8-bit CPU control logic: Part 2

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Genre: Education

Family friendly? Yes

Wilson score: 0.9921

Rating: 4.9815 / 5

Engagement: 2.51%

Ben Eater

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Shared April 30, 2017

In this video, we talk about the difference between instructions and microinstructions. Then we build a ring counter to keep track of where we are in the instruction cycle. Finally, we hook up a quick-and-dirty implementation of the fetch cycle to demonstrate instruction fetching. See https://eater.net/bbcpu8-control-logic for more.

Support me on Patreon: https://www.patreon.com/beneater


Henrik Danielsson

The way you speed up the video but keep your explanations perfectly synced is beautiful! That and the mesmerizing "slotting in" of perfectly fitted wires makes this thing looking like it's developing, almost evolving, so effortlessly and naturally I don't think anyone could have done it better.

2 years ago | [YT] | 161


You should make a microopcode o reset the counter at the end. Therefore different insrructions will use a different amount of clock cycles. The microopcode that resets will take no time.

2 years ago | [YT] | 54

Amean Abdelfattah

his tone and his ability to enunciate clearly is comforting.

2 years ago (edited) | [YT] | 151


I love how Ben's videos often have no dislikes at all.

2 years ago | [YT] | 56

Esteban Escareno

Haha YES! I just found your channel and started binge watching this entire series yesterday and finished the last video about an hour ago and was so sad. Then, as I was explaining the series to my wife I go to show her one of the videos and see this! Perfect timing!

2 years ago | [YT] | 92

Iron Core

I would like to thank you for this tutorial
I'm a computer engineer student and I found it very helpful to really understand how computer work

2 years ago | [YT] | 24


2 years late to the party, but would it be worth having the microinstruction counter reset by the EEPROM as well? Some of those instructions only have a few additional cycles, it would seem like a grand ol' waste to just pulse the clock and have it do nothing.

6 months ago | [YT] | 4

Paddy Robinson-Griffin

Another cliffhanger! Thanks so much for these Ben, really enjoying them.

2 years ago | [YT] | 8

Eli D

Oh, I have been waiting so patiently for this! I can put off sleep for another video from Ben.

2 years ago | [YT] | 3

Guy Cothal

this is the geek version of a TV show's "to be continued"

2 years ago | [YT] | 1

Ben Harper

Great seeing the comments on the last video incorporated into this video =D Next optimization I'd love to see (that doesn't make the computer significantly harder to understand) is getting the microinstruction counter reset integrated into the control lines so that short instructions can reset the microinstruction counter earlier and move more quickly to the next instruction.

So glad to get the new video, thank you for the whole series!

2 years ago | [YT] | 41


Wouldn't you be able to set an end flag for each instruction which tells the cpu the command has finished and the counter should be reset? it would add a step to each command but would allow you to write longer instructions but not have to count to the max length each time

2 years ago | [YT] | 17


8:36 They are pulling you over.

1 month ago | [YT] | 0

Rolf Jander

4017 chip can replace these two chips. It directly counts by turning on just one output at a time.

2 years ago | [YT] | 2


This is a brilliant, informative course of videos. I'm actually started to build this, it's a great demonstration of what you can make with 'glue' logic..... and flashing LEDs!

2 years ago | [YT] | 2

Random Bit

I was doing some small assembly language programs on my Vic20 (back in the day ;-) and now I know what was going on! THANKS!!

2 months ago | [YT] | 0


wow.I love these montages that you do. They look great. Greetings.

5 months ago | [YT] | 0

Robert Eliassen

Brilliant! It's getting increasingly harder to comment on your videos Ben, because I'm running out of superlatives. ;-) Thanks!

2 years ago | [YT] | 3

Harry V

Wonderful explanation, smooth speaking, beautiful presentation!

1 year ago | [YT] | 0

Михаил Кулагин

You electrically combined the CO and MI signals, so when the IO MI step is to be implemented, the CO signal will also be raised.
I think it was necessary to connect CO and MI on separate inverters.

2 years ago | [YT] | 1