XeO3: Whoops! -*Blush*-

See.... Now I just feel really embarrassed! The reason it only shows up when I have 2 or 3 (or as it turns out less than 8) cache entrys, is because the INIT loop uses the number of cache blocks to clear the array! Heres the start of the init() call..

InitCache:
           ;
           ; First wipe cache data
           ;
           ldx  #0
           txa
!Lp1       sta  Cache_GraphicLo,x
           sta  Cache_GraphicHi,x
           sta  Cache_Index,x
           sta  Cache_8Bytes,x       ; clear first 8 bytes (and more) of cache
           inx
           cpx  #CacheBlocks         ; number of cache entrys to init
           bne  !LP1
    

Now.... those Cache_8Bytes are really important, thats how I scroll sprites up and down cleanly without having to store/cache extra space for vertical movement. Because sprites are stored in characters you have to rotate horizontally, and in theory - vertically; so that you can move it around the screen.

However.... if you store the sprites in columns (which is faster anyway), you can then store 8 bytes before the sprite and just move the start address backwards a byte to move the sprite downways a pixel. This means you only ever copy in 3x2 characters, and the rest are blank. Each column moves up 1 byte and gets the space from the previous column (a colunm is 24 pixels high, but the lower 8 bytes are always blank).

Now the next sprite in the cache doesn't have these 8 spare bytes in front, it does however have the previous sprites last empty character - so uses that!

So... anyway... turns out this bug wasn't clearing the whole of the 8byte cache space simply because I had lowered the number - DOH! I could "fix" it so that it will cope with caches lower than 8, but thats virtually useless anyway, and this way uses less memory - so as long as I now know WHY its doing it, I'm happy.