=== 0Dh - Volume Envelope Shape (4bit) ===
Writing to this register (re-)starts the envelope. Both components of the envelope's phase are reset. The first step of the envelope has full duration every time. It is never shorted. [https://forums.nesdev.org/viewtopic.php?p=236672#p236672 Source]
The written value specifies the envelope shape, the four bits have the following meaning:
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. Both components of the envelope's phase are reset. The first step of the envelope has full duration every time. It is never shorted. [https://forums.nesdev.org/viewtopic.php?p=236672#p236672 Source]
=== 0Eh - External Dataregister Port A ===