/* Atari VCS Color Bars Program */

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

zeropage char clrbkg; //Color Background
zeropage char clrofs; //Color Offset
zeropage char clrmsk; //Color/B&W Mask

/* Generate Vertical Sync Signal */
void vtsync() {
  X=49;     //49*64 Cycles (37 Scanlines)
  A=2;      //Set Bit 2 (D1)
  WSYNC=A;  //Wait for end of Scanline
  VSYNC=A;  //Turn On Vertical Sync
  TIM64T=X; //Set Timer
  WSYNC=A;  //Wait 2 More Scanlines
  WSYNC=A;
  A=0;      //Clear Bit 2 (D1)
  WSYNC=A;  //Wait for End of 3rd Scanline
  VSYNC=A;  //Turn Off Vertical Sync
  return;
}

/* Execute Vertical Blank Code */
void vtblnk() {
  clrmsk = (SWCHB & 8) ? $FF : $0F;
  if (SWCHB & 64) clrofs--; else clrofs++;
  A=0; //Clear All Bits
  return;
}

/* Execute Kernel Code */
void kernel() {
  do WSYNC=A; while(INTIM);
  VBLANK=0; //Turn Off Vertical Blank
  X=192; //Draw 192 Scanlines
  do {
    clrbkg = X;
    if (!SWCHB & 128) clrbkg<<;
    A = clrbkg + clrofs & clrmsk; //Set Background Color
    WSYNC = A;  //Wait for end of Scanline
    COLUBK = A; //Set Background Color
    X--;
  } while (X);
  return;
}

/* Execute Overscan Code */
void ovrscn() {
  WSYNC=A;     //Wait for end of Scanline
  A=2;         //Set Bit 2 (D1)
  VBLANK=A;    //Turn On Vertical Blank
  TIM64T = 32; //Delay 32*64 Cycles (27 Scanlines)
  do {
    WSYNC=A;  //Wait for end of Scanline
  } while(INTIM);
  return;
}

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