Friday, 6 June 2014

Game Over and Apple II colours

Firstly, I've ported the GAME OVER spinning animation. It was quite straightforward (although I had to convert the graphics format) but it looks pretty cool. Curiously enough, achieving a high score means you don't get to see the animation!

A few other fixes and enhancements; added support for the game freeze (pause) function, and fixed the key debounce on the extra life cheat and a few other CTRL key commands. I also added support for the Coco arrow keys, and also the Z & X keys for digging.

In preparation for colour mode, I've re-arranged the memory map to accommodate the larger screen memory required. To do so, I had to relocate the program code from $4000 to $8000, and the graphics data from $8000 to $C000. This presented a new challenge for me - how to load code at $8000, over the BASIC ROM.

I may have mentioned that you can load segments of a BIN file into arbitrary memory banks by having the previous segment load directly into the MMU bank registers to switch them into the 6809 address space. This is the method I've been using to load the graphics data into high memory, by writing a small utility to effectively wrap a custom .BIN file format around the raw binary data.

With the program code, however, the linker already produces a .BIN file which loads at $8000 - as I mentioned - directly over the BASIC ROM. So I wrote another small utility that relocates the .BIN file to an area of empty RAM, prepends a segment to set up the MMU bank registers accordingly, and finally appends a small loader that simply switches in the previously loaded memory pages and then jumps to the Lode Runner main program. The end result is a single binary that can load itself (effectively) over ROM.

With that working, I could turn my attention to implementing actual colours. Since I've been toying with the idea of reverting the title screen graphics data to the original 7-bit format to save some memory, I decided that I would (also) convert the graphics on-the-fly from 1bpp monochrome to 2bpp colour. After some preliminary investigation into how the Apple II colour artifacting actually works, I modified the (still 8-bit) title display routine to display 4 colours.

4-colour title screen generated on-the-fly from the original Apple II data

Not a bad result but the artifacting around the white characters is clearly emphasised, and doesn't gel with the results in MESS. I will probably need to do more thorough research, but it's a pretty good result for what amounts to a few dozen lines of assembler.

I should also note that I am preserving the ability to build 1bpp (mono), 2bpp (mono) and 2bpp (colour) versions of the game - all via .define directives. I could in theory use the same method to (inefficiently) generate the in-game graphics on-the-fly for the 4bpp modes, but there's probably little point. So my next task will be to produce 4bpp mono and colour versions of the tile data and update the rendering routines accordingly. I shouldn't imagine this will be particularly difficult, providing the colour generation falls out easily.

Once I have colour working I'll (hopefully) be able to add some more levels and release a new Beta.

Finally, there is a report of the game crashing (twice) in VCC. At this point I don't have a lot to go on so please, if you do get a crash under emulation, I'd be interested in a screenshot (if there's any corruption) at least. And if you happen to be running it under the debug build of MESS, a full dump (SAVE) of the memory space would be much appreciated!

No comments:

Post a Comment