Run your programs against a real 6502. 6502 As A Service.
The 6502 is now available in the Chiplab! This allows you to upload your programs to be queued for running against a real 6502 chip. For each cycle of execution, you can observe the values of all the busses of the physical chip.
This can be used in place of an emulator for a highly accurate way to test your 6502 programs. It can also be used to research the behavior of this chip, which I believe will be an effective way of developing highly accurate emulators. Towards the end of the article I’ll share a repo in case you’d like to help.
You can run your programs against a real 6502 chip today. The rest of this post will cover some technical details involved in getting the chip online.
Using the 6502 Chiplab.
The system is designed to allow you to write code in assembly language, or upload your pre-assembled programs. These will be queued for execution on the chip. After it is your turn, your program will be allowed to run for 100 cycles, which takes around 1 second.
Once your program is finished running, you will be provided with a trace. This shows the values of the address(16 bits) and data(8 bits) busses. Your programs can write to memory if you want to see some result captured in the trace.
How does it work?
The 6502 interacts with all peripherals though the memory bus. The 6502 has some internal state that cannot be viewed directly, but otherwise can be throught of as reading and writing from memory repeatedly.
The system, included the 6502 and other peripherals like memory, is synchronized through a signal called the clock. When the clock signal changes, all connected peripherals take this as a signal that they can do one extra step of work. At each change of the clock, the chiplab monitors the address bus, and updates the data bus to provide the value corresponding with your program at that address. In this way the lab simulates the surrounding environment for the 6502.
Sort of like “The Matrix” but for the 6502. You can read more about the details in this previous post on emulating program rom for the 6502.
Why do this?
Through these reads and writes, we can get glimpses into the internal operation of the chip. Now for the 6502 this is a bit unnecessary, as the internal operations of the chip are already well known.
But there are many systems for which the inner workings are not well understood. This chiplab, initially tested with the 6502, should provide a way to learn and build models for these less well understood chips.
Attempting to rebuild a 6502 with this setup is sort of a test for more advanced systems in the future. Help wanted.
There are also at least a few chips which involve an embedded 6502 and some other custom electronics. So building the 6502 is a direct stepping stone for modeling those other chips too.
Building a digital 6502 model
The behavior of the real test can be used to generate test cases for a simulated version of this chip. Fortunately, the 6502 (and most other ICs) have many pieces of independent functionality. This allows a virtual representation of the chip to be up incrementally.
Small isolated tests, such as those for a single instruction, can be created by writing 6502 programs that use only that instruction. By capturing a cycle-by-cycle trace of the busses as a real chip runs, we know what the behavior of our model should look like.
Want to help? Take a look at the 6502_model repository on Github. The model is written in a simple subset of Rust. The goal is to capture the behavior of the system, not necessarily build something that can be synthesized for silicon. Traditional programming languages are much more expressive than typical hardware description languages (HDL). Even if you dont know Rust, most of the logic should be easy to follow if you have experience with one other programming language.
Questions / comments? Send me an email, which you can find in the site footer. Thanks for reading!