The term "extended addressability" refers to the ability of a program to use virtual storage that is outside the address space the program is dispatched in. Synchronous cross memory communication describes how a caller uses the PC instruction to call a program in another address space and run there under the caller's TCB. It describes the two cross memory instructions (MVCS and MVCP) that move data from primary to secondary and from secondary to primary.

Access registers provide you with a different function from cross memory. You cannot use them to branch into another address space. Through access registers, however, you can use assembler instructions to manipulate data in other address spaces and in data spaces. You do not use access registers to reference addresses in hiperspaces.