Denial

Code: Select all

   processor 6502

   include <macros.lm>
   include <macros_16.lm>

   include <vic20.lm>
   
   org BASIC_RAM

   basic start
      10 sys {tapemon}
   basic end

dim tape_old_irq as word

const IRQ = $314
const VIA2_COUNTER = $9124
const VIA2_ACR     = $912b
const VIA2_PCR     = $912e

const TIMER_VALUE = $ffff

const COLOR1 = 3*16+3+8
const COLOR2 = 1*16+1+8

sub tapemon()

   ; not sure: disable all interrupts
   sei
   lda #$7f
   sta $911e  ; VIA 1 Interrupt Enable Register
   sta $912e  ; VIA 2 Interrupt Enable Register
   sta $912d  ; VIA 1 Interrupt Flag Register

   ; Start tape motor
   lda $911c
   and #$fd
   sta $911c

   ; Set IRQ vector
   ld16 tape_old_irq, IRQ
   ld16 IRQ, #bit_received

   ; Initialise VIA2 Timer 1 (cassette tape read)
   ld16 VIA2_COUNTER, TIMER_VALUE   ; Restart timer
   ld VIA2_ACR, #%00000000          ; One-shot mode
   ld VIA2_PCR, #%10000010          ; CA1 IRQ enable (tape pulse)

   ; Let the IRQ handler do everything
   cli
   clc   
   do : loop while not carry

; this routine gets triggered when the edge of the tape bit 
; is detected or when the timer counter underflows

bit_received:
   
   ; an actual bit decoding routine should look at underflow bit
   ; lda $912d       ; Get timer underflow bit.
   ; asl             ; Move underflow bit into carry.
   ; asl             ;
   ; it should also look into the counter value?

   ; color effect   
   lda $900f
   if a=#COLOR1 then 
      ld a, #COLOR2
   else
      ld a, #COLOR1
   end if
   sta $900f
       
   ; sound effect
   lda 36878
   if a=#15 then 
      ld a, #0
   else
      ld a, #15
   end if
   sta 36878

   ld16 VIA2_COUNTER, TIMER_VALUE        ; Restart timer.

   ; not sure: prepare for re-trigger?
   lda #$7f
   sta $912d

   ; complete the interrupt routine
   jmp $eb18

end sub

Code: Select all

   sei
   lda #$27
   sta $9128
   ldx #$01

loop:
   lda #$02
   do
      bit     $912D
   loop while zero

   ; ???
   lda $912D
   stx $9129
   bit $9121   
   asl     
   asl     
   asl     

   ; color effect   
   lda $900f
   if a=#COLOR1 then 
      ld a, #COLOR2
   else
      ld a, #COLOR1
   end if
   sta $900f

   ; sound effect
   lda 36878
   if a=#15 then 
      ld a, #0
   else
      ld a, #15
   end if
   sta 36878
   
   jmp loop