Picture of How to Build an 8-Bit Computer
Building an 8-bit TTL computer sounds like a daunting and complicated task, or at least it did to me when I started out on my journey to understand the architecture of a basic CPU. When it comes down to it, a CPU is fairly simple in operation once you learn the fundamentals behind all of its processes. This project is intended to help anyone interested in building their own computer and gaining the wonderful knowledge that comes along with the process. Don't be afraid to try, you can only learn.

This project will start off by describing the basics of electronics. After that, the fundamentals of binary and boolean logic will be described. Lastly we will then move onto the function of the various parts of a simple-as-possible computer (with a few modifications) as described in Malvino's text Digital Computer Electronics. This means that the end product of this Instructable will be a computer that you can program with a unique instruction set. This project also leaves many of the design aspects of the computer up to you and serves as a guide for building your own computer. This is because there are many ways to approach this project. If you already have a sound understanding of boolean logic and the workings of binary feel free to skip to the meat of the project. I hope that you all enjoy and get something out of a build like this, I know that I sure did.

For this project you will need:

1.) A power supply
2.) Breadboards + lots of wires
3.) LED's for output
4.) Various logic IC's (discussed later)
5.) Free time
6.) A willingness to mess up and learn from mistakes
7.) A lot of patience

Optional (but very useful):

1.) Oscilloscope
2.) Digital multimeter
3.) EEPROM programmer
4.) Sonic screwdriver

Useful Links for a Project Like This:

Digital Computer Electronics: 
TTL Cookbook: http://www.amazon.com/TTL-Cookbook-Understanding-Transistor-Transistor-Integrated/dp/B0049UUV38
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john henry12 hours ago

thanks for all the info and references.
your 8 bit computer looks like a pile of wires lol.
its cool though.

teknohawk6 months ago
It is awesome! One question though, I didn't quite understand what your computer is capable of doing. What can it actually do?

Presumably execute simply binary programs. Adding numbers, subtracting numbers, multiplication, division, maybe some more complex things like calculating square roots or powers. Anything that a standard computer can do within the limited memory it has (256 bytes).

Thanks for letting me know.

Are logical operations like AND , OR, NOR, or XOR essential in performing complex operations like square rooting, or a game. I just wanna know if they are needed. If so, which operation is used the most.

Every single kind of computation your computer does relies fundamentally on the operations of AND, OR, NOR, NOT and XOR. Actually, even AND, XOR and NOR can be composed simply of OR and NOT gates.

i want to ask about about the uses of this 8 bit computer

Since its 8-bit, you can do anything that requires 16 bytes of memory ( depending on your RAM ). Unless you add some external drive... which would allow you to do so much more. But just with 16 bytes of RAM you can probably do operations like:

A+B=C, then



I think.

But with an external drive of some sort your 8 bit computer will be able to do stuff like this https://www.youtube.com/watch?v=qYvr0b8jqbg ...i think?-e-dah-puzi-1998

16 bytes? How did you get that? Surely an 8-bit address buffer should be able to address 2^8=256 bytes of memory?

What ROM ic's did you buy.

dark321 month ago

Can anyone tell me which RAM IC I should buy?

Medb13 months ago

i like your instructable and i'am building it i'am wondering if it's possible to calculate big numbers with this computer or i am limited to 8 bit numbers only?

in 32 bit computer you can calculate bigger number than 32 bit, i think it's a software thing but anyone can make it clear how it's done ?

Sorry for my english.

zv_odd Medb12 months ago

You certainly can, you just have to do the calculations over a series of operations, also storing and loading from RAM. As opposed to having the CPU's ALU do it all for you. Just like how you would have to manually calculate Floating Point operations on old CPUs. Yes it is much slower!

Cyanicsky09 Medb13 months ago
lots of registers and a big ALU would allow bigger number calculations to be done.

how many wires do I need?

Cyanicsky093 months ago
just an idea, couldn't you use a decoder for the binary to decimal conversions, instead of using an arduino (using another computer I think is technically cheating)?

I think someone forgot to cable manage :P

haha sonic screwdriver. This is awesome

falloist5 months ago

this is so awesome i am about to cry. congrats

MasoodZ6 months ago

This is amazing will try this hmm.. maybe next year

SajithP6 months ago

I love it. Thanks a lot. Nicely explained. Awesome. :D

techno guy6 months ago
cool, you've made the Apollo flight computer (not really but still cool :p)
Chakuli6 months ago

cost to make computer?

bdraško6 months ago

Where can i buy those nice leds? :D

JonathanM96 months ago

I clicked this link, saw the image of the bread boards swathed with a gazillion wires, and literally laughed out loud. Clearly not an easy task, but clearly you've shown it is achievable. Thanks a lot for sharing this!

such skill

eudoxus8 months ago

This is an amazing project, thank you for sharing!

Would it make sense to replace the JMP instruction with some kind of conditional jump? Such as "Jump to a certain instruction in memory at the specified address *if* the accumulator is non-zero"? Otherwise I cannot figure out how to program conditionals (such as a program which outputs 0 if the accumulator contains the value 0, and 1 otherwise). In particular, I can't figure out how to program multiplication with your instruction set (which would be easy with a conditional jump). All of the arithmetic tricks that I know for getting a conditional jump out of a non-conditional jump require multiplication, such as encoding "if A then B else C" with A*B+(1-A)*C in the case that A is either 0 (false) or 1 (true).

Am I missing something obvious? Thanks again!

joelhacks9 months ago
Incredible instructable!
MisterFrogger9 months ago

I think clock can be a simple button at beginning :)

Great project, but I have a question, why always I see 8 bit calculator or 4 bit or such numbers ( 1-2-4-8-16... ), I mean for an 8 bit calculator it can do numbers up to 255, so why not a 7 bit, with limit of 128? I mean the whole bit thing is just summing of multiple full adders, that will work like a chain reaction, or I'm missing something?

a bit is the smallest form of computer memory

so basically 8 is its maximum storage and processing power which isn't much for a computer but it is all it need's for calculating numbers also why would you limit the ability of a calculator?

csleong981 year ago
This is awesome !!!

do you have a multisim or any other simulation files of the end result? if so could you please mail it to me at aus284@gmail.com if possible.

DrifterNL1 year ago

What a great idea using the XOR and C-in to preform a subtraction!

xtjacob03 years ago
How many of each chip did you order?

I know right. i want to know that too!!!

If you are asking that, it means that you do not have understood this document. Keep studiing it !

spel3o (author)  xtjacob03 years ago
It depends on what schematic you end up using for your computer. In the end, it is really up to you. To know how many of each chip you need to buy it is best to look at the datasheet. For instance, if you need an 8-bit register and the datasheet for the chip that you are buying says that the part only has 4 bits of storage, you would need to buy two of those parts for one register.
If I'm not mistaken, Zilog have dumped the ever-reliable Z-80 chip, but I do believe that there is Z-80-H or higher still in their catalog.
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