Looks like a fun project to code.

Another day, another rewrite! 😁

So I fundamentally changed how the simulation works. I'm not anymore going through all the pin networks and "logic devices" (that should have immediate output based on input), calculating their values (potentially multiple times until they stabilize) and then going through all the "clocked devices" to let them do their job and changes to the pins.
Instead, I rewrote the system into a "reactive" style. Each device can register pins they want to react to (doesn't have to be all input pins, just clock for example) and they will be triggered only when their registered pin changes value.

As a result, performance jumped quite a bit!

The scenario is a 3 device setup: 6502 CPU, 64kB of SRAM and small device (I call it "chipset") that translates memory related control signals from 6502 to SRAM compatible values. (Think of simple 74LS glue-logic, GAL chips or extremely under-utilized FPGA).

Before the rewrite I got a cycle performance of about 25kHz on debug mode and 45kHz on release mode.
Now I get about 120kHz on debug mode and 170kHz on release mode.

And while I didn't have to change much how I had already implemented for the devices (CPU, RAM, glue logic), I did add the trigger pin definition to each device and I was even able to simplify their implementation a bit. (Pins became stateful with this rewrite, so no need to re-write to the pins repeatedly for multiple cycles if you want to keep values on the bus for a while)

Pretty nice! And because it is reactive system now, I can model a huge design/hardware set, as devices are only triggered when necessary. Earlier system became slower with every component I added, even if they never did any work. (Like the "synthetic" device that functions as a set of pull-up and pull-down resistors.)

Btw, did I mention that it runs real 6502 assembly program from memory? It is fully functional.

EDIT:

NO WAY!

More optimizing and i got 350kHz on debug mode and 1.4MHz on release mode!

You just need state machines. This is basic stuff.

kingcake wrote on 2026-01-30, 20:36:

You just need state machines. This is basic stuff.

Yet, I have to build one myself because nobody has written one yet! It is that basic!

GigAHerZ wrote on 2026-01-30, 20:41:

kingcake wrote on 2026-01-30, 20:36:

You just need state machines. This is basic stuff.

Yet, I have to build one myself because nobody has written one yet! It is that basic!

Wut. State machines are a basic computer science concept and are everywhere. You should look into industrial control state machine frameworks for C. You basically just define all the states and the framework generates the rest.

I don't know if anything like that exists for .NET languages, but I would start by porting one of those first. it will make what comes next much faster, and portable.

Sure-sure, kingcake.

Anyways...

I've set up the cc65 development into my solution side-by-side with the C# circuit simulation. So when I run the project, I both compile the hardware design project as well as the assembly code for the 6502. With a single "F5" button, I have the full cycle of development in 2 levels (hardware design vs 6502 software) done. I see a bug, change the code and 1 second later I'm running the new machine. (You can't even flash a ROM chip that fast!)

I've now quickly built a tiny terminal component (using raylib) and connected it to the machine. I also found an enhanced WozMon (EWOZ) and ported it to my machine.

As a result, I have a full working 6502 computer, fully defined/designed in code. At any point I can pause the whole thing and look into every chip and every bus and every line and see what value it has. I can watch the values and single step not the machine, but the processing of a single component, when debugging.

Elementary in software development, but crazy in the context of hardware design.

And speed hovers around 1-1.2MHz. (I let it run as fast as possible)
Similar to VIC-20 and Commodore 64 clock speeds.

EDIT: I missed one silly unoptimized code part. Now it runs at around 1.5MHz speed.