CheesySong.asm

; -----------------------------------------------------------------------------
;
; The available notes are:
; G3, A4, B4, C4, D4, E4, F4, G4, A5, B5, C5, D5
;
; Low byte of note frequencies
*=  $FE7C
.byte $3A, $FB, $C3, $AA, $7B, $52, $3F, $1C
.byte $FD, $E1, $D4, $BD

; High 3 bits of note frequencies plus (fixed) length counter
*=  $FE88
.byte $CA, $C9, $C9, $C9, $C9, $C9, $C9, $C9
.byte $C8, $C8, $C8, $C8

; Cheesy song
; Uses 4-bit packing, each nibble is the index of the next note to play
; for each of the two pulse channels
; The index is 1-based, and 0 means don't play a note
; Also, this is played backwards: from the last byte to the first
*=  $FE94
.byte $00, $00, $00, $46, $00, $35, $68, $57
.byte $00, $46, $00, $68, $79, $8A, $46, $68
.byte $00, $79, $00, $8B, $46, $57, $68, $46
.byte $00, $13, $00, $57, $46, $57, $68, $46
.byte $00, $13, $00, $35, $24, $35, $46, $57
.byte $00, $00, $00, $58, $00, $00, $00, $58
.byte $00, $46, $00, $68, $79, $8B, $9C, $8B
.byte $00, $00, $00, $8B, $00, $00, $00, $8B
.byte $00, $00, $00, $46, $35, $57, $68, $57
.byte $00, $46, $00, $68, $79, $8A, $79, $68
.byte $00, $46, $00, $57, $68, $79, $8B, $68
.byte $00, $57, $00, $57, $46, $35, $00, $35
.byte $00, $14, $00, $25, $46, $57, $68, $57
.byte $00, $46, $00, $57, $68, $8B, $00, $68
.byte $00, $40, $00, $50, $60, $70, $00, $70
.byte $00, $60, $00, $60, $50, $40, $00, $10

; -----------------------------------------------------------------------------
;
*=  $FE14
StartMusic:
    lda #$03    ; Enable pulse channels only
    sta ApuStatus_4015
    lda #$01    ; Starting index (0 means end reached)
    sta MusicIndex
    rts

; -----------------------------------------------------------------------------
;
*=  $FE1E
PlayMusic:
    lda $39     ; This is sometimes set to #$FF to stop the music
    beq _Play

    ; Music "paused"
    lda #$7F
    sta MusicCounter
    jmp _Done

_Play:
    ldx MusicIndex
    dex
    bne _ReadNext
    ; End reached: loop
    ldx #$80

_ReadNext:
    stx MusicIndex
    lda $FE93,X     ; Read one byte of data

    pha
    ldy #$00    ; Channel 0
    lsr A       ; Left nibble
    lsr A
    lsr A
    lsr A
    jsr _PlayNote

    pla
    ldy #$04    ; Channel 1
    and #$0F    ; Right nibble

; --------sub start--------
_PlayNote:
    beq _NoteEnd
    tax
    lda #$17    ; Fixed duty and volume
    sta Sq0Duty_4000,Y

    lda $FE7B,X
    sta Sq0Timer_4002,Y

    lda $FE87,X
    sta Sq0Length_4003,Y

_NoteEnd:
    lda #$08
    sta MusicCounter

_Done:
    rts