The ARM Evaluation Star

Author:   atsampson
Category: Christmas Challenge
System:   Acorn ARM Evaluation System
Language: Assembler
Len source code: 560
Len exe file:    108
Len code only:   108
Instructions:
  armstar.ssd is a BBC Micro DFS floppy image containing the executable.
  Start the B-Em emulator with the ARM coprocessor and the disk image:
    b-em -tx 1 armstar.ssd
  Then type STAR at the prompt.
Description:
  The star is drawn by considering it as four intersecting triangles. The code
  iterates through the X/Y coordinates within the star, calling the code at
  CHECK to test if X/Y is within the triangle for each possible reflection of
  the X/Y axes.

  The program was developed using BBC BASIC V's built-in assembler on a BBC
  Master 128 with a PiTubeDirect (Raspberry Pi 3B+) coprocessor. The executable
  works correctly both on the ARM Evaluation System's ARM2 (1986), and on the
  PiTubeDirect's native BCM2837 (2018).
Comments:
  The executable is 27 instructions -- the encoding's not very efficient
  because the ARM2 doesn't have Thumb mode, although it does use some
  conditional instructions that Thumb doesn't have to make up for it.