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|
;;;
;; Repeat a scale of notes on the triangle channel.
.segment "HEADER"
.byte 'N', 'E', 'S', $1A
.byte $02, $01
.res $0A, $00
.segment "VECTORS"
.addr nmi, reset, irq
.segment "CHARS"
.incbin "../assets/basic.chr"
.segment "CODE"
.include "../shared/asm.s"
.include "../shared/apu.s"
.include "../shared/oam.s"
.include "../shared/ppu.s"
.include "../shared/clear.s"
.scope Vars
zp_flags = $20
;; Boolean value stating whether the currently indexed note should be
;; delivered to the triangle channel or not.
zp_next_note = $21
;; Index to the 'notes_{low,high}' tables, pointing at the current note.
zp_index = $22
;; Simple counter increased/reset on NMI code.
zp_counter = $23
.endscope
;; Where the scale of notes should start and end.
INITIAL_NOTE = 40
HIGHEST_NOTE = 50
.proc reset
sei
cld
ldx #$40
stx APU::m_frame_counter
ldx #$FF
txs
inx
stx PPU::m_control
stx PPU::m_mask
stx APU::m_dmc
bit PPU::m_status
@vblankwait1:
bit PPU::m_status
bpl @vblankwait1
ldx #0
lda #0
@ram_reset_loop:
sta $000, x
sta $100, x
sta $300, x
sta $400, x
sta $500, x
sta $600, x
sta $700, x
inx
bne @ram_reset_loop
lda #$EF
@sprite_reset_loop:
sta $200, x
inx
bne @sprite_reset_loop
OAM_WRITE_SPRITES
@vblankwait2:
bit PPU::m_status
bpl @vblankwait2
lda #$3F
sta PPU::m_address
lda #$00
sta PPU::m_address
lda #$0F
ldx #$20
@palettes_reset_loop:
sta PPU::m_data
dex
bne @palettes_reset_loop
__fallthrough__ main
.endproc
.proc main
CLEAR_SCREEN
;; Initialize the note index, reset the counter and ensure that the first
;; note is delivered.
lda #INITIAL_NOTE
sta Vars::zp_index
ldx #0
stx Vars::zp_counter
inx
stx Vars::zp_next_note
;; Same as beep.s
lda #$0F
sta APU::m_status
;; Enable the triangle channel.
lda #%10000001
sta APU::m_triangle_control
cli
lda #%10001000
sta PPU::m_control
lda #%00011110
sta PPU::m_mask
@main_game_loop:
;; Should we change the note? If not, then skip this altogether, or
;; otherwise sending the same note over and over will result in a
;; blurry/ugly sound.
lda Vars::zp_next_note
beq @skip_note
;; Fetch the new note and send it to the triangle channel. Note that notes
;; on both tables are defined with the square channels in mind. Sicne the
;; triangle channel is an octave below, we need to divide the given value by
;; two.
ldx Vars::zp_index
lda notes_low, x
lsr
sta APU::m_triangle_low
lda notes_high, x
lsr
sta APU::m_triangle_high
@skip_note:
lda #%10000000
ora Vars::zp_flags
sta Vars::zp_flags
@wait_for_render:
bit Vars::zp_flags
bmi @wait_for_render
jmp @main_game_loop
;; NOTE: notes taken from: https://www.nesdev.org/wiki/APU_basics.
;; NOTE: only valid for NTSC. For PAL we would need another set of values
;; which can be found in other places.
notes_low:
.byte $f1,$7f,$13,$ad,$4d,$f3,$9d,$4c,$00,$b8,$74,$34
.byte $f8,$bf,$89,$56,$26,$f9,$ce,$a6,$80,$5c,$3a,$1a
.byte $fb,$df,$c4,$ab,$93,$7c,$67,$52,$3f,$2d,$1c,$0c
.byte $fd,$ef,$e1,$d5,$c9,$bd,$b3,$a9,$9f,$96,$8e,$86
.byte $7e,$77,$70,$6a,$64,$5e,$59,$54,$4f,$4b,$46,$42
.byte $3f,$3b,$38,$34,$31,$2f,$2c,$29,$27,$25,$23,$21
.byte $1f,$1d,$1b,$1a,$18,$17,$15,$14
notes_high:
.byte $07,$07,$07,$06,$06,$05,$05,$05,$05,$04,$04,$04
.byte $03,$03,$03,$03,$03,$02,$02,$02,$02,$02,$02,$02
.byte $01,$01,$01,$01,$01,$01,$01,$01,$01,$01,$01,$01
.byte $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00
.byte $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00
.byte $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00
.byte $00,$00,$00,$00,$00,$00,$00,$00
.endproc
.proc nmi
bit Vars::zp_flags
bpl @next
pha
txa
pha
tya
pha
;; Increase the counter and do nothing if we are not at the desired value yet.
inc Vars::zp_counter
lda Vars::zp_counter
cmp #30
bne @skip_reset
;; Reset counter, move the note index and state that we need to update the
;; triangle channel with this new note.
lda #0
sta Vars::zp_counter
inc Vars::zp_index
inc Vars::zp_next_note
;; Wrap around the note index if needed.
lda Vars::zp_index
cmp #HIGHEST_NOTE
bne @done
lda #INITIAL_NOTE
sta Vars::zp_index
bne @done
@skip_reset:
;; In the general case, avoid the current note to be delivered.
lda #0
sta Vars::zp_next_note
@done:
lda #%01111111
and Vars::zp_flags
sta Vars::zp_flags
pla
tay
pla
tax
pla
@next:
rti
.endproc
.proc irq
rti
.endproc
|
