Huge 74181 is a Classic ALU You Can Actually Understand

You can no longer buy a brand-new 74181, they’ve been out of production for years. All is not lost though, for [Dave’s Dev Lab] have created a facsimile of one on a printed circuit board, using modern single-gate 74-series chips.

Why on earth would you want an oversized replica of an outdated logic chip from nearly five decades ago, we hear you ask? The answer lies in education. If you were to embark on learning about the internals of a microprocessor by taking a modern example such as the one that powers the device on which you are reading this, you would find it to be a daunting task. Over six decades of progress in computer technology have delivered the performance enhancements that put a supercomputer in your smartphone, but at the expense of a contemporary microprocessor being an extremely complex machine which you can’t peer into for any level of understanding.

Simple enough to work your way through the logic

The starting point for the student of microprocessor internals often lies in the past. The technology of the early 1970s holds the fundamentals from which a modern processor can be understood, but remains simple enough to grasp in its entirety as a beginner. Registers, instruction decoders, counters, and an arithmetic/logic unit, or ALU. And for decades the 74181, as an all-in-one 4-bit ALU on a chip that you might have found in a minicomputer at the turn of the 1970s, represented the most convenient way to teach the operation of these devices. Electronic engineers and computer scientists of all ages will have encountered them as they gained their qualifications.

The PCB version of the 181 faithfully follows the original, but with modern 74LVC gates laid out as they would be in the circuit diagram of the chip, and LEDs to show logic state at the different parts of the circuit. Thus when it is used to teach ALU operation it can show every part of the device in detail in a way a real 74181 would never have done.

If the 74181 has caught your interest, we’ve previously brought you [Ken Shirriff]’s reverse engineering of the device in detail using breathtaking images of the silicon.

21 thoughts on “Huge 74181 is a Classic ALU You Can Actually Understand

  1. I love these types of builds! I’ve aways been a learn by seeing/learn by doing type. I can’t just read about it and immediately get it. This is exactly the type of stuff that flip on those light bulbs for people like me. Awesome job!

    1. I was thinking … a overlaid schematic (logic diagram) and lots of LEDs on the gate inputs/outputs.

      Then I was thinking perhaps a small CPLD with toggle buttons for the inputs and a 4 bit up down counter with buttons for up and down for the select. Then it becomes two many pins for a small CLPD so perhaps even 74xx would do it.

      It would be a challenge to get the current for the gate input LEDs as the fanout is 10 or more in some cases and there are many signal paths.

      Now I’m tempted to make an emulator in a browser.

        1. With 87 discreet signals it would have to be a 100 pin CPLD. The Altera MAX II EPM240 could probably do it if you only had LEDs on gate outputs as it can sink 25mA per pin but the total current per I/O bank (130mA) would be the limit.

          Open collector transistor arrays like the ULN2803 are probably just as pin deficient as buffers.

          Some fanouts are over ten. Putting 10 LEDs in series would require to high a supply voltage so a parallel system would be better if buffers are used.

    1. In that case then, study how the DEC LSI-11 was built. Also study the DG Nova 4 and DG Eclipse families. All of those used the AM2900 family members. And in fact AMD made members of the AM29 logic series for DEC for their different systems based on the LSI-11.

      1. The DEC LSI-11 does not use AM2900 family devices – it uses devices from Western Digital (better known these days for making hard drives). As far as I know the only CPU DEC built using AM2900 family devices was the VAX-11/730 although it used them for other things than CPUs (such as the FP11-A floating point processor for the PDP-11/34).

    1. No, a triangle by it’s self represents a buffer or null operation. The circle on the output means not. So a simple NOT gate. Look at the symbols for NAND vs AND and OR vs NOR. The circle is the NOT part.

    1. Yes, there are plenty of New Old Stock (NOS) out there, but there are no current silicon manufacturers who produce new 74181 parts. This has lead to a booming business for selling the NOS parts to students who are talking college classes that require a 74181. My hope was that these could be used in those classes instead…

  2. “Over six decades of progress in computer technology have delivered the performance enhancements that put a supercomputer in your smartphone, but at the expense of a contemporary microprocessor being an extremely complex machine which you can’t peer into for any level of understanding.”

    No matter how much Anandtech tries. :-p

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