Logisim - A logic simulator I've built an 8-bit CPU in

[Matte]

I don't know with you, but in the last few weeks, wiremod has only been a portal for me to build things with wire logic gates. That is, until I discovered Logisim.

Logisim is a schematic based electronics simulator. It works pretty much like wiremod. You spawn a gate, then wire it to other gates. It has a couple of advantages over wiremod, though. No lag. There is only paint-like graphics to render, and you don't have to compute physics and all that stuff Garry's Mod requires. You can also create subcurcuits. The ability to make subcurcuits lets you create a logic circuit, then turn it into a chip, so you don't need to repeat. I love this program so much, and I've spent the last two days making an 8-bit programmable CPU. By looking at the following images, you may get a picture of how complicated a CPU is, and how many thousand logic gates a simple CPU requires.

Here's a picture of how my CPU currently looks. As you can see, this picture does not contain a single logic gate. That's because it's only built up of subcircuits:



Do you see that rectangular microchip to the right there? That's the ALU. Some of you might remember the 8-Bit ALU I built in Garry's Mod. Well, this is an extension containing Addition, Subtraction, Multiplication, Division, Modulus, Exponention, Comparing (greater than, equal to, less than), AND, OR, XOR, NOT and random. Here's a picture of how it is built:



Still no logic gates, but remember that all the subcircuits here is made up of tens, maybe hundreds of logic gates.

All the microchips in that picture is handling the data simultaniusly, then a multiplexer is selecting the data based on an opcode recieved from the Instruction Reader.
If you scroll up and look at the CPU, see that square subcircuit in the middle there? That's the instruction decoder and sequencer. Let's see how that subcircuit looks:



This circuit is reading the program code from a ROM (read-only memory) then decoding the instructions and sequencing them, then it runs the code. You can barely see any logic gates here, either. So, I'm gonna show you how one of the mutliplexers is built. See that microchip marked "MUX"? That's a multiplexer. I'm gonna zoom in on that multiplexer to see how many logic gates such a simple chip requires.
Here's a picture of a 4x8-bit multiplexer:



As you can see, all this subcircuit does is to wire up 4 8-bit multiplexers. The 8-bit multiplexer looks like this:



Think that every subcircuit you've seen on the last pictures requires as many gates as this. You will now have a vague impression of how many logic gates a simple CPU really requires. The first ever CPU commercial, the Intel 8080, was an 8-bit processor, and it had 6000 transistors. CPU's today have about 125 million.

[ryland]

Nice, what did you make the schematics with?

[Matte]

Nice, what did you make the schematics with?

That's how the program looks. And that's what is so great with it. It doesn't require a monster of a computer to render unnecessary heavy graphics

[ryland]

That's how the program looks. And that's what is so great with it. It doesn't require a monster of a computer to render unnecessary heavy graphics

Cool Ive used altium to do that before.

[Chrono101]

I use Quartus II 9.0 Web Edition for this kind of stuff. It's free, and it does everything that Logisim does, except it also has support for Verilog and VHDL. Most professional electronics engineers don't design circuits in a graphical simulator anymore, because it is too time consuming. They use Verilog or VHDL and just "program" their circuits. Quartus II can convert Verilog and VHDL to schematic diagrams, and vice-versa, and then lets you simulate inputs and stuff as well.

[Nikita]

I liked playing with Logisim, but there is just one big problem: It keeps whining about "Oscillation apparent" and stops the simulation.

[Matte]

I use Quartus II 9.0 Web Edition for this kind of stuff. It's free, and it does everything that Logisim does, except it also has support for Verilog and VHDL. Most professional electronics engineers don't design circuits in a graphical simulator anymore, because it is too time consuming. They use Verilog or VHDL and just "program" their circuits. Quartus II can convert Verilog and VHDL to schematic diagrams, and vice-versa, and then lets you simulate inputs and stuff as well.

I know, but I find it more fun to place chips and wire them up

I liked playing with Logisim, but there is just one big problem: It keeps whining about "Oscillation apparent" and stops the simulation.

That happens when you create an "infinite loop". It always takes some time for a gate to handle a data input and then output, so if you are creating a circuit like this, you will have an infinite loop:

[Whosdr]

The site seems to be down.
Can we get a download?

Pinging gave timeout.

[N00bDud3]

I found it on Sourceforge by searching Google.

[GUN]

matte can we get a save for that cpu? i want to see how you made all that crazy stuff plz :P