CoBo-CPU : Introduction

Question: What do you do when you have started too many projects and have yet another idea?

Answer: You tell yourself this is too great of an idea to not add it to the pile of unfinished things.

Why? Because I can!

The Idea

What if a Turing Machine would actually be a (somewhat) useful CPU?

When Alan Turing layed out his idea of a universal computing machine he designed it as simple as possible. He, being a mathematician made his life “easy” by defining some of the parts of that machine as infinite: The memory for example. He also decided to separate the “Data Storage” from the “Program Storage” which is not ideal for a simple implementation of a CPU.

So, unlike the classic definition of a Turing Machine my implementation does not use infinite amount of memory and does not separate data from machine code. In fact, my design uses just 16 bits of address space which is quite far away from infinity.

The CoBo Design

CoBo stands for “Cobbled And Bodged”. Which describes the hardware implementation perfectly.

One of the reasons I started this project was that I have tons of LS74HC*** logic chips lying around which I wanted to use for – something.

One of the goals – a challenge, rather – was to only use my existing stock and not buying any more logic chips for that project. Because of this situation, some of the solutions seem quite strange and cumbersome. For example, the implementation of the ALU, which could be done using actual full adders but is instead implemented with “discrete” AND and XOR logic chips.

The Architecture

As you can see, it’s not much of an architecture compared to other simple CPU designs. But the goal was to only add the things that turn the impractical design of the turing machine into a (somewhat) practical CPU.

That includes more than 1 bit of number range and an ALU that can do some level of actual computation. The things that are similar to the turing machine are the instruction counter which represents the “state” of the machine, the data pointer which represents the position of the “read head” and the data register which represents the contents read by the “read head”. Things like the instruction register and the instruction decoder are more or less necessary “infrastructural” elements of the system.

Some details about the architecture:

  • All registers are 16bit wide
  • The instruction counter is an actual counter that can be parallel loaded like a register
  • The ALU is only capable of the following operations: +1, -1, AND, OR, XOR, Shift left/right
  • There’s a carry and a zero flag
  • Every instruction can be conditional
  • Separate memory and IO address space
  • Every instruction is executed in one cycle
  • Interrupt input (must be queried in software)

There are a few more features that are not shown in the diagram, like moving data directly between registers or being able to write the contents of the status register to memory.

So Far So Good

Before I started to design the actual hardware I designed the entire thing in LogiSim to see if it would even work. It does. It also gave me the opportunity to iron out some kinks in the system that aren’t immediatly obvious. Like, where do I get the second operand from when doing a two-operand computation while writing the result into memory. Hence the [0x0000 / 0xFFFF] block in the diagram above. One of the two values can be chosen as the second operand. The first operand is always the data register.

Gate Level Implementation

System Level Implementation

Spontaneous Lamp Build

Sometimes projects go from “whoo, that’s gonna be awesome looking, be fine built craftsmenship and I’ll be proud of it” to “yeah whatever, let’s get it over with”.

The results speak for themselves. Unfortunately.

And, no, I don’t care that it looks like a life hazard. It’s bright, that’s the only requirement.

Also, it IS a life hazard, so appearances don’t deceive here. How thoughtful.

But: none of the large copper areas are connected to the solder pads that carry the components. So it’s save to touch the “case”. It’s not save to touch the LEDs, though.

And, also, no, I particularly don’t care that I could have bought an 8W lamp easily.

I built it, because I can.

C64 Clock – Prototyping

Second Prototype

In the spirit of this site, this would probably be the final cartridge (pun intended).

Currently my successes on etching double sided PCBs are a little nonexistent. So I “quickly” milled a “few” slots in a raw PCB board with a Dremel. I did the rest using variboard.

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Test Site

Prototype investigation.  … of an unfinished project … naturally.

That’s 2 NAND-JK-Flip-Flops built in DTL-Logic, making up a 4 Bit Johnson-Counter

To clear up some details and anwser all possible question about that circuit, here’s the carefully drawn schematic.

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One Day Build

DMX512 Data Rate “Limiter”

Final assembly on variboard

The Problem

You get what you pay for. So if you pay squat, you get squat.
In my case, we bought super cheap LED spots to light our small-ish techno parties via DMX512. The money saved went into a pretty neat and modern lighting console. The result is that the cheap LED spots can’t keep up with data rate of the not-so-cheap console. Which causes the LED spots to do exactly … diddly-squat.

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