=== 0Dh - Volume Envelope Shape (4bit) ===
Writing to this register (re-)starts the envelope. Additionally, the The written value specifies the envelope shape, the four bits have the following meaning:
Bit 0 Hold (1=stop envelope past first cycle)
1110 0Eh /\/\/\/\/\
1111 0Fh /_________ (volume remains quiet)
We can observe that there are only 8 resulting shapes even though they are selected by using a 4-bit value.
When using the volume envelope generator, the volume is always increased from 00h to 0Fh (or vice versa), it is not possible to specify a starting/ending point (like from 00h to 07h).
Writing to this register (re-)starts the envelope.
=== 0Eh - External Dataregister Port A ===