This was much harder than scrolling right. If I had chosen to scroll downward, it might have been easier, because scrolling up is like going backwards in the code.
Skipping over the y values of 0xf0-ff wasn’t that bad as long as you use my functions add_scroll_y() and sub_scroll_y() that handles this for you. The code assumes that Y position is between 0 and $ef… so you need to skip over the values 0xf0-0xff. These 2 functions can do that for you.
int add_scroll_y(unsigned char add, unsigned int scroll);
This adds an 8 bit value with the 16 bit scroll value, and returns a new scroll value.
.
int sub_scroll_y(unsigned char add, unsigned int scroll);
This subtracts an 8 bit value from the 16 bit scroll value, and returns a new scroll value.
.
But, I also had to use them with the BG collision code, since you can be half in one nametable and half in another.
.
Since scrolling up is going backwards, I’m starting at the maximum scroll position, and going backwards to 0.
I had to build the array of room arrays backwards too.
const unsigned char * const Rooms[]= {
Room5, Room4, Room3, Room2, Room1
};
Anyway, I made 5 more rooms, in Tiled, and exported 5 more csv files, converted to C arrays, and imported those into the code.
And after a few days of debugging (yikes) it finally works. I have to give some due respect to the guys who made games like Kid Icarus, or Metroid, or Ice Climber, because scrolling upward without bugs was not easy.
Note: we are using the horizontal mirroring (vertical scrolling) cfg file.
https://github.com/nesdoug/16_Scroll_Up/blob/master/scroll_up.c
nesdoug 