## It’s alive!

So it turns out my 1 bit ALU–which has both circuitry for our accumulator and a tristate circuit to write to our bus, works correctly! Over the next week my plan is to assemble 8 of them and hook them up to my ALU.

## My accumulator boards just showed up!

They’re nowhere near as complicated as the 1 bit ALU boards, but they do contain the accumulator circuit as discussed in our “Introduction to Digital Computers Part 4 video, along with a single tristate circuit for allowing the contents to be written to our bus, as discussed elsewhere in the same video.

These accumulator circuits are used as-is with the accumulator of our transistor-based circuit, and with additional circuitry for our flags register.

Of course this will take a while to build–and I have another 1,000 transistors and resistors on order (because ordering in bulk is cheap)–so hopefully I will be able to demonstrate a working accumulator with the ALU circuit built earlier.

Fortunately the boards (which implement the logic for a 1 bit accumulator) are not quite as complicated.

This is starting to get exciting, because instead of just showing addition we can show state, that is, we can show intermediate results being stored away and used for subsequent operations.

## Announcing a new Quick Video: Building an ALU.

So this one is a little different; in this video I show the steps to go from the design of an arithmetic logic unit to actually demonstrating a working 8-bit ALU, made entirely out of 2N3904 transistors.

The show notes include links to circuit diagrams and files in case you may want to build your own.

Here’s the video, and thanks for watching.

## I am unreasonably happy that my 1 bit ALU board works.

One thing about my “Introduction to Digital Computers” series I wanted to verify was that all the circuits designed in the series in fact works correctly.

And one aspect of that–which we introduced in the first part of the fourth video–was the complete ALU.

While I’ve taken the liberty to do some minor redesigns (so as to reduce the number of transistors–things like realizing we don’t need a separate XOR circuit if we already have A XOR B embedded in the adder circuit)–I’ve built a circuit that is entirely made of transistors and resistors (with a couple of status LEDs to show the outputs visually on the board) that actually is a full 1 bit slice of my 8-bit ALU.

And I’m unreasonably happy that it (the second iteration of the circuit board design) works flawlessly.

Of course I have 7 more blank boards and a whole bunch of resistors and transistors on order.

And when they arrive I plan to wire it all up to demonstrate that the principles introduced here in fact work as advertised.