> > Megadrive has 2 scroll planes A and B, with 4-bit 8x8 tiles from 4 palettes.
> > Colour 0 is transparent. A tile can be selected high or low priority by a bit
> > and can be flips x and y. All stuff I'm sure you know, but just to remind you
> > ;-)
>
> Is the character (pattern) data packed like graphics in the NES (i.e. one byte
> holds the lsbits of a tileline and other bytes hold other bits)?
4-bytes holds 8 pixels like this:

static int TileNorm(unsigned char *pd,int addr,int pal)
{
  unsigned int pack=0; int t=0;

  pack=*(unsigned int *)(Pico.vram+addr); // Get 8 pixels
  if (pack==0) return 1;

  t=pack&0xf; pack>>=4; if (t) pd[3]=(unsigned char)(pal|t);
  t=pack&0xf; pack>>=4; if (t) pd[2]=(unsigned char)(pal|t);
  t=pack&0xf; pack>>=4; if (t) pd[1]=(unsigned char)(pal|t);
  t=pack&0xf; pack>>=4; if (t) pd[0]=(unsigned char)(pal|t);
  t=pack&0xf; pack>>=4; if (t) pd[7]=(unsigned char)(pal|t);
  t=pack&0xf; pack>>=4; if (t) pd[6]=(unsigned char)(pal|t);
  t=pack&0xf; pack>>=4; if (t) pd[5]=(unsigned char)(pal|t);
  t=pack    ;           if (t) pd[4]=(unsigned char)(pal|t);

  return 0;
}