SteveF's VC3-2023 entry

Author: SteveF
Category: Christmas Challenge
System:   BBC Micro, Acorn Electron
Language: 6502 assembler
Len source code: 3194 bytes
Len exe file:    63 bytes
Len code only:   63 bytes
Instructions:
  To run in the jsbeeb online emulator, go to https://bbc.godbolt.org/, open the
  hamburger menu at the top right and choose Discs->From examples or
  local->Browse, then pick the star.ssd disc image file. Then press Shift-F12 to
  boot the disc.
Description:
  The main idea here is to apply the classic "bouncing ball" animation (without
  erasing the ball, so we get trails) to a 19x19 sub-region at the top left of
  the screen. We have three "balls" which together generate the complete pattern.
  The code just runs forever, because why not?
Comments:
  star.asm is the assembly language source
  star.bin is the actual machine code+data
  star.ssd is a bootable Acorn DFS disc image containing star.bin
  Thanks very much for organising this competition again!