//Atari VCS Color Bars Program

#pragma origin $F800    //2k Cartridge
#include <vcshead.h02>  //TIA and RIOT Registers

/* Generate Vertical Sync Signal */
void vtsync() {
  A=2;     //Set Bit 2 (D1)
  WSYNC; //Wait for end of Scanline
  VSYNC=A; //Turn On Vertical Sync
  WSYNC; //Wait 2 More Scanlines
  WSYNC;
  A=0;     //Clear Bit 2 (D1)
  WSYNC; //Wait for End of 3rd Scanline
  VSYNC=A; //Turn On Vertical Sync
}

/* Execute Vertical Blank Code */
void vtblnk() {
  X=37; //Delay 37 Scanlines
  do {
    WSYNC;  //Wait for end of Scanline
    X--;
  } while (*);
}

/* Execute Kernel Code */
void kernel() {
  A=0; //Clear All Bits
  WSYNC;  //Wait for end of Scanline
  VBLANK=A; //Turn Off Vertical Blank
  X=0; //Draw 192 Scanlines (256-64) 
  do {
    if (X & 3) {
      WSYNC;  //Wait for end of Scanline
      COLUBK = X; //Set Background Color
    }
    X--;
  } while (*);
}

/* Execute Overscan Code */
void ovrscn() {
  WSYNC;  //Wait for end of Scanline
  VBLANK=2; //Turn On Vertical Blank
  X=27; //Delay 27 Scanlines
  do {
    WSYNC;  //Wait for end of Scanline
    X--;
  } while(*);
}

start:
  asm("","SEI","");
  asm("","CLD","");
  A = 0;
  X = A;
  Y = A;
  do {
    X--;
    asm("","TXS","");
    push A;
  } while (*);
irqbrk: //Code to Execute when BRK Instruction is Encountered
main:  //Start of Program Code
  vtsync(); //Generate Vertical Sync Signal  
  vtblnk(); //Generate Vertical Blank Signal  
  kernel(); //Execute Kernal Code
  ovrscn(); //Execute Overscan Code
  goto main;

#include <vcsfoot.h02>  //Finalization Code