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).
==== Bit 1 (Alternate) and Bit 0 (Hold) ====
These 2 bits determine what happens to volume and direction at the end of a period.
volume = ~volume if
direction = 1 if else -1 if else 0
=== 0Eh - External Dataregister Port A ===