git.sur5r.net Git - cc65/blob - libsrc/atari5200/irq.s

   1 ;
   2 ; IRQ handling (ATARI 5200 version)
   3 ;
   4
   5         .export         initirq, doneirq
   6         .import         callirq
   7
   8         .include        "atari5200.inc"
   9
  10 ; ------------------------------------------------------------------------
  11
  12 .segment        "ONCE"
  13
  14 initirq:
  15         lda     VVBLKD
  16         ldx     VVBLKD+1
  17         sta     IRQInd+1
  18         stx     IRQInd+2
  19         ldy     #<IRQStub
  20         ldx     #>IRQStub
  21         jmp     SETVBV
  22
  23 ; ------------------------------------------------------------------------
  24
  25 .code
  26
  27 doneirq:
  28         ldy     IRQInd+1
  29         ldx     IRQInd+2
  30         ;jmp     SETVBV
  31         ; fall thru
  32
  33 ; ------------------------------------------------------------------------
  34 ; Set deferred vertical blank interrupt
  35 ; logic copied from Atari computer ROM
  36
  37 SETVBV: txa
  38         ldx     #5
  39         sta     WSYNC                   ; waste 20 CPU cycles
  40 @1:     dex                             ; to allow VBLANK to happen
  41         bne     @1                      ; if this is line "7C"
  42         sta     VVBLKD+1
  43         sty     VVBLKD
  44         rts
  45
  46 ; ------------------------------------------------------------------------
  47
  48 .segment        "LOWCODE"
  49
  50 IRQStub:
  51         cld                             ; Just to be sure
  52         jsr     callirq                 ; Call the functions
  53         jmp     IRQInd                  ; Jump to the saved IRQ vector
  54
  55 ; ------------------------------------------------------------------------
  56
  57 .data
  58
  59 IRQInd: jmp     $0000