Simple CPU

http://www.simplecpu.com/Binary.html

by michael_fine • 12 years ago

617 62 12 years ago

alain94040• 12 years ago

Anyone who cares about this topic is welcome to contribute to https://en.wikibooks.org/wiki/Programmable_Logic/Verilog_for...

I started it at the request of some HN users, and I hope it continues to hopefully enlighten some software folks who wonder how the CPU they rely on actually works.

jwise0• 12 years ago

I'll rant for a moment here, because I think this is something that the Simple CPU link has the potential to get right, and that I think is a common pitfall with digital design tutorials.

I think one of the pain points that you commonly see with tutorials is the concept that the hard part is the language. As a digital designer, I think that's explicitly not the case: just like teaching functional programming, the syntax is not the challenge, but the mode of thinking. Digital design is a skill that really requires deep thought about what a given piece of syntax will compile down to; it's possible to skim over the "building blocks" of software (for instance, it is possible to be a very good JavaScript programmer without knowing what the JIT will transform your code to), but in order to build effective (or even possible-to-synthesize!) hardware designs, I believe that it is very important to start with the fundamentals of how hardware actually works.

So, I plead: if you have cause to write a digital design book, don't write it as a "Verilog for Computer Scientists"! Verilog is a big language, and not all of it is good for writing hardware. What you really want is "Digital Design for Computer Scientists".

sliverstorm• 12 years ago

Yes, the two key hurdles to HDL are (in my opinion):

- Being able to think parallel. Software is sequential by default, HDL is parallel by default.

- Understanding what you want to build. If you can't draw what you want, you probably can't write good HDL. There's a reason it's called Hardware Description Language, and not "JS2GATES". In a sense, HDL is really just text-based schematics.

Case in point:

http://www.allaboutcircuits.com/vol_5/chpt_7/8.html

When you think "HDL" you probably think of Verilog or VHDL, but SPICE netlists is also an HDL.

niklasni1• 12 years ago

I'm not sure how and where or indeed if to contribute, but as a programmer who learned digital electronics (stack of free 74xx chips) before diving into programmable logic, one thing that really helped make these things click for me was thinking about hardware as transistors and learning the very basics of electricity and related physics. This helped me see digital electronics not as a sequential process but as a consequence of natural laws, like something dropping due to gravity.

Thinking of a transistor as a voltage controlled switch (obviously simplified to the point of being wrong but works for understanding digital) and seeing how you can use those to build logic gates, and then use gates to build muxes and flip-flops and so on up was essential for my understanding HDLs.

(Incidentally, all that has also made how computers and machine code work much clearer to me).

I don't think I could've really done it without that foundation. And I certainly couldn't have done it by just thinking of Verilog with my programmer's idea of concurrency.

It's obviously very low level and you'd have to gloss over a bunch of things, but I'd definitely start with transistors if I had to explain programmable logic to somebody.

foobarian• 12 years ago

I had a similar experience. Digital logic is very clean and neat, but the part where things broke down for me are things with feedback loops like latches. It's a bit tricky to tell what's going to happen in such devices without some analog background.

virtualSatai• 12 years ago

The gates with the simple toggle functions are really good and simple to use. It reminds me of playing around with redstone in minecraft.

Knowing most if not all of this already I cannot say how well it teaches, but it does describe what I spent much longer on learning.

orf• 12 years ago

I'm in my 4th year of University and this just explained the concepts in the best way I've ever seen, far better than any of my lecturers. Thank you.

acqq• 12 years ago

Here Feynman explains the basic logic behind building of all of the computers. He is inspiring even if you know more details about the topics he talks about, because he stimulates you to a fresh view. The youtube title is, IMHO, misleading. The lecture is about the basic hardware concepts and not about heuristics, so I think it fits perfectly with the OP.

http://www.youtube.com/watch?v=EKWGGDXe5MA

Edit: At the start the original title is visible it's "Richard Feynman: The Computers From the Inside Out" and the lecture is from 1985.

The major quote:

"Computers are not primarily computing machines, they are actually filing systems."

AlexanderDhoore• 12 years ago

Listening to Feynman talking about computers on a sunday evening. Life is good.

tinco• 12 years ago

Thanks for that. Man, Robin Williams would've played the heck out of a Feynman movie..

rdc123• 12 years ago

Saying that is torturous

kemist• 12 years ago

Thank you

ca98am79• 12 years ago

Listening to the Q&A, I am surprised that, despite how much computers have changed in the last 30 years, how little has changed:

https://www.youtube.com/watch?v=EKWGGDXe5MA&feature=youtu.be...

