another location. The remaining four bits offered a similar functionality by selecting a second register as an index register, allowing memory to be stepped through by changing the value in the fast register rather than in slower main memory.

The registers in the PDP-6 were simply the first 16 memory locations of main memory. Most, if not all, PDP-6 systems were equipped with the optional Type 162 "Fast Memory", which constructed these 16 memory locations from