Some low hanging fruit to reduce allocations: the discriminated unions in Instructions.fs could be [<Struct>], reusing field names to reuse internal fields.
Also, minor nitpick but I'm confused about some of the registers. They are already of type byte, the setters with `a &&& 0xFFuy` don't add anything over `member val A = 0uy with get, set`. I'm guessing this changed over time.
There is some hope for humanity after all I suppose.
Even when I want code written in a different language (e.g., C/C++), I often still start by making a prototype in F#. This helps me nail down the logic without having to worry about things like allocation or layouts. Perhaps I could ask an AI to do this second step for me, and then use the F# implementation as an oracle. Anyway.
For example, I usually like using the `Map` data structure, and that's a pretty neat immutable structure and is usually fine for most stuff, but when performance becomes critical, it's easy enough to break into a boring imperative loop with a regular hash map. If I keep everything contained into one function, I usually can avoid feeling super dirty about it.
There are basically two points to programming with immutable-first data. One, eliminate certain classes of data race concurrency bugs. Two, less mutable state in a given context makes it easier to reason about.
So, if you're inside a function scope and you aren't launching any concurrent operations from inside that function, you don't have to worry about benefit #1. If you're inside a function (and you're not reaching out for global mutable state), then the context you need to keep in your working memory is likely fairly small, so a few local mutable variables doesn't significantly harm "understandability" of the implementation (in most cases). So, you really don't have to worry about #2, either. Make your functions black boxes with solid "APIs" (type signatures), and let the inside do whatever it needs to make it work the best.
Just because premature optimization is the root of all evil, it doesn't mean we need to jump right to premature pessimization...
https://gbatemp.net/threads/no-gmb-2-5-dos-full-version.6039...
I've got fond memories of using this to get a preview of Pokemon Gold before it was released in NA!
I mean, ideally, we'd run different color test patterns through, in different lighting conditions, to build a really detailed model, right?
I've been going through a lot of very old stuff recently and a lot of it is well preserved in a way but given enough years everything changes.
I don't think any original Gameboys have been made in twenty years or more.