https://sta.c64.org/cbm64krnfunc.html
https://www.youtube.com/watch?v=wAo4dePJaCE Maus in Basic

https://www.chibiakumas.com/6502/platform3.php#LessonP30
assembly
SID_VOL     = $d418
V1_FREQ_LO  = $d400
V1_FREQ_HI  = $d401
V1_CONTROL  = $d404
V1_ADSR1    = $d405
V1_ADSR2    = $d406

* = $c000         ; Start address in memory

start:
    lda #15       ; Set max volume
    sta SID_VOL

    lda #$c5      ; Frequency low byte
    sta V1_FREQ_LO
    lda #$28      ; Frequency high byte
    sta V1_FREQ_HI

    lda #$09      ; Attack/Decay (fast attack)
    sta V1_ADSR1
    lda #$00      ; Sustain/Release
    sta V1_ADSR2

    lda #$21      ; Sawtooth wave + Gate ON (Bit 0 = Gate)
    sta V1_CONTROL

    rts           ; Return to BASIC or monitor