The best way to show how address resolution in primary mode compares with address resolution in AR mode is through an example. Figure 1 and Figure 2 show two ways an MVC instruction works to move data at location B to location A.

In Figure 1, the move instruction, MVC, is in code that is running in primary mode. The MVC instruction uses GPRs 1 and 2. GPR 1 is used as a base register to locate the destination of the MVC instruction. GPR 2 is used as a base register to locate some data to be moved.

Figure 1. The MVC instruction in primary mode

In Figure 2, the MVC instruction, in code that is in AR mode, moves the data at location B in Space Y to location A in Space X. GPR 1 is used as a base register to locate the destination of the data, and AR 1 is used to identify space X. GPR 2 is used to locate the source of the data, and AR 2 identifies Space Y. In AR mode, the MVC instruction is in code that is running in AR mode. The MVC instruction moves data from one address/data space to another. Note that the address space that contains the MVC instruction does not have to be either Space X or Space Y.

Figure 2. The MVC Instruction in AR Mode

Addresses that are qualified by an ALET are called ALET-qualified addresses.