Ken, thanks for yet-another amazing and dense article. Somewhat off topic, but y...

kens · on Oct 25, 2020

I have a Four Phase chip that I'll write about at some point. (Not the AL1 arithmetic/logic chip, but an I/O chip.) I think the main reason that Four Phase's AL1 didn't get more attention is that the processor was used as part of a proprietary computer, so it didn't have much visibility. In comparison, the 8008 was sold as an individual product with extensive documentation and support (essentially creating the microcomputer industry). The other factor is that improvements in NMOS made four-phase logic the losing technology, so it was mostly forgotten.

FullyFunctional · on Oct 25, 2020

Thanks, that makes sense. Maybe I should have asked differently: at the time of its introduction, it seemed like four phase logic (the technology) ought to have had superior absolute performance and even performance/area. It seems like a static version of Intrinsity's Fast14.

EDIT: clarified that "it" is the logic approach, not the end product.

kens · on Oct 25, 2020

My impression is that in 1967 four phase logic was the superior approach, but by the 1970s technology had changed enough that it wasn't. The problem is how to implement the pull-up on your gates. The four phase approach precharged the gates using 4 different clock phases. This was fairly efficient since you just wasted one charge. In contrast, using a pull-up transistor meant that you had a constant current flow, wasting power. The disadvantage with four phase logic is that each gate level takes two clock cycles. If you use static logic, on the other hand, you have the propagation delay through the gates. Another disadvantage of four-phase logic was the more complex design. I think it also required one more transistor per gate.

The main technology changes between 1967 and 1975 were the switch to silicon-gate transistors, the switch from PMOS to NMOS, and the use of depletion-load transistors. I think these considerably reduced the advantage of four-phase logic. The common microprocessors of that era (8080, 6502, Z-80, etc) used depletion-load NMOS logic, although the IMP-16 and TI TMS9900 apparently used four-phase logic. I'm currently looking at the Mostek 4116 memory chip which uses weird precharge logic, probably not four-phase, but I haven't figured out yet what's going on there.

I've read some of the papers on four-phase logic, but haven't studied it closely enough to be sure of the above. So I welcome corrections and more information about the demise of four-phase logic.