Rocket to me, baby!

Some eye candy today. I've now got the rockets launching and following their pre-described path. Every 64 interrupts the code scans the ground-based objects to see if there are any rockets likely to hit the player (within a hard-coded window towards the left side of the screen) and if so, launches the first one it finds.

There are a maximum of 4 in-flight objects at any one time, and that includes rockets, ufos and even fireballs. Yes, hard to believe that there are only ever 4 fireballs on the screen in stage 3, and that they cause so much trouble!

Rockets launching

Each so-called 'in-flight' object has a table that describes their flight path, which is used to apply a delta to its position every interrupt. Not surprisingly, the path table for the rocket consists of a single entry that moves it up the screen.

Rendering the ceiling and the floor

It's interesting that you don't notice the imperfections in the game until you're debugging a transcode. The rocket sprites actually overwrite the progress bar before disappearing - confirmed on MAME.

You tend not to notice the gaudy palette when you're playing...

Next up is to add the routines to handle ufos and fireballs, and also to render the base (a ground-based object). Aside from the fuel gauge, which is currently not updated, that'll be about all I can do before adding the player ship and handling collision detection & explosion to round out the demo.

All that will leave handling of player controls, bullets and bombs, things exploding and game mechanics suchs as coin-up, scoring, high score updating etc. If I had to take a guess, I think I'm approaching the halfway point in the transcode. It's hard to estimate at this stage because the ASM-to-C code line ratio varies considerably with the code itself.