* external audio input (for sound mixing with external signal sources)
* random number/modulation generator
<br>
==SID registers==
{| class="wikitable"
|-
! Category
! Regs
! Name
! 7
! 6
! 5
! 4
! 3
! 2
! 1
! 0
|-
| rowspan="7" | Voice 1 || $00 (W) || FREQLO1 || colspan="8" style="text-align:center;" | Channel 1 Frequency Low-Byte
|-
| $01 (W) || FREQHI1 || colspan="8" style="text-align:center;" | Channel 1 Frequency High-Byte
|-
| $02 (W) || PWLO1 || colspan="8" style="text-align:center;" | Channel 1 Pulse Width Low
|-
| $03 (W) || PWHI1 || colspan="4" style="text-align:center; background:#E0E0E0;" | unused || colspan="4" style="text-align:center;" | Channel 1 Pulse Width High
|-
| $04 (W) || CR1 || style="text-align:center;" | NOISE || style="text-align:center;" | PULSE || style="text-align:center;" | SAW || style="text-align:center;" | TRI || style="text-align:center;" | TEST || style="text-align:center;" | RING || style="text-align:center;" | SYNC || style="text-align:center;" | GATE
|-
| $05 (W) || AD1 || colspan="4" style="text-align:center;" | Channel 1 Attack || colspan="4" style="text-align:center;" | Channel 1 Decay
|-
| $06 (W) || SR1 || colspan="4" style="text-align:center;" | Channel 1 Sustain || colspan="4" style="text-align:center;" | Channel 1 Release
|-
| rowspan="7" | Voice 2 || $07 (W) || FREQLO2 || colspan="8" style="text-align:center;" | Channel 2 Frequency Low-Byte
|-
| $08 (W) || FREQHI2 || colspan="8" style="text-align:center;" | Channel 2 Frequency High-Byte
|-
| $09 (W) || PWLO2 || colspan="8" style="text-align:center;" | Channel 2 Pulse Width Low
|-
| $0A (W) || PWHI2 || colspan="4" style="text-align:center; background:#E0E0E0;" | unused || colspan="4" style="text-align:center;" | Channel 2 Pulse Width High
|-
| $0B (W) || CR2 || style="text-align:center;" | NOISE || style="text-align:center;" | PULSE || style="text-align:center;" | SAW || style="text-align:center;" | TRI || style="text-align:center;" | TEST || style="text-align:center;" | RING || style="text-align:center;" | SYNC || style="text-align:center;" | GATE
|-
| $0C (W) || AD2 || colspan="4" style="text-align:center;" | Channel 2 Attack || colspan="4" style="text-align:center;" | Channel 2 Decay
|-
| $0D (W) || SR2 || colspan="4" style="text-align:center;" | Channel 2 Sustain || colspan="4" style="text-align:center;" | Channel 2 Release
|-
| rowspan="7" | Voice 3 || $0E (W) || FREQLO3 || colspan="8" style="text-align:center;" | Channel 3 Frequency Low-Byte
|-
| $0F (W) || FREQHI3 || colspan="8" style="text-align:center;" | Channel 3 Frequency High-Byte
|-
| $10 (W) || PWLO3 || colspan="8" style="text-align:center;" | Channel 3 Pulse Width Low
|-
| $11 (W) || PWHI3 || colspan="4" style="text-align:center; background:#E0E0E0;" | unused || colspan="4" style="text-align:center;" | Channel 3 Pulse Width High
|-
| $12 (W) || CR3 || style="text-align:center;" | NOISE || style="text-align:center;" | PULSE || style="text-align:center;" | SAW || style="text-align:center;" | TRI || style="text-align:center;" | TEST || style="text-align:center;" | RING || style="text-align:center;" | SYNC || style="text-align:center;" | GATE
|-
| $13 (W) || AD3 || colspan="4" style="text-align:center;" | Channel 3 Attack || colspan="4" style="text-align:center;" | Channel 3 Decay
|-
| $14 (W) || SR3 || colspan="4" style="text-align:center;" | Channel 3 Sustain || colspan="4" style="text-align:center;" | Channel 3 Release
|-
| rowspan="4" | Filter || $15 (W) || FCLO || colspan="5" style="text-align:center; background:#E0E0E0;" | unused || colspan="3" style="text-align:center;" | Filter Cutoff Low
|-
| $16 (W) || FCHI || colspan="8" style="text-align:center;" | Filter Cutoff High
|-
| $17 (W) || Res/Filt || colspan="4" style="text-align:center;" | Filter Resonance || style="text-align:center;" | Filt Ex || style="text-align:center;" | Filt 3 || style="text-align:center;" | Filt 2 || style="text-align:center;" | Filt 1
|-
| $18 (W) || Mode/Vol || style="text-align:center;" | Chan 3 Off || style="text-align:center;" | High Pass || style="text-align:center;" | Band Pass || style="text-align:center;" | Low Pass || colspan="4" style="text-align:center;" | Volume
|-
| rowspan="4" | Other || $19 (R) || POTX || colspan="8" style="text-align:center;" | Potentiometer X
|-
| $1A (R) || POTY || colspan="8" style="text-align:center;" | Potentiometer Y
|-
| $1B (R) || OSC3 || colspan="8" style="text-align:center;" | Channel 3 Oscillator
|-
| $1C (R) || ENV3 || colspan="8" style="text-align:center;" | Channel 3 Envelope
|}
<br>