snowflake 

Author: Sergey Kiselev
Category: Christmas Challenge
System:   IBM PC or compatibles
Language: Assembler (TASM/MASM)
Len source code: 2218 bytes
Len exe file:    90 bytes
Len code only:   90 bytes
Instructions:

  On a real hardware:
  Copy vccc2025.com file to a floppy drive, boot the system into DOS, insert
  floppy disk into the system, and type "vccc2025" (without quotes).
  
  In an emulator:
  Use supplied vccc2025.img image, which is an image of a MS-DOS 3.30 bootable
  diskette, with the vccc2025.com and vccc2025.asm copied to it.
  For example, in PCjr https://www.pcjs.org/machines/pcx86/ibm/5160/cga/,
  press "Browse..." button at the bottom of the emulated display, browse and
  find vccc2025.img. Next press "Mount" button to mount the image, and then
  "Reset" button to reboot the system and boot from the image. Finally, once
  MS-DOS boots up, type "vccc2025" to run the snowflake program.

Description:

  The code uses -9 to 9 grid in both horizontal (X) and vertical (Y) dimensions
  It relies on the following observations:
  - Symmetry: 1/8 of the image can be mirrored, and then rotated 4 times
    to draw the entire image
  - There are solid horizontal and vertical lines at X = 0 and Y = 0
  - There are solid diagonal lines at Y = |X| && |X| <= 7
  The solid lines are printed using simple coordinates comparison
  The remaining 6 asterisks are printed by comparing their absolute coordinates

Comments:

I did a few attempts at reducing the code size. For example, in the submitted
code, SCASW instruction is used to compare the coordinates, which reduced
the code size versus simple CMP / JE combination.
I also tried packing absolute coordinates in a single byte - a nibble for
X and Y respectively, but that did not result in the code size reduction.
Similarly, I tried using an algorithmic approach to calculate absolute values,
but ultimately simple comparison and conditional jumps produced the smallest
code.