
Building a 4 bit computer from Transistors - dholowiski
http://www.waitingforfriday.com/index.php/4-Bit_Computer
======
geuis
Lots of comments denigrating this demonstration. I'd like to say that as
someone who understands some fundamental things like AND/NOT/OR etc only at a
high level, its very illuminating to see how the math is implemented in
hardware.

You have to remember that 30-40 years ago people were building entire
businesses building computers by knowing this stuff and wiring everything
together by hand. Many younger folks now have little to no idea how the
devices they use every day work at a fundamental level, and its videos like
this that could let a young hacker-to-be pick up a few parts inexpensively and
easily wire up their own basic logic circuits. Its this kind of hands-on
education that helps curious people learn and explore.

~~~
marshray
Well, 30 years ago was 1981 and the IBM PC was kicking off that part of
history. 40 years ago was 1971. There were still many commercial
transistorized systems but most new designs were going to integrated circuits
quickly.

This <http://en.wikipedia.org/wiki/IBM_1401> is probably a representative
example of the transitorized computer, produced from 1959 to 1971. It's a good
bet that the people who designed it knew how to solder but were different from
those who assembled it. They knew the transistors, but probably designed it in
terms of higher-level blocks of logic (adders, memory, etc).

Not many people have designed and built full computers from scratch at the
transistor level. Enthusiasts didn't really get into it until microprocessors
were available. Even then, they required hundreds of solder connections to be
able to even use lights and switches for IO.

Fun times.

~~~
stcredzero
NAND to Tetris -- build your own computer system from NAND gates on up to your
own Object Oriented language, operating system, and simple games.

Just doing this one course to tie together a Computer Science education puts
you a step ahead of 50% of the coders out there.

[http://www.catonmat.net/blog/video-lecture-from-nand-to-
tetr...](http://www.catonmat.net/blog/video-lecture-from-nand-to-tetris-
in-12-steps/)

~~~
marshray
When I was a kid, my hero was David L. Heiserman. <http://www.beam-
wiki.org/wiki/Heiserman,_David_L>. His book "Build Your Own Working Robot"
(ISBN 0-8306-6841-1) I must have read a hundred times.

It wasn't quite as low as transistor logic, but it was designed from discrete
logic gates.

I also remember him writing "How to design & build your own custom TV games",
which was basically NAND to Pong, without a CPU.

------
jws
What‽ It isn't even Turing complete.

How about a 16 bit computer made from 3000 transistors[1] that runs at 500kHz?
<http://www.6502.org/users/dieter/mt15/mt15.htm>

[1] umm, and a couple 74 series chips to generate the clocks and buffer the
busses.

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msarnoff
"Code: The Hidden Language of Computer Hardware and Software" by Charles
Petzold does an excellent job of explaining everything (from logic gates to
CPU architecture to graphics) to the layperson. If any nontechnical person (my
dad, for example :p) asked me how computers worked, I would tell him/her to
read this book.

Even for a technical reader it's still an enjoyable book. It inspired me to
design and make a computer of my own.

------
RodgerTheGreat
Calling this a computer is rather misleading- it's really a 4-bit adder.

~~~
marshray
Let's give this guy credit where due. It's probably the most awesomest
transistorized 4-bit adder handmade in the US since 1973.

~~~
RodgerTheGreat
I will say it's a very clean build and a nice writeup.

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jcw
Thanks, I've been looking for a good tutorial on this for a while. Going to
try to implement this in Minecraft (it's funny how Minecraft has incidentally
made circuitry easier to get into).

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joshu
Lots of half-gates means lots of pulldown/pullup resistors. Probably uses a
ton of power (per bit, or whatever.)

We had to build a full 16-bit machine in school. Thankfully, we were allowed
to use 290x ALU slices.

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RiderOfGiraffes
Site's down for me - has it been slashdotted by HN? (so to speak)

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vitaIiy
This is a dumb 4-bit adder, something every CS student builds using
breadboards, in a middle of some Digital Logic course.

~~~
RiderOfGiraffes
Perhaps in some schools, and perhaps in some courses, but not usually with
transistors.

~~~
iuytryuik
Well they would have played with a javascript simulation of a half-adder, or
at least seen a youtube video of a powerpoint presentation describing it -
that's good enough for CS these days.

