If a program is part of a subsystem that offers services to many users, it might want to set up addressability for programs executing in other address spaces to reference data in its address space. Figure 1 shows a subsystem's home address space as AS1. Setting up addressability to the home address space requires no action. The PC routines that run in address spaces AS2, AS3, and AS4 all run under the same TCB, that of PGM1. Addressability to AS1 is through the special ALET of two. PGM2, PGM3, and PGM4 can place the value 2 in an AR of a AR/GPR pair to reference data in the home address space.

Figure 1. Using the ALET for the Home Address Space