34 byte christmas star in Assembler

Author: serato / finnish gold
Category: Christmas Challenge
System:   C64
Language: C64 Assembler
Len source code: 1126
Len exe file:    36
Len code only:   34
Instructions:
Load PRG,8,1 and for best results clear screen, then SYS4121
Description:
The required octogram may be constructed as the union of four right triangles reflected four ways in x and y axes. A subroutine renders one row of the triangle, plus its horizontal mirror image. The main loop iterates over normal and mirror image screen rows. The result is all four triangles being rendered on top of each other.
Comments:
Saved a byte with unintended 6502 opcode ISC which combines INC and SBC to both increment a counter and load (16 minus) its value into A.