In access register address space control (ASC) mode, a program can use the full set of assembler instructions (except MVCP and MVCS) to manipulate data in another address space or in a data space. Unlike cross memory, access registers allow full access to data in many address spaces or data spaces.

ASC mode determines how the processor resolves address references for the executing program. In primary ASC mode, the processor uses the contents of general purpose registers to resolve an address to a specific location. In access register ASC mode, an access register (AR) identifies the space the processor is to use to resolve an address. The processor uses the contents of an AR as well as the contents of general purpose registers to resolve an address to a specific location.

In AR ASC mode, a program can move, compare, or perform operations on data in other address spaces or in data spaces. It is important to understand, however, that ARs do not enable a program to transfer control from one address space to another. That is, you cannot use ARs to transfer control from a program in one address space to a program in another address space. For that, you need cross memory.

You can, however, use ARs without using cross memory. If your application needs to manipulate data in other address/data spaces but does not need to transfer control to other address spaces, use ARs. If your application needs to transfer control to routines in other address spaces but does not need to manipulate data, use cross memory. If your application needs both the transfer of control and the manipulation of data, use both cross memory and ARs.