AndrewDucker• 12 years ago

The memory scrolling doesn't work in FF - you can only see the initial 6 locations... And he's fixed that. Awesome!

Also - it would be very neat to have a "reset" on the Instruction Pointer, so that you don't have to refresh the current page if you want to start your program over...

michaelmior• 12 years ago

This is fixed in the sense that I can now scroll through the memory, but the page scrolls at the same time, which makes it a little awkward.

rogertux• 12 years ago

At 1st year of CS they taught us to design and understand the simple machine, a simple computer with just a CPU and a control unit. They also taught us how MIPS architechture worked (quite simplified). I still remember the struggle to rotate a matrix in mips.

javajosh• 12 years ago

I really like this presentation and content. It's accurate, interactive, and brief. Feynman, I think, would be pleased. What these few pages describe are precisely the "atoms" that underly computation, which is truly counter-intuitive, remarkable, and (these days) ubiquitous. (Which describes a lot of things! Black body radiation is my current favorite).

Two suggestions to the author: first, identify yourself on the site! Second, interpret "0" as "STOP" or "RESET" to prevent the simulation from running to infinity. Alternatively, adjust the UI to indicate that the PC is off-screen.

mockery• 12 years ago

This is awesome.

"You can scroll through memory as well" --> Would be nice to have a scroll-bar because I missed this at first and wondered why there were only 6 bytes of memory. (Would also be nice to see a bit more of the memory at once, although then the instructions might be easy to miss. Maybe a "show more memory" check-box?)

Would also definitely be nice to reset / modify the instruction pointer.

And of course the ability to save/load the CPU state (eg. simple copy/paste to the clipboard.)

Next steps would be to teach about basic I/O, since that's a fundamental part of any computer. (Start with some simulated LEDs/toggle switches that map directly to a special address space?)

jhdevos• 12 years ago

A button to execute a single instruction would also be nice.

ajkjk2• 12 years ago

Very nice.

One request: don't introduce the concept of 'nibble' on the first page. It's jargon that no one needs in this general introduction. And it just sounds silly.

noonespecial• 12 years ago

I don't know. I heard about nibbles right at the beginning and then promptly forgot them. I even remember thinking that the word was silly. But it set me up for a very nice epiphany the first time I saw bytes written as hex and played a part in my instantly grasping that.

ajkjk2• 12 years ago

I'd be curious to poll people's experiences, but, I can say that I had never heard the term until today and I've known this stuff for a number of years.

And it sounds silly and adds nothing. That too.

(edit: my education on this stuff was in the 00's. Sounds like it was more common in the 80s. I'd definitely support obsoleting the term entirely now, though! It adds unnecessary complexity.)

olive_• 12 years ago

I wish they would give us such a summary at the college before talking about all those confusing stuff. Excellent work!

ygra• 12 years ago

They did for us; right in the first two semesters we had 4–6 hours a week just low-level stuff. They also introduced us to algorithms and data structures with Pascal, not Java (which I'm very thankful for).

It was quite enlightening to see over the course of three weeks or so how a (simple) CPU could be built from ever-larger building blocks, but starting at the very beginning with transistors.

orbifold• 12 years ago

In the third semester at university in Germany there was a course "Introduction to technical computer science", where one would learn about things like Karnaugh Diagrams, compression algorithms, D-Latches and so on and built a 16bit optionally pipelined RISC CPU in lab assignments. The professor teaching the course had developed custom signal processors for the LHC, so that was pretty cool aswell.

greenster• 12 years ago

Reminds me of this course: http://nand2tetris.org/

Easily accessible, fun, and if you find it too simple, you can always introduce others to it.

stevek• 12 years ago

I also really enjoyed the course. The book behind this course is called "The Elements of Computing Systems". I've been recommending it to lots of people.

wazoox• 12 years ago

It's very similar to Charles Petzold's "CODE" book. An excellent read.

http://www.charlespetzold.com/code/

MichaelGG• 12 years ago

That book made it all snap together for me. It really is fantastic. After reading it, I felt I knew enough to build hardware from first principles, even I'd be slow and terrible at it.

rdc123• 12 years ago

I found it easier to understand how the d-latch worked from this they Petzold's book, probably because I could see the changes vs working it out and having to convince myself I was reading it all right.

peteri• 12 years ago

Just as an aside the D latch you're showing is the more traditional NMOS circuit rather than CMOS (see the diagram at the bottom of page 510.

http://books.google.co.uk/books?id=5zUABAAAQBAJ&lpg=PA511&ot...

bprater• 12 years ago

If you want a comprehensive read on this topic, check out the book Code by Charles Petzold: http://www.amazon.com/Code-Language-Computer-Hardware-Softwa...

