Christmas star

Author: Mira
Category: Christmas Challenge
System:   Atari
Language: Assembler (mads)
Len source code: 2958 bytes (with comments)
Len exe file:    149 bytes
Len code only:   82 bytes(net code size; subtract the bytes for header/start address; assembler: also subtract the BASIC stub)
Instructions:
  Use Altirra, v 4.10
  Menu: File -> Open Image -> *.obx
Description:
  Antic chip (responsible for converting screen data in RAM into screen signal) in 8-bit Atari computers
  can be programmed in a way that each text screen line can star on an arbitrary address in RAM.
  The program for Antic is called a "display list".
  So, I created a display list for Antic to repeat the lines, according to the specified pattern.
  All the pattern consist of 4 distinct lines l1, l2, l3, and l4 only:
    l1:    *     *     *
    l2:   * *   * *   * *
    l3:  *   * *   * *   *
    l4: *     *     *     *
  And I place them on the screen in this order:
    l1, l2, l3, l4, l3, l2, l1, l2, l3, l4, l3, l2, l1, l2, l3, l4, l3, l2, l1
  Then my code needs to fill in only 4 lins the memory, which simplifies the rest of the program.

  The code itself is in 6502 assembly language, I used MADS cassembler to compile it to the executable image (object) file.
  Register X stores a screen code for asterisk ('*").
  To correctly print a character on a specified position on the X-axis, I use this principle:
    1. I use indirect addressing of the screen memory (I use a 16-bit pointer DISPPTR stored in zero page memory) with Y register: (DISPPTR), Y.
    2. DISPPTR points to the first asterisk on the current line.
        a) that means, that when moving to the next line, I need to move it down (address += 40) and also left (address -= 1). Therefore, I add 39 to DISPPTR
        b) as those 4 lines occupy less than 1 page (256 bytes), I can update only lower byte of DISPPTR pointer
    3. I use Y register as the relative offset from the first (leftmost) asterisk on the line.
  As I need to calculate correct Y only relative to DISPPTR, the pattern that I need to get simplifies to this:
    l1: *     *     *
    l2: * *   * *   * *
    l3: *   * *   * *   *
    l4: *     *     *     *
  An outer cycle uses YCOUNT variable (also in zero page memory) to correctly count 4 repettitions (for 4 lines to render).
  In the cycle, the first asterisk is printed and XCOUNT variable (also in zero page memory) is initialized - it specifies the number of X-axis pattern repetitions.
  The repetitions consist of updating value of Y register by subsequently adding 2 values, after each step an asterisk is prented on the screen.
  For line 1, these values are 0 (technically, it prints the asterist on the same position twice) and 6.
  For line 2, they are 2 and 4. For line 3, they are 4 and 2. And finally, for line4 they are 6 and 0.
  The second number is 6 minut the first number (as it is obvious from the pattern).

Comments:
  It is a great challenge. I tried a couple of approaches. This one seemed to be teh "smartest", that's why I want to share it with others.