6581 Sound Interface Device
                       ===========================

The 6581 SID (Sound Interface Device) is used in the Commodore 64 and the
BeebSID add-on.

+----------+---------------------------+-----------------------------+
|          |      Write                |         Read                |
+==========+===========================+=============================+
|          |   Voice 1:                |                             |
| base+&00 | Frequency Low             |                             |
| base+&01 | Frequency High            |                             |
| base+&02 | PW Low                    |                             |
| base+&03 | PW High                   |                             |
| base+&04 | Control register          |                             |
| base+&05 | Attack/Decay              |                             |
| base+&06 | Sustain/Release           |                             |
|          |                           |                             |
|          |   Voice 2:                |                             |
| base+&07 | Frequency Low             |                             |
| base+&08 | Frequency High            |                             |
| base+&09 | PW Low                    |                             |
| base+&0A | PW High                   |                             |
| base+&0B | Control register          |                             |
| base+&0C | Attack/Decay              |                             |
| base+&0D | Sustain/Release           |                             |
|          |                           |                             |
|          |   Voice 3:                |                             |
| base+&0E | Frequency Low             |                             |
| base+&0F | Frequency High            |                             |
| base+&10 | PW Low                    |                             |
| base+&11 | PW High                   |                             |
| base+&12 | Control register          |                             |
| base+&13 | Attack/Decay              |                             |
| base+&14 | Sustain/Release           |                             |
|          |                           |                             |
|          |   Filter:                 |                             |
| base+&15 | FC Low                    |                             |
| base+&16 | FC High                   |                             |
| base+&17 | Reset/Filter              |                             |
| base+&18 | Mode/Volume               |                             |
|          |                           |                             |
|          |                           |   Misc:                     |
| base+&19 |                           | POTX                        |
| base+&1A |                           | POTY                        |
| base+&1B |                           | OSC3/Random                 |
| base+&1C |                           | ENV3                        |
+==========+===========================+=============================+

Base  Device
----------------------------------------------
FC20  BeebSID