Here's a classic screensaver - the bouncy ball. While written as a couple overlays using my overlay loader, it's small enough that it would fit in ROM with only a little rearranging of the code. ~~I haven't managed to get it quite small enough to be loaded from disk as one unit by the built-in bootloader though, even by making assumptions on the start-up values of registers it takes a word or two too many. Might be possible with some tricks, but I haven't quite gotten there.~~ (Edit: Not sure why I bothered, but I figured it out, so here's the code for a version that the built-in bootloader can (barely) load.)

Here's the code:

OVERLAY init MOVI 13 @local:y_two MOVI 12 @local:x_two MOVI 9 @local:pos MOVI 8 @local:dir MATH 3 3 1 MATH 2 2 1 SET 2 3 @overlay:main_loop JMP 3 9 pos: DATAC 0b11_1000_100... dir: DATAC 0b00_0001_001... y_two: DATAC 0b00_0000_010... x_two: DATAC 0b00_0010_000... OVERLAY main_loop loop: MATH 2 2 1 PMOV 9 2 6 8 23 1 // R2 = Y pos SET 3 3 7 // R3 = Y max IFJMP 1 @local:not_y_max 1 // if not at Y_max, skip reversing MATH 13 8 1 // R8 -= R13 : direction -= Y_two, so go from +Y to -Y not_y_max: SET 3 3 0 // R3 = Y min IFJMP 1 @local:not_y_min 1 // if not at Y_min, skip reversing MATH 13 8 0 // R8 += R13 : direction += Y_two, so go from -Y to +Y not_y_min: PMOV 9 2 2 5 26 1 // R2 = X pos // SET 3 3 0 : R3 = X min : R3 is already 0 IFJMP 1 @local:not_x_min 1 // if not at X_min, skip reversing MATH 12 8 0 // R8 += R12 : direction += X_two, so go from -X to +X not_x_min: SET 3 3 15 // R3 = X max IFJMP 1 @local:not_x_max 1 // if not at X_max, skip reversing MATH 12 8 1 // R8 -= R12 : direction -= X_two, so go from +X to -X not_x_max: MATH 9 2 5 // R2 = R9 : copy old position PMOV 15 2 0 1 0 0 // R2[0:1] = R15[0:1] : clear color MATH 8 9 0 // R9 += R8 : update position SETDATA 0 3 2 // Erase old ball position SETDATA 0 3 9 // Draw new ball position JMP 1 @local:loop

EDIT: Gah. The code has empty lines to make it clearer which lines go together, but this site's post editor apparently doesn't like that... (Yet another annoyance to add to the list about it I guess...)