[tailrecursion]

The CPU for this computer is microcoded and is made out of a small number of discrete logic chips, plus an EEPROM. The data and address bus are 8 bits wide but the NOR unit operates on one bit only. The microcode has to loop in order to perform an 8 bit add.

I was looking for a hardware shifter in the diagram, because I figured it'd be necessary to access bits D1-D7. But there is none. He uses a lookup table in EEPROM to do the shift.

If you're interested in nice CPU designs, here's another one: the Novix NC4000 (also RTX2000), in which the bits in the instructions lead directly to control various muxes and units inside the chip. There's almost no decoding.