All the above got more complicated with the System/3 model 15, and the Systems/34 and 36. Though still using 16-bit addressing, all these systems could support well over 64K of main storage (up to 512K and theoretically more), so address translation was used to swap from one 64K address space to another. Address Translation Registers were set to define the actual address space in use at any one time, their contents being concatenated with the 16-bit address used by a program to produce a real address. These "ATRs" were privileged, available only to the operating system.