Thursday, March 10, 2016

Compact it, Compress it, Recycle, Make Less of it!

My mission this week was to compress things. To compact it. Make less of it.  I was running out of space, and I needed more.

First pass was to go through my code looking for macros that I could convert to proper subroutines. I found a couple, which freed up a little space, maybe a couple hundred bytes total.

Next pass was to do with enemies what I had previously done with room layouts:  In my room definitions, I had a pointer to the enemy type that would be in that room. Pointers in 6502 are 2 bytes. I have less than 256 enemies, so it made sense to, instead of storing a pointer in each room definition, just use an index.  It took a tiny bit more code, but quite a bit less total space. That saved another couple hundred bytes.

The biggest savings, though, was redoing my room layouts.  Because of how the atari playfield registers work, and 6502 indexing works, it's easiest to store 3 chunks of data for each room layout -- one for the outside edge, one for the middle of each half-screen, and one for the middle of the screen.  So I stored 23 bytes of layout data for each of those chunks, for each room.

Well, it turns out that a lot of room layouts share similar chunks. For example, either they have completely blank outside edges, or nice square doors on the outside, etc. I had quite a few chunks that were repeated throughout my graphics data. So instead of clustering the 3 chunks together, I added yet another lookup table, where for each room layout, I store the indices of the 3 different chunks that make up that room.   That freed up about 400 bytes.

The bonus of this is that now I can make different permutations of my existing chunks, for nearly free. This gives me a lot more options for making different room layouts and keeping the game at least slightly interesting.

So, that's done.  Now back to content.

(Although some of the nice folks on AtariAge have convinced me that I should try to support the SaveKey, a hardware doodad that someone made, which plugs into the player 2 joystick port, and has some flash memory to allow game saves.  I guess it takes between 100 and 200 bytes of code to use it. Now that I've freed this up space, I may try to do it. We'll see)

I'll leave you with this nice picture of things being broken as I tried debugging my new room layout code.


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