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I am a software developer trying to understand how computers work at a lower level.

I understand that there is a difference between an Integrated Circuit, a MicroProcessor and a MicroController. A Microcontroller is an integrated circuit but an IC is not necessarily a Microcontroller.

I have had a look through my book, which shows lots of examples of logic gates used in IC's. Is there a one to one relationship between a logic gate and an IC? The IC's I have looked at suggest that there is not, but the circuit diagrams suggest that there is i.e. on the circuit diagram, the IC is replaced by a single logic gate.

Also, I read somewhere that a NOR gate has four Transistors. A Microcontroller has a billion Transistors, so based on this logic; does a microcontroller have millions of logic gates?

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Microcontroller would be in the range of 50000 transistors, plus 4 or 6 more for each bit of memory. Z80 (workhorse CPU of the 8-bit days) was something like 4500 gates. Not so bad, eh? Laptop CPU now, that's a different matter.... – Brian Drummond Dec 20 '12 at 12:11

3 Answers

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There's no relationship, it's like asking about the relationship between cars and nuts and bolts - one contains the other, but there's more to it...

Have a look at www.nand2tetris.org which is a course starting at the basic building block (the NAND gate) and working up to a microcontroller playing tetris.

You can of course go further "back" than a NAND gate by breaking it down to its component transistors, etc., but that's probably not overly helpful to a software engineer unless the question is about the actual physics of system performance.

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Thanks, I am looking at a circuit diagram where the IC is replaced with a single OR gate. Does this mean that the IC has one AND gate or is this some level of abstraction? – w0051977 Dec 20 '12 at 10:56
+1 for the link. – w0051977 Dec 20 '12 at 11:51
The diagram is probably abstraction - logic IC's usually contain more than one gate, plus they must have power and ground connections, smoothing capacitors across the power rails, etc. to work, but that stuff is not important when you are trying to show the function of the system, as opposed to the full electrical circuit. Likewise, the gate is shown as a gate rather than a collection of transistors, as that's not important. – John U Dec 20 '12 at 12:49
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IC is just a package - they put whatever they like on it. It could be a single gate in IC, several gates, a microcontroller (which is effectively a complete computer), a processor, memory or just some analog circuit. There might be not only transistors in IC, but passive compenents like resistors and capacitors too.

IC stands for "integrated circuit" and that is all that is - a circuit made to fit in a small form factor and packaged in a small bug-like thing.

Logic components like NOR, NAND, etc are basic building blocks for computer, so yes, any processor or memory is built of logic gates.

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I was going to suggest getting hold of "The TTL Data Book" (from Texas Instruments) in one of the single volume editions, but YOW! the prices they are fetching on Amazon Marketplace!

Which I suppose reinforces what I was going to say about them - they are a goldmine of basic information that you won't easily find elsewhere - including transistor-level circuits for simple gates, and gate-level circuits of everything up to multipliers. (bipolar transistors of course!)

You can have mine (4th European Ed, 1980), for its weight (1123 grams) in gold...

Or keep an eye on eBay...

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I got you beat. I've got the second edition copyright 1976. Inside on the title page it says "$4.95 (in U.S.A.)". That's about $20 today. If I remember right, I got it for free by asking a TI rep, which is how I think most people got them. The price was just to make the favor look good. No way I'm selling it for $20 though. Mine is 1126 grams. Give me that much gold and you can have it. I'll even throw in free shipping! – Olin Lathrop Dec 20 '12 at 12:23
I have a Basic Electronics book from Radio Shack that I had since I was a kid, on the cover it has a sticker $2.95, my friend just bought the same book for his son, $24.99! – Garrett Fogerlie Dec 20 '12 at 12:40

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