This SID detection routine is based on the fact that there is a one cycle delay in the oscillator on 8580 compared to 6581 when turned on.
;SID DETECTION ROUTINE ;By SounDemon - Based on a tip from Dag Lem. ;Put together by FTC after SounDemons instructions ;...and tested by Rambones and Jeff. ; - Don't run this routine on a badline sei ;No disturbing interrupts lda #$ff cmp $d012 ;Don't run it on a badline. bne *-3 ;Detection itself starts here lda #$ff ;Set frequency in voice 3 to $ffff sta $d412 ;...and set testbit (other bits don't matter) in VCREG3 ($d412) to disable oscillator sta $d40e sta $d40f lda #$20 ;Sawtooth wave and gatebit OFF to start oscillator again. sta $d412 lda $d41b ;Accu now has different value depending on sid model (6581=3/8580=2) lsr ;...that is: Carry flag is set for 6581, and clear for 8580. bcc model_8580 model_6581: [...] model_8580: [...]