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n65/sound_engine.asm

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;------------------------------------------------------------------------------
; An NES sound engine that understands the binary stream outputted from my
; MIDI converter :)
;;;;
; Create an iNES header
.ines {"prog": 1, "char": 0, "mapper": 0, "mirror": 0}
;;;;
; Include all the symbols in the nes library
.inc <nes.sym>
;;;;
; Let's put a data structure to control the sound engine in the zero page
.org $0000
.scope sound_engine
.space stream_read_ptr_lo 1
.space stream_read_ptr_hi 1
.space stream_write_ptr_lo 1
.space stream_write_ptr_hi 1
.space delta 1
.
;;;;
; Open the prog section bank 0
.segment prog 0
;;;;
; Setup the interrupt vectors
.org $FFFA
.dw vblank
.dw main
.dw irq
;;;;
; Here is our code entry point, which we'll call main.
.org $C000
.scope main
; Disable interrupts and decimal flag
sei
cld
; Wait for 2 vblanks
wait_vb1:
lda nes.ppu.status
bpl wait_vb1
wait_vb2:
lda nes.ppu.status
bpl wait_vb2
; Now we want to initialize the hardware to a known state
lda #%00
ldx #$00
clear_segments:
sta $0, x
sta $100, x
sta $200, x
sta $300, x
sta $400, x
sta $500, x
sta $600, x
sta $700, x
inx
bne clear_segments
; Reset the stack pointer
ldx #$FF
txs
; Disable all graphics and vblank nmi
lda #$00
sta nes.ppu.control
sta nes.ppu.mask
; Initialize sound engine structure
; To read from $D000, and to write to $40**
lda #$D0
sta sound_engine.stream_read_ptr_hi
lda #$00
sta sound_engine.stream_read_ptr_lo
sta sound_engine.stream_write_ptr_lo
; Make the first delta happen immediately
lda #$01
sta sound_engine.delta
lda #$40
sta sound_engine.stream_write_ptr_hi
jsr init_sound
; Resume interrupts and NMI and loop here forever
lda #%10000000
sta nes.ppu.control
cli
forever:
jmp forever
.
;;;;
; Initialize the APU to enable Pulse1
; Bitfield: ---D NT21
.scope init_sound
lda #$00
sta nes.apu.pulse1.control
sta nes.apu.pulse1.ramp_control
sta nes.apu.pulse1.ft
sta nes.apu.pulse1.ct
lda #$01
sta nes.apu.channel_enable
rts
.
;;;;
; VBlank reads our music buffer
.scope vblank
; Backup our registers
pha
txa
pha
tya
pha
jsr sound_engine_driver
; Restore the registers
pla
tay
pla
tax
pla
rti
.
;;;;
; Sound driver that updates the APU registers
; via a stream of APU write commands
.scope sound_engine_driver
dec sound_engine.delta
bne done
read_event:
; Load the new delta from the stream
ldy #$00
lda (sound_engine.stream_read_ptr_lo), Y
sta sound_engine.delta
; Read pulse1 control register value
ldy #$01
lda (sound_engine.stream_read_ptr_lo), Y
sta nes.apu.pulse1.control
; Read the value for pulse1.ft
ldy #$02
lda (sound_engine.stream_read_ptr_lo), Y
sta nes.apu.pulse1.ft
; Read the value for pulse1.ct
ldy #$03
lda (sound_engine.stream_read_ptr_lo), Y
sta nes.apu.pulse1.ct
; Advance the 16-bit stream pointer by number of bytes read
lda sound_engine.stream_read_ptr_lo
clc
adc #$04
sta sound_engine.stream_read_ptr_lo
bcc done
inc sound_engine.stream_read_ptr_hi
; If the very next event is 0 delta away, do it now too
; Read the value for pulse1.ct
ldy #$04
lda (sound_engine.stream_read_ptr_lo), Y
beq read_event
done:
rts
.
;;;;
; IRQ Does nothing
.scope irq
rti
.
;;;;
; Include the music buffer stream
.org $D000
music_buffer:
.incbin "data.mus"