// ================================================
//
// antic_nmien = $40
//
// %01000000 $40 VBI
// %10000000 $80 DLI
// %11000000 $c0 VBI + DLI
//
// ================================================
//
// pia_portb = $fe
//
// PORTB_BASIC_OFF + PORTB_SELFTEST_OFF + %01111100
//
// PORTB_SELFTEST_OFF = %10000000; portb bit value to turn Self-Test off
// PORTB_BASIC_OFF    = %00000010; portb bit value to turn Basic off
// PORTB_SYSTEM_ON    = %00000001; portb bit value to turn System on
//
// ================================================
//
// Now the routine that lets you get to
// the RAM that is under the OS.
// There are actually 2 memory areas
// present:
//    4K at $C000 to $CFFF, 49152 to 53247
//   10K at $D800 to $FFFF, 55296 to 65535
//
// The last 6 bytes of the 10K area are not
// usable, since that is where the interrupt
// routines are located.  Therefore do not
// use any RAM above $FFF9 (65529) or you
// will crash the system.
//
// ================================================

byte nmien = $c0

byte rti       @ $15 // default routine for VBI & DLI
word vbivec    @ $10 // vector for VBI
word vdslst    @ $16 // vector for DLI

// simple display list; LMS = $e000
const array(byte) dl align(32) = [
  $70,$70,$70,
  $42,$00,$e0,2,2,$f0,2,2,2,$f0,2,2,2,
  $41,@word[dl.addr]
]

// init procedure
void system_off(){
  asm { sei }                 // turn off IRQ
  antic_nmien = 0             // turn off NMI
  pia_portb = $fe             // turn off ROM
  rti = $40                   // set RTI opcode
  vbivec = rti.addr           // set address for VBI routine
  vdslst = rti.addr           // set address for DLI routine
  os_NMIVEC = nmi.addr        // set address for custom NMI handler
  antic_nmien = nmien
}

// custom NMI handler
asm void nmi(){
  bit antic_nmist             // test nmist
  bpl .vblclock               // if 7-bit not set handle VBI
  jmp (vdslst)                // indirect jump to DLI routine
  .vblclock:                  // RTCLOK maintainer
  inc os_RTCLOK.b2
  bne .tickend
  inc os_RTCLOK.b1
  bne .tickend
  inc os_RTCLOK.b0
  .tickend:
  jmp (vbivec)                // indirect jump to VBI routine
}

// example dli
interrupt asm void dli_first(){
  pha
  lda #$2a
  sta gtia_colpf2
  sta antic_wsync
  lda #<dli_second.addr
  sta vdslst.lo
  lda #>dli_second.addr
  sta vdslst.hi
  pla
  rti
}

// example dli
interrupt void dli_second(){
  gtia_colpf2 = $de
  antic_wsync = $de
  vdslst = dli_first.addr
}

// wait for VBLANK
asm void pause() {
  lda os_RTCLOK.b2
  .rt_check:
  cmp os_RTCLOK.b2
  beq .rt_check
  rts
}

// wait 0-255 frames
noinline asm void wait(byte register(a) f) {
  clc
  adc os_RTCLOK.b2
  .rt_check:
  cmp os_RTCLOK.b2
  bne .rt_check
  rts
}

// example vbi
interrupt void vbi(){
  gtia_colpf2 = os_RTCLOK.b2
}

// main procedure
void main(){
  system_off()                // turn off OS

  wait(100)                   // waint 2 sec on PAL for fun
  antic_dlist = dl.addr       // set custom display list
  wait(100)                   // waint 2 sec on PAL for the lulz
  vbivec = vbi.addr           // set custom VBI
  wait(100)                   // waint 2 sec on PAL because we can
  vdslst = dli_first.addr     // set custom DLI

  while(true){
    wait(100)
    nmien ^= %10000000        // toggle DLI
    antic_nmien = nmien
  }
}