.include "validation.a"
      .include "sync_apu.a"
      
; Set up APU with square 1 enabled and unhalted
setup_apu:
      sei
      lda   #0
      sta   result
      lda   #$40        ; mode 0, interrupt disabled
      sta   $4017
      lda   #$01        ; enable square 1
      sta   $4015
      lda   #$10        ; unhalt length
      sta   $4000
      lda   #$7f        ; sweep off
      sta   $4001
      lda   #$ff        ; period
      sta   $4002
      rts
      .code

; Count number of length clocks required until $4015 AND A becomes 0
; then return result in A.
count_length:
      ldy   #0
      bit   $4015
      beq   count_length_end
      ldx   #$c0
:     stx   $4017
      iny
      beq   count_length_overflow
      bit   $4015
      bne   -
count_length_end:
      tya
      rts
count_length_overflow:
      lda   #$ff
      rts
      .code

; Synchronize with DMC
sync_dmc:
      lda   #$00
      sta   $4010
      lda   #1
      sta   $4013
      lda   #$10
      sta   $4015
:     bit   $4015
      bne   -
      rts