Quick Video: The Accumulator

A new quick video with a bit of a status update: I now have a working accumulator register tied to my ALU. (I’m also working on the flags register as well as the 4 8-bit registers B through E introduced in the “Introduction to Digital Computers Part 4” video.

This includes video of the ALU in action.

As a quick aside, each board of my ALU contains 66 transistors each, handling addition, subtraction, and, or, xor, not, left-shift, right-shift and passing the bus through to the output, as well as driving the status LEDs.

Each accumulator board contains 37 transistors: 1 to drive the LED, 8 to provide a tri-state gate for writing the accumulator to the bus, and 28 transistors to provide the three S/R flip flips and associated latching logic so we load on the rising edge of the clock line, and present the results to the output on the falling edge. And we need 11 boards total: 8 for the accumulator and 3 for the flags. There is an additional flags logic board which contains 43 transistors providing glue logic to make the flags work with the ALU.

And each register board contains 36 transistors: 1 to drive the LED, 19 for the 2 S/R flip flips making the D-latch, and two tri-stage gates using 8 transistors each; one to output the register to the data bus, and another to the address bus. (This is so we can combine two registers, such as B and C, into a single 16-bit address value.)

Each board now takes me about an hour to assemble–and I’ve got 20 register boards to build, as well as the flags logic board (which should be delivered from Seeed Studio in a couple of weeks, along with a board that contains 8 LEDs which will display the contents of the bus line (so we can see better what’s going on with our circuits than me waving vaguely towards a bunch of toggle switches).

So that’s our update.

I do have the next video outlined; basically I intend to roughly follow the evolution of the control circuits from older computers, starting with the ENIAC–which used a bank of mechanical toggle switches and a mechanical counter to control the circuits that made up the ENIAC’s math circuits and registers. That will progress to using a transistor-based counter circuit and decoder circuits to get instructions, and show how a modified transistor-based counter with data latching abilities can be used for jumps and conditional jumps as we send signal pulses to our registers, accumulator and ALU.

Thanks for watching.

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William Woody

I'm a software developer who has been writing code for over 30 years in everything from mobile to embedded to client/server. Now I tinker with stuff and occasionally help out someone with their startup.

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