Sprite timing question

[MrProper]

Hi everyone!

 

I have been trying my hand at writing a little bit of code after reading through most of the "2600 programming for Newbies" but I ran in to a problem with the positioning for my sprites. In trying to test my knowledge I wanted to simply draw box (P0) on a predefined position on the screen. But my timing seemed off. I wrote some code to experiment with timing and RESP0 but it seems to contradict what I read. Can some one help me?

 

After the vertical sync I wrote this to set the player position

sta WSYNC
        REPEAT 15
        nop
        REPEND
        sta RESP0

Now from what I read this should take 15*2*3 = 90 colour cycles. Which should position the player at 90-68 = 22.But what gets drawn is this:

 

(The white lines are the most left and right pixel of the player. The rest is just playfield for help counting pixels)

 

So I'm off by what seems like 5 cpu cycles. Can anyone explain what goes wrong in my calculations?

Thanks in advance!

[+SpiceWare]

You left out the 3 cycles for sta RESP0 - (15*2 + 3) * 3 =  99.

As for the rest of the difference I believe it's caused by a propagation delay within TIA, but don't know specifics.

 

I do know the results are slightly different for the ball and missiles, I cover that in Step 11 - add the ball object of my tutorial:

Quote

I then modified RandomLocation to set all objects to the same location for comparision:

RandomLocation:
...
    ; for alignment test, set to (100, 100)
        lda #100
        sta ObjectX,x
        sta ObjectY,x
        rts

 

 

This revealed a minor quirk with TIA - namely that when objects are set to the same X position, missiles and the ball end up 1 pixel to the left of where a player ends up (player1 is the green square and it's directly on top of the red ball).

 

This is a known issue and the solution is to increase the X value by 1 to compensate.