Here's a brief summary of the SID demo:

What it plays: A continuously looping three-voice piece. Voice 1 carries a short melody that cycles through the SID's three main waveforms each repeat — triangle (soft), sawtooth (bright/buzzy), pulse (hollow) — so you hear the chip's tone colors. Voice 2 holds a sawtooth bassline, and Voice 3 plays a sustained harmony a fifth up, routed through the filter with a sweeping cutoff for that signature SID "wah."

How it works: I poke the SID's registers at $D400–$D418 directly from assembly. Each voice gets a frequency, a waveform, and an ADSR envelope (attack/decay/sustain/release — the swell-and-fade shape of a note). A note sounds when I set the frequency and flip the voice's gate bit on; clearing the gate triggers the release. Timing comes from a raster-synced tick (wait_frame) that fires once per video frame (~50Hz), the standard heartbeat for C64 music. The filter sweep is just nudging the cutoff register up and down a little each frame.

; sid_demo.asm - three-voice SID sound demonstration for the C64
;
; Assemble: acme -f cbm -o sid_demo.prg sid_demo.asm
; Run:      x64sc sid_demo.prg
;
; What it does:
;   Voice 1 (melody)   - plays a short phrase, switching WAVEFORM every pass so
;                        you hear triangle -> sawtooth -> pulse -> back again.
;   Voice 2 (bass)     - a steady low sawtooth bassline under the melody.
;   Voice 3 (harmony)  - sustained notes a fifth above, with the SID FILTER
;                        sweeping open and closed so you hear the filter effect.
;
; Timing is done with a simple frame-counted delay loop driven off the VIC-II
; raster (a crude ~50Hz tick). No CIA/IRQ player - kept readable on purpose.
;
; SID register map (base $D400):
;   +0/+1  voice1 freq lo/hi      +7/+8  voice2 freq lo/hi   +14/+15 v3 freq
;   +2/+3  voice1 pulse width      ...
;   +4     voice1 control (waveform + gate)
;   +5/+6  voice1 attack/decay , sustain/release
;   (voices 2 and 3 follow at +7 and +14 offsets)
;   +21/+22 filter cutoff lo/hi   +23 filter resonance/routing   +24 vol+mode

SID      = $D400
V1FREQ   = SID+0
V1PW     = SID+2
V1CTRL   = SID+4
V1AD     = SID+5
V1SR     = SID+6
V2FREQ   = SID+7
V2PW     = SID+9
V2CTRL   = SID+11
V2AD     = SID+12
V2SR     = SID+13
V3FREQ   = SID+14
V3PW     = SID+16
V3CTRL   = SID+18
V3AD     = SID+19
V3SR     = SID+20
FCUTLO   = SID+21
FCUTHI   = SID+22
FRESON   = SID+23
FVOLMODE = SID+24

RASTER   = $D012

; zero page scratch
note_idx = $02        ; index into melody table
wave_sel = $03        ; which waveform voice1 currently uses (control byte hi bits)
sweep_dir= $04        ; filter sweep direction (0 = up, 1 = down)
sweep_val= $05        ; current filter cutoff (hi byte)
tmp       = $06

        * = $0801
        !byte $0b,$08,$0a,$00,$9e,$32,$30,$36,$31,$00,$00,$00  ; 10 SYS 2061

start:
        sei

        ; ---- clear all SID registers ----
        lda #0
        ldx #$18
clr:    sta SID,x
        dex
        bpl clr

        ; ---- master volume = max, filter mode set later ----
        lda #$0f
        sta FVOLMODE

        ; ---- envelopes ----
        ; voice1 melody: quick attack, medium decay, mid sustain, short release
        lda #$09          ; attack=0 (fast), decay=9
        sta V1AD
        lda #$80          ; sustain=8, release=0
        sta V1SR
        ; voice2 bass: punchy
        lda #$0a
        sta V2AD
        lda #$90
        sta V2SR
        ; voice3 harmony: slow swell so the filter sweep is audible
        lda #$48          ; attack=4 (slowish), decay=8
        sta V3AD
        lda #$a0          ; sustain=A, release=0
        sta V3SR

        ; voice1 + voice3 pulse width (used when pulse waveform selected) ~50%
        lda #$00
        sta V1PW
        lda #$08
        sta V1PW+1
        lda #$00
        sta V3PW
        lda #$08
        sta V3PW+1

        ; ---- filter: route voice3 through it, low-pass, some resonance ----
        lda #%00000100    ; bit2 = filter voice3
        sta FRESON        ; (low nibble = routing; high nibble = resonance)
        lda #%11110100    ; resonance=$F (high), and re-assert voice3 routing
        sta FRESON
        ; FVOLMODE: low nibble volume=$F, bit4=low-pass enable ($10)
        lda #$1f
        sta FVOLMODE

