Kiwi's Blog

There is a trick to know how much time are you wasting inside vertical retrace interrrupt.

 

At start of interrupt (after saving the register and everything) load border register with white color, and at end of interrupt reload border with black.

 

The white color will show you how much screen time are you using inside interrupt.

 

Typically the white color should not exceed the first 32 lines, but that depends on your game.

This is very interesting trick. I tried it with 3 different game loops with 3 different results. I used it like so.

paper(2);Set the background color by using the VDP register 7
delay(1);wait specified VBLANKs
paper(14);I used dark green and gray to keep from having a seizure. 

 

1st result, This part of the game uses heavy bgtile updates. I think it takes about 2-4 frames to update the entire screen. The player controls are in the NMI loop so they can move and jump when the screen get complicated when there's a lot of tiles to be update at a time. The result using the border trick, the screen flashes back from green and gray rapidly while the 4 top pixel remains green, about 4-16 pixels are gray. The graphic does occasionally corrupt part of the pattern table.

 

The next stage uses sprite majority. I did try to keep the bg scrolling to the minimum in this stage and used sprite majority for the objects. The border is solid gray and maintain full framerate. Only when the game is updating the bgtile scrolling at 1 per 3 frame, the enemy sprites movement are halved. I'm thinking when the border is solid gray is when there is just about to do a 2nd retrace.

 

In this project with 20 double sized sprite trees scrolling along with the grass. Majority of the border is green. The gray border is about 32 pixel tall reside on the top of the screen.

 

This is very interesting and useful. Thank you for letting me know about this trick.