The 6100 was a 12-bit processor, which had three registers: the PC, AC (accumulator), and MQ. All 2-operand instructions read AC and MQ and wrote back to AC. It had a 12-bit address bus, limiting RAM to only 4K. Memory references were 7-bit, offset either from address 0, or from the PC page base address (PC AND 7600 oct).
4K RAM was pretty much hopeless for general purpose use. The 6102 support chip (included in the 6120) added 3 address lines, expanding memory to 32K the same way that the PDP-8/E expanded the PDP-8. Two registers, IFR and DFR, held the page for instructions and data respectively (IFR was always used until a data address was detected). At the top of the 4K page, the PC wrapped back to 0, so the last instruction on a page had to load a new value into the IFR if execution was to continue.