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Bit idea : The Witcher 3 written in assembly with 40 billion lines of code so you can run it on an old windows 98 desktop
(hexbear.net)
I don’t wanna any nerds in here explaining why this is impossible I choose to believe it could be done in theory
I was alive and played games during that era, so I am aware! I'm very personally aware how much work we crammed from a tiny slow processor and a few kb.
And though we probably wouldn't describe those games as 'poorly optimised', there's still absolutely huge room for optimisations. As you say, compilers have gotten a lot better, and we have better algorithms and data structures. In Quake especially, they basically did have order-of-magnitude gains by writing some parts in assembly. And they'll have undoubtedly have written most of those parts very imperfectly.
I may be skirting the bounds of reality more than you, by talking about what's 'theoretically possible' compared to what's possible by today's knowledge and human practices. We agree there'd be some graphical compromise, maybe we're just disagreeing over details of the scale of that compromise and the hardware that would be 'permittable' by this scenario.
I was imagining something that looks and plays almost exactly like Witcher 3 except at lower resolution, which doesn't seem possible to me. I'd say if you simplify the graphics (replace dynamic lighting with pre-baked lightmaps, do gouraud shading instead of PBR, remove most post-processing effects etc etc) and make all models look like half-life assets, you could definitely render something at an acceptable frame rate. However asset streaming would still be a huge issue for a large and detailed open world like Witcher 3, considering memory limits and slow disk reads