ah, no, no, I only have 4 directions! I initially wanted to do 8 directions, but the number of tiles needed was insane! So, the way it works is like this:
- Every track has defined a set of "rails" along which the camera can move.
- The camera can ONLY move along those rails
- In this way, I can limit the number of tiles needed, since I know in which parts of the track I need tiles for horizontal scroll and in which parts I only need tiles for vertical scroll
- I considered having diagonal "rails", but that bumped my tile count too high. It is doable in principle, but I just ran out of RAM, since I am keeping the whole map in RAM (about 8KB) and a few other things, so, I had no space for all the additional sets of extra tiles I needed.
Looking forward to what you are planning to do!! 
That's how I managed to have the tile count so low. And also, there was a lot of tweaking to make the base tiles more or less align to a 2-pixel grid, so that the number of tiles is minimized as well (e.g. the number of empty space above/below/left/right of the decoration element tiles is always a multiple of two).
Hi gdx!! which MSX did you try it on? I have reports of people having it work fine on an VG8020.
And thanks for the suggestion!! I take note! But I am not sure that is the problem, since I have been careful to place the call to the read joystick routine in a place where the interrupt just happened (very close to a "halt"), so that the interrupt will never occur in between. But you are right in any case, and I might protect the routine with di/ei! 
But you are right in any case, and I might protect the routine with di/ei!
Unfortunately you can't do that since the routine is:
DI OUT (#A0),A PUSH AF LD A,E EI OUT (#A1),A POP AF RET
