How to work with button presses

How to work with button presses


I always had problems with button presses, until I found this post. Though I can just search for "Button Press" every time I need that information, I think it's a better idea to make a real guide out of it (also, I'm bored right now :V).


To make a button test, for example if you want a check for the button B (this will be the example in this guide), you need to choose which method you wanna use: Is the button being held or just being pressed? will continue with the held one. (If you want the code for the pressed one, scroll down a bit.)


Check if a Button is being held


For this you will need the RAM adress ($FFFFF602).w, as it's the one for the held buttons. It works with a btst command (BiTeST), which actually does what the name suggests: Checking if a specific bit in a specific RAM adress is set or not. Choose the button you want in this list:



Start = 7
A = 6


C = 5


B = 4


Right = 3


Left = 2


Down = 1


Up = 0



An example with the button B would be:



btst #4,($FFFFF602).w ; check if B is being held
bne.s IfBIsBeingHeld ; if so, branch



In case you don't know how to work with branches, you just need to change the bne.s to a beq.s to check for the opposite:



btst #4,($FFFFF602).w ; check if B is being held
beq.s IfBIsNotBeingHeld ; if not, branch



And that's pretty much it.


Check if a Button is being pressed


This one doesn't work with btst, but with and.b. I don't really know how that works, but shobiz does, and I don't think he's lying (=P). Anyway, in case you don't know the difference between a button being held and pressed: While held checks if you are keep holding a specific button and keeps redoing your code until you release the button, pressing just does your code one time, no matter how long you hold a button. For this one you will need the RAM address ($FFFFF603).w (basically the next one from held). Again, choose the button you want from this list:



Start = $80
ABC = $70


A = $40


C = $20


B = $10


Right = $08


Left = $04


Down = $02


Up = $01



An example with button B again would be:



move.b ($FFFFF603).w,d0 ; move the current button press to d0 (we may not and d0 directly for whatever reason)
and.b #$10,d0 ; and the your value on it ($10/B in this case)


bne.s IfBIsBeingPressed ; if B is being pressed, branch



The version if B is not being pressed:



move.b ($FFFFF603).w,d0 ; move the current button press to d0 (we may not and d0 directly for whatever reason)
and.b #$10,d0 ; and the your value on it ($10/B in this case)


beq.s IfBIsNotBeingPressed ; if B is not being pressed, branch



And there you go! Have fun with making your own... button presses... whatever.


Credits:


- Me (Selbi) for the guide


- shobiz for the original guide and actually the whole information

 

Actually, instead of picking numbers, a set of macros can be made (I'll talk about it and stuff to you in private), but nevertheless, a good explaination.

 

He could also use that list as equates if they're not in use:



btst B,($FFFFF602).w ; check if B is being held



The SVN version already implements this I believe.