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I want to build a little 4 bit computer, out of logic circuits. I want RAM and a CPU and all that good stuff. My main problem is, should I work on RAM or CPU, which one would "teach" me more so that I could build the other, and could someone give me a circuit map? Thanks, if you helped!

Since I have asked this question, I have learned much more about electronics. So on October 1st 2015 I will do a live stream where I will be designing a 4-16 bit computer with logic gates, transistors and nothing else really! Go here to watch.

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    \$\begingroup\$ Go here and start learning. The linked Coursera course accompanied with this site is one of the best (actually - the best one) I've seen on the subject. \$\endgroup\$ – Eugene Sh. Jul 9 '15 at 18:59
  • \$\begingroup\$ RAM will show you the basics. Do that before trying a CPU... \$\endgroup\$ – Brian Drummond Jul 9 '15 at 19:47
  • \$\begingroup\$ @Brian Drummond Could you give me a logical gate map? \$\endgroup\$ – ender_scythe Jul 9 '15 at 19:53
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    \$\begingroup\$ A computer (even the simple one) is not a "basic schematic". \$\endgroup\$ – Eugene Sh. Jul 9 '15 at 20:09
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    \$\begingroup\$ @EnderScythe Know that your project is cool and there's nothing wrong with trying to find a schematic to base your design on. But this question isn't a good fit for this site. It's too broad, and our goal here is to answer specific questions, not design entire circuits for anyone. \$\endgroup\$ – skrrgwasme Jul 9 '15 at 21:28
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Yes, you can build a computer from scratch. 4-bits is a good choice, since it uses considerably less hardware (but obviously more than half) of an 8-bit computer. The first commercially available microprocessor, the 4004, was a 4-bit device. Its successor was the 4040, also a 4-bit device.

In the home-brew computer field, 4 and 8-bits are the norm. 16-bit computers are less common, and I have never seen a 32-bit home-built computer. (Note -- I am limiting this discussion to designs where the entire computer is built from scratch, i.e. they don't use a conventional microprocessor chip; however some homebrew designs do use the 74181 4-bit ALU, commonly found in minicomputers such as the PDP-11).

Here are five 4-bit computers that others have built. In three of the examples, complete schematics are available.

Fourbit, 4-bit homebrew CPU design, and schematic available

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4 bit computer built from discrete transistors design available

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Build this 4 bit CPU from TTL ICs. design and schematic available

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APOLLO181 Homemade 4-bit TTL Processor design and schematic available, uses 74181 ALU

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Homebrew 4-bit Computer using TTL ICs Youtube video only

One of first things you will need to do is figure out the instruction set. This is the fun part, as far as I'm concerned, as you visualize how your computer will run your programs. With just a 4-bit memory path, you will certainly need some instructions longer than 4 bits. So do you make all instructions double-wide? Or use variable length instructions? You choice.

Check out the instruction sets for the computers listed above, in addition to the 4004 and 4040, for ideas.

There are a lot of examples in the Homebuilt CPUs WebRing, which include some more 4-bit as well as 8-bit and 16-bit home-built computers. There's even a couple of relay computers.

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  • \$\begingroup\$ It's pretty cool to see these, I can't imagine the perseverance and ambition of people back in the day to just go and make these things happen. I take so much for granted these days, it's sad. Saw the movie "Jobs" recently, and that was basically it!! \$\endgroup\$ – KyranF Jul 10 '15 at 18:45

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