Enables a host (hypervisor) to access any location in any guest zone in a large memory, when host and guest operands have small addresses that cannot access locations outside of their own zones. System hardware/microcode provides a particular number of windows for host use. Each CPU in the system has one or more window access registers (WARs), and one or more window registers (WRs). The host uses a load WAR instruction to designate each page frame (PF) in the host zone to be used as a host window, and each PF is associated with a respective window number. When the host receives an interception signal requiring the host to access a guest location represented by a guest zone identifier and a guest small address, the host designates one of its window numbers for an access to this guest location. Then, the host executes an activate WR instruction which invokes CPU hardware/microcode that generates a large absolute address for accessing this guest location in the large memory and stores it in a WR associated with the window number. When the host thereafter executes any instruction with an operand small address accessing the host window PF associated with that WR, and CPU hardware/microcode automatically substitutes that guest large address in the WR for the host operand small address for accessing the guest location.