processor 6502
        include "vcs.h"
        include "macro.h"
        include "xmacro.h"

	org  $f000

; This program demonstrates a VCS music player based on tracks
; and patterns. A pattern is a list of variable-length notes,
; each of which is defined by a pitch and duration.
; There are two tracks, one for each audio channel.
; Each track consists of a list of patterns, each entry being
; a byte offset into the Patterns array.

; The patterns in the tracks are played in-order until one ends,
; and then both tracks are restarted. It's up to the composer
; to make sure the durations in each track line up properly.

; Patterns consist of NOTE or TONE commands. TONE sets the
; tone of the channel (the AUDCx register) and NOTE plays a note
; with a duration taken from a lookup table.
; TONE 0 ends a pattern.

; Both channels share the same logical array for tracks and patterns,
; so both tracks can take up to 255 bytes total, and all patterns
; can use up to 255 bytes total.
; The music player uses 8 bytes of RAM (not counting stack).

Trk0idx		equ	$e0	; offset into tracks for channel 0
Trk1idx		equ	$e1	; offset into tracks for channel 1
Pat0idx		equ	$e2	; offset into patterns for channel 0
Pat1idx		equ	$e3	; offset into patterns for channel 1
Chan0dur	equ	$e4	; current note duration channel 0
Chan1dur	equ	$e5	; current note duration channel 1
Chan0note	equ	$e6	; current note pitch channel 0
Chan1note	equ	$e7	; current note pitch channel 1

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Usage: NOTE pitch duration
; Plays a note in a pattern.
; pitch = 0-31
; duration = 1-7, uses DurFrames lookup table
	MAC NOTE
.pitch	SET {1}
.durat	SET {2}
	.byte (.pitch+(.durat<<5))
        ENDM

; Usage: TONE tone
; Changes the tone in a pattern.
; tone = 1-15
	MAC TONE
.tone	SET {1}
	.byte .tone
        ENDM

; Usage: PATTERN address
; Plays a pattern in a track.
	MAC PATTERN
.addr	SET {1}
	.byte (.addr-Patterns)
        ENDM
        
; Usage: ENDTRACK
; Marks the end of a track.
        MAC ENDTRACK
        .byte 0
        ENDM

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

Start
	CLEAN_START
        jsr ResetTrack

NextFrame
	VERTICAL_SYNC
	TIMER_SETUP 37
        ldx #0
        jsr MusicFrame
        ldx #1
        jsr MusicFrame
        TIMER_WAIT
	TIMER_SETUP 192
        TIMER_WAIT
	TIMER_SETUP 30
        TIMER_WAIT
        jmp NextFrame

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

ResetTrack
	lda #0
        sta Trk0idx
        sta Pat0idx
        sta Pat1idx
        sta Chan0dur
        sta Chan1dur
        lda #Track1-Track0
        sta Trk1idx
NextPattern
	ldy Trk0idx,x
	lda Track0,y
        beq ResetTrack
        sta Pat0idx,x
        inc Trk0idx,x
MusicFrame
	dec Chan0dur,x		; decrement note duration
        bpl PlayNote		; only load if duration < 0
TryAgain
	ldy Pat0idx,x		; load index into pattern table
	lda Patterns,y		; load pattern code
        beq NextPattern		; end of pattern?
        inc Pat0idx,x		; increment pattern index for next time
        pha			; save A for later
        clc			; clear carry for ROL
        rol
        rol
        rol
        rol			; rotate A left by 4 (same as ROR by 5)
        and #7			; only take top 3 bits
        beq NoteTone		; duration zero? tone instruction
        tay			; Y = duration
        lda DurFrames,y		; look up in duration table
        sta Chan0dur,x		; save note duration
        pla			; pop saved value into A
        and #$1f		; extract first 5 bits
        sta Chan0note,x		; store as note value
PlayNote
	lda Chan0note,x		; get note pitch for channel
	sta AUDF0,x		; store frequency register
	lda Chan0dur,x		; get note duration remaining
        clc
        ror			; divide by 2
        cmp #16
        bcc NoHighVol
        lda #15			; make sure no greater than 15 (max)
NoHighVol
	sta AUDV0,x		; store volume register
	rts
; This routine is called for duration 0 (TONE) codes
NoteTone
	pla
        and #$f
        beq NextPattern
        sta AUDC0,x
        jmp TryAgain


Patterns
	TONE 0		; byte 0 of patterns array is unused

Pattern00
	TONE 3
	NOTE 16,4
	NOTE 2,4
	NOTE 16,4
	NOTE 30,4
	NOTE 16,4
	NOTE 4,4
	NOTE 30,4
	NOTE 16,4
        TONE 0

Pattern10
        TONE 6
        NOTE 6,4
        TONE 12
        NOTE 16,4
        NOTE 18,4
        NOTE 19,4
        NOTE 22,4
        NOTE 23,4
        NOTE 26,4
        NOTE 23,4
        NOTE 26,4
        NOTE 23,4
        NOTE 26,4
        NOTE 23,4
        NOTE 22,6
        TONE 0
Pattern11
        TONE 6
        NOTE 6,6
        NOTE 6,4
        TONE 12
        NOTE 16,4
        NOTE 18,4
        NOTE 19,4
        NOTE 22,4
        NOTE 23,4
        NOTE 26,4
        NOTE 23,4
        NOTE 26,4
        NOTE 26,4
        NOTE 22,7
        TONE 11
        NOTE 0,7
        NOTE 0,2
        TONE 0
Pattern12
        TONE 11
        NOTE 0,5
	TONE 12
        NOTE 18,5
        NOTE 18,3
        NOTE 18,5
        NOTE 16,6
        NOTE 19,5
        NOTE 22,3
        TONE 6
        NOTE 4,5
        NOTE 4,4
        TONE 12
        NOTE 11,5
        NOTE 11,3
        NOTE 11,5
        NOTE 10,6
        NOTE 10,4
        NOTE 17,3
        NOTE 17,5
        NOTE 16,5
        TONE 0

Track0
	PATTERN Pattern00
	PATTERN Pattern00
	PATTERN Pattern00
	PATTERN Pattern00
	PATTERN Pattern00
	PATTERN Pattern00
	PATTERN Pattern12
        ENDTRACK
Track1
	PATTERN Pattern10
	PATTERN Pattern11
	PATTERN Pattern10
	PATTERN Pattern12
        ENDTRACK

DurFrames
	.byte 0,4,8,12,16,24,32,48

; Epilogue
	org $fffc
        .word Start
        .word Start