Assembly experiment: bouncing block

        ld de,(test)
        ld a, d
        xor 0xFF
        ld d,a
        ld a, e
        xor 0xFF
        ld e,a
        inc de 
        ld (test),de
test:   dW 0x1000

Nice understanding of 2’s complement math. This can be key to some smart optimisations. Here, instead of xor 0xFF you can use cpl which is a dedicated instruction for inverting all the bits.

However I think subtracting from 0 directly is more readable:

ld de,(test)
ld hl,0
and a
sbc hl,de
ld (test),hl

It also happens to be a few cycles faster. But cycle micro-optimisations generally don’t matter much unless it’s executed frequently, in an inner loop. So I wouldn’t focus on cycles too much, and just make sure to write your code in such a way that you still understand it a month from now.

E.g. you could write the ld hl,0 / and a part here as xor a / ld l,a / ld h,a to save 1 more cycle, but readability suffers and the gain is irrelevant imo. Although of course on the forum finding the fastest solution is always fun, and I reckon also educational.

ld hl,test
xor a
sub (hl)
ld (hl),a
inc hl
ld a,0
sbc a,(hl)
ld (hl),a

Is less readable but should be faster and save some bytes.

Good call Grauw, on readability. I'm a big fan of making source code clean and understandable. For us, assembler geeks, it sometimes is a job that seems out of reach. But with a bit of effort, you can make beautiful, readable, and fast code. Like you just demonstrated.

I've seen my share of Italian code, even my own. #cleancode

Ok, managed to find it by myself, with some help of a debugger

I found some mistakes and further cleaned up my code. That solved the problem that the maximum value of the y-coordinate was exceeded

In the end, there turned out to be no mistake in the bouncing algoritm. The first time the coin falls it is 'dropped' from a higher place and reached the maxspeed, but after that it does not get so high anymore and when it hits the floor it has not reached the maximum speed. So then it won't get that high anymore, causing an even lower speed the next time it hits the floor, and so on.

I've decided to keep it the way that it bounces endlessly: I just take the maxspeed and make that negative.

Newer version here: https://msxpen.com/codes/-M_BBaGgL7oWzVNCKWgx

Thanks. I made the final version in where I've implemented the other suggestions by ARTRAG about VDP vram pointer auto-increments. I only did this for the initial display of the sprites. It did not make my program smaller on updating x and y coordinates and speed is not in issue for this little demo.

I started just over 500 bytes and now it's exactly 400 bytes. I'm happy with that and I'm happy with my first real program in assembly for the MSX. I've added a lot of comments, hopefully it is helpful for other beginners.

Final version can be found here: https://msxpen.com/codes/-M_CtHD19xjHdPi_p5wI

Thanks for the help, I've learned a lot by doing this. On to the next project.