HanyouHottie• 12 years ago

I thought this was pretty clever:

  00000001 <-- this line is both an instruction and data.
  00000011
  00000001 <-- this line is both data and an address.
  00000011
  00000111
  00000000
  11111101

A program that doesn't work if you move its location in memory is pretty hilarious.

Danieru• 12 years ago

You may be interested in the story of Mel: http://www.pbm.com/~lindahl/mel.html

userbinator3• 12 years ago

Overlapping code/data is a trick used in practice in highly size-constrained systems, like older game consoles (e.g. http://andrewkelley.me/post/jamulator.html#dirty-assembly-tr... , discussed here at https://news.ycombinator.com/item?id=8405214 ) and in the tiniest categories of the demoscene.

muyuu• 12 years ago

That was extremely common in 8-bit days. But then again it was commonplace to destroy userspace upon loading of programs and you finished your session by either resetting the computer or turning it off (1~2s boot times helped though).

moron4hire• 12 years ago

Just goes to show how good my reading comprehension is. I completely failed to realize that the subtract operation was operand B from operand A, not the other way around.

timbt2• 12 years ago

This is my first real introduction to uber-low level computer science. I managed the following super simple program:

1. 00000000

2. 00000001

3. 00000000

4. 00000001

5. 00000010

6. 00000011

7. 00000011

8. 00001010

9. 00000000

10. 00000001

The CPU jumps to the instruction at #4, which tells it to add the value of #2 to #3. The instructions at #7-#10 reset the instruction pointer to 00000001, so that it'll increment to #4 again. The result is that #3 is continuously incremented up by one.

I've usually preferred to stick to really high level stuff (Like JS/Node.js), but trying this out makes me want to go learn assembly or something!

maxerickson2• 12 years ago

This makes it into a bit of a game:

https://microcorruption.com/

voltagex_• 12 years ago

I feel like there's a few tutorials needed in between writing your first opcodes and passing the first level of microcorruption ;)

timbt2• 12 years ago

I tried to sign up, but I got a "Sorry, but this form is no longer accepting submissions." :(

KeriWarr2• 12 years ago

I was attempting to create the most simple infinite loop that I could, and found that the instructions do not work in the way that I thought they would. Here are a couple programs I wrote. If someone could explain why they work the way they do..?

00000011

00000100

00000000

00000001

Looking at this, I expected that it would read line one, then move the value addressed by line two to the value addressed by line three. i.e. set the instruction pointer to 00000001. This didn't work. What did was:

00000000

00000011

00000111

00000000

00000001

This program advances to line 4 then remains there. Could someone explain why program two is an infinite loop, but program one isn't? Thanks!

r00nk• 12 years ago

Hi, I made this site.

The problem is that it moves that value into the instruction pointer, then the instruction pointer increments itself by 3.

the instruction pointer always increments itself by 3 after every instruction.

KeriWarr2• 12 years ago

Thanks all, for the explanations :)

hundt• 12 years ago

Your first program does as you expect, but then the instruction pointer gets incremented by 3, so it goes on.

If you modify the last line to "11111101" then when the instruction pointer is increased by 3 it will overflow back to 00000001 and you will get your infinite loop.

Vendan• 12 years ago

The CPU runs the opcode(instruction), then adds 3 to the IP. Therefore, your first program sets IP to 1, then it gets 3 added to it, setting IP to 4. Your second program is exactly the same, but setting IP to 4 means infinite loop.

friendzis2• 12 years ago

As nothing is moving visually it seems to be doing nothing. 0001: 0000 0010: 0000 0011: 0000 0100: 0000 0101: 0000 0110: 0000 0111: 0011 1000: 1010 1001: 0000 1010: 0001

markbnj• 12 years ago

Very well done. I'll be passing it on as a great introduction to logic and binary operations.

natch• 12 years ago

This is really cool.

But. (Is there always a but?) it's very unapproachable. It has no welcome mat.

"Let's dive in" is normally an appreciated attitude, but here it makes it sound like we are coming in in the middle, and have missed the introduction. So, what is this thing? The page doesn't really say. There's an about page, but most people are too lazy to click. They want to see the value explained immediately, clearly, succinctly, on the landing page... or they're gone.

If they do click the About page, the writing is stilted, formal. But they don't, in most cases, I suspect.

When there are clickable images, it's not clear they are clickable. The text indicates you should click, but there are these huge arrows right there, and the obvious interpretation of the text is that you're supposed to click the arrows to see the image do its animation. Nope.

CPU is very jargonish as well. A better title would be something like "how computers work."

Still, nice design, beautifully made, good chunking of stuff into bite-sized pieces.

Simple CPU

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