The memory may be cumbersome to access, as in the 8088 series, but it is available. A large memory space is important for big data structures and programs. The MMU is the mechanism by which the 64180 can access an address space larger than 64k, but still maintain compatibility with the Z80 instruction set. All instructions, in all modes, always issue 16 bit addresses. The MMU converts every one of these 16 bit addresses to 20 bits.
Every address generated by a user's program is a logical address.
The MMU's role is to translate these logical addresses into physical ones. If, for example, the user's program tries to reference location 0, this is a reference to logical address 0. Every address reference made by a program is passed through the MMU before being placed on the CPU's pins. If the address matches a range previously programmed into the MMU, then the MMU will add an offset to that address, forming the physical address. Since the offset is a 20 bit number the device can access up to 1 Mb of memory.