAR OA07708 installed. When the facility is enabled bit CVTALR in byte CVTFLAG2 of the CVT data area is 1.
Primary address space: The address space whose segment table is used to fetch instructions in primary, secondary, and AR ASC modes. A program in primary mode fetches data from the primary address space.
Primary ASC mode: The ASC mode in which the system uses the GPRs, but not the ARs, to resolve an address in an address space. In primary ASC mode, the system fetches instructions and data from the primary address space.
PC number: A number that identifies a PC routine. The service provider creates the number, by using MVS services, and supplies it to the user. The user specifies the number in a PC instruction to identify the PC routine that is to be invoked.
PC routine: A program that receives control as the result of a PC instruction's executing and performs a service for the caller.
Secondary address space: The address space whose segment table the system uses to access data in secondary ASC mode.
Secondary ASC mode: The ASC mode in which the system fetches instructions from the primary address space and data from the secondary address space.
Space switch routine: A program that issues a PC instruction that causes the primary address space to change.
Stacking PC: Transfers control to another program, the PC routine. The stacking PC uses the linkage stack for storing the caller's status. It provides more options and more automatic function than the basic PC instruction. The PC routine can be in the same address space as the program that issues the PC instruction, or a different address space. IBM® recommends using the stacking PC instead of the basic PC.