Moving a sprite!

[Ypsilon]

Hello!

I need some help with the task of moving a sprite. The problem is that, with some directions, the sprite moves faster than it should... I thing it has something to do with the carry flag, but I tried to CLC before any arithmetic operation to clear the carry bit, and it was still happening.

Thanks.

[tokumaru]

Need to see code. More details would be god too:

- how does the movement differ across the specific directions?
- how much did you expect it to move and how much is it moving?

[Ypsilon]

Ok sorry, these are the routines that move the sprite left/right and up/down. The quantmov variable is 2 or 1 whether the B button is down or not. But that's not the problem. That's just to make it run when B is pressed, and when you press it, it twices both directions.

http://www.copypastecode.com/25047/

Also, don't care about the CLC ops. That was just to try.

[MetalSlime]

Hello!

I need some help with the task of moving a sprite. The problem is that, with some directions, the sprite moves faster than it should... I thing it has something to do with the carry flag, but I tried to CLC before any arithmetic operation to clear the carry bit, and it was still happening.

Thanks.

Clear the carry before adding, set the carry before subtracting:


;adding
CLC
ADC #$02 ;or whatever

subtracting
SEC
SBC #$02

[Ypsilon]

Okey thanks! But I really don't understand why. I know it has something to do with the two's complement, maybe the sign?

[tepples]

You guessed correctly that it has something to do with two's complement. When you subtract n on a 6502, you actually add 255-n. To make the subtraction work as expected, you need to set the carry so that you add 255-n+1 = 256-n = -n (mod 256).

[Ypsilon]

You guessed correctly that it has something to do with two's complement. When you subtract n on a 6502, you actually add 255-n. To make the subtraction work as expected, you need to set the carry so that you add 255-n+1 = 256-n = -n (mod 256).

Okey! That's perfect. Thanks everyone!

[Noobiscus]

You guessed correctly that it has something to do with two's complement. When you subtract n on a 6502, you actually add 255-n. To make the subtraction work as expected, you need to set the carry so that you add 255-n+1 = 256-n = -n (mod 256).

Okey! That's perfect. Thanks everyone!

Did you get it working?