I learned about shifting to divide or multiply when writing my game boy emulator. Something I find fascinating is the lack of precision needed for games of that fidelity.
Say you’re animating a jump and the character slows down towards the apex. I’ve seen an approach where it’s just “every X frames halve the speed until it’s zero.” This is done with the `SLA` (shift-left, LSB to 0) and `SRL` (shift-right, MSB to 0)` operations, which are very fast (8 cycles!) The bonus is that these opcodes will set the Zero Carry flag to 0 if the result is zero, so you know when your jump animation (up) is done if there is no ceiling collision, without having to check the value of the byte.
(I’ve also seen a set of hard coded speeds to animate through, and cases where they don’t change the speed at all, making the animation feel very rigid. It’s amazing what little details are important)
I also wrote a game boy emulator recently with the purpose of educating myself. I haven't looked that deep into what games do wrt math, but I was really impressed with all the creative ways they abuse the PPU by changing its various registers mid-frame. "Batman return of the Joker" for example uses this interesting effect of "compressing" the upper portion of the screen when you enter the options menu. It does this by changing the scroll Y to specific values on specific lines.
Though I do have some math-related problems in there. I spent several hours on it but I couldn't figure out how the SBC instruction should affect the carry flags. No matter what I do I can't pass the test ROMs.
But even with that, many of my childhood games are 100% playable. Writing a video game console emulator is a rewarding experience!
Say you’re animating a jump and the character slows down towards the apex. I’ve seen an approach where it’s just “every X frames halve the speed until it’s zero.” This is done with the `SLA` (shift-left, LSB to 0) and `SRL` (shift-right, MSB to 0)` operations, which are very fast (8 cycles!) The bonus is that these opcodes will set the Zero Carry flag to 0 if the result is zero, so you know when your jump animation (up) is done if there is no ceiling collision, without having to check the value of the byte.
(I’ve also seen a set of hard coded speeds to animate through, and cases where they don’t change the speed at all, making the animation feel very rigid. It’s amazing what little details are important)