        ; ---- init state ----
        lda #0
        sta note_idx
        sta sweep_dir
        lda #$10          ; voice1 waveform start = triangle ($10)
        sta wave_sel
        lda #$40
        sta sweep_val

        cli

; ================= MAIN LOOP =================
mainloop:
        ; --- play current melody note on voice 1 ---
        ldx note_idx
        lda mel_lo,x
        sta V1FREQ
        lda mel_hi,x
        sta V1FREQ+1
        ; gate ON with current waveform
        lda wave_sel
        ora #$01          ; set gate bit
        sta V1CTRL

        ; --- bass note on voice 2 (sawtooth), one per two melody notes ---
        lda bass_lo,x
        sta V2FREQ
        lda bass_hi,x
        sta V2FREQ+1
        lda #$21          ; sawtooth ($20) + gate ($01)
        sta V2CTRL

        ; --- harmony on voice 3 (triangle through filter) ---
        lda harm_lo,x
        sta V3FREQ
        lda harm_hi,x
        sta V3FREQ+1
        lda #$11          ; triangle ($10) + gate ($01)
        sta V3CTRL

        ; --- hold the note for a while, sweeping the filter as we wait ---
        jsr note_delay

        ; --- gate OFF (release) all voices ---
        lda wave_sel
        sta V1CTRL        ; gate bit cleared -> release
        lda #$20
        sta V2CTRL
        lda #$10
        sta V3CTRL

        ; short gap
        jsr short_delay

        ; --- advance to next note ---
        inc note_idx
        ldx note_idx
        lda mel_lo,x
        cmp #$ff          ; $ff in lo table marks end of phrase
        bne mainloop

        ; phrase finished: reset index and switch voice1 waveform
        lda #0
        sta note_idx
        jsr next_waveform
        jmp mainloop

; ---- cycle voice1 waveform: triangle -> saw -> pulse -> triangle ... ----
next_waveform:
        lda wave_sel
        cmp #$10
        bne nw_try_saw
        lda #$20          ; triangle -> sawtooth
        jmp nw_set
nw_try_saw:
        cmp #$20
        bne nw_pulse
        lda #$40          ; sawtooth -> pulse
        jmp nw_set
nw_pulse:
        lda #$10          ; pulse -> back to triangle
nw_set:
        sta wave_sel
        rts

; ---- note_delay: hold ~24 frames, advancing the filter sweep each frame ----
note_delay:
        ldy #24
nd_loop:
        jsr wait_frame
        ; sweep filter cutoff up and down
        lda sweep_dir
        bne nd_down
        inc sweep_val
        lda sweep_val
        cmp #$f0
        bcc nd_apply
        lda #1
        sta sweep_dir
        jmp nd_apply
nd_down:
        dec sweep_val
        lda sweep_val
        cmp #$20
        bcs nd_apply
        lda #0
        sta sweep_dir
nd_apply:
        lda sweep_val
        sta FCUTHI
        dey
        bne nd_loop
        rts

short_delay:
        ldy #6
sd_loop:
        jsr wait_frame
        dey
        bne sd_loop
        rts

; ---- wait_frame: spin until the raster reaches line $FA (250), which happens
;      exactly once per frame -> a clean ~50Hz (PAL) / 60Hz (NTSC) tick.
;      We first wait for it to LEAVE 250 (in case we're already there), then
;      wait for it to come back, guaranteeing one full frame elapses.
wait_frame:
wf_leave:
        lda RASTER
        cmp #$fa
        beq wf_leave      ; wait until we are NOT on line 250
wf_reach:
        lda RASTER
        cmp #$fa
        bne wf_reach      ; wait until raster hits line 250
        rts

; ================= NOTE DATA =================
; Frequencies are 16-bit SID values (PAL). $ff in mel_lo ends the phrase.
; Melody: a little rising/falling phrase.
mel_lo:
        !byte $25,$1c,$d6,$48,$d6,$1c,$25,$00,$ff
mel_hi:
        !byte $11,$15,$11,$1c,$11,$15,$11,$00,$00
; Bass: lower octave roots
bass_lo:
        !byte $25,$25,$d6,$d6,$d6,$25,$25,$25,$00
bass_hi:
        !byte $08,$08,$06,$06,$06,$08,$08,$08,$00
; Harmony: a fifth above the melody, sustained
harm_lo:
        !byte $b8,$ad,$1c,$8e,$1c,$ad,$b8,$00,$00
harm_hi:
        !byte $19,$1f,$15,$23,$15,$1f,$19,$00,$00