nullsound: check validity of 8bit pointer addition
In order to optimize speed and size of Z80 code, one can do
8bit addition on 16bit pointers, assuming the result of the addition
will not change the MSB of the pointer.
This works as long as nullsound data are linked appropriately in
the Z80 address space by using the right .bndry statements.
To verify that nullsound binaries have the expected boundary
properties, add a validating script that checks that data surrounded
by labels _state_XXX_start to _state_XXX_end are always allocated
by sdld to addresses with the same MSB.
nullsound: new NSS opcode to simulate auto-envelope
Furnace has a feature that reconfigures the SSG envelope based
on the current note's frequency and a configurable quotient
numerator/denominator. This maps into the YM2610's envelope
register.
We don't want to do math on the Z80 at runtime, so we have
to simulate this feature by precomputing the quotient prior
to playing the note at runtime.
Add a new opcode SSG_ENV_PERIOD that is inserted automatically
by nsstool.py in a new compilation pass.
This requires nsstool to know the autoenv configuration for the
instrument in use, so make nsstool parse the instrument section
of the Furnace module before generating NSS data.
nullsound: rework SSG macros and how they are applied
Two major updates to how SSG macro work in nullsound:
1.
The logic of update_ssg_macro used run macro steps for each
SSG channel and then load the mirrored state of all channels.
This is not ideal as it cannot detect when a macro is finished,
and thus it keep uploading mirrored state into the YM2610 at
every step even when not necessary.
Now the algorithm group update and load together, so it won't
load data into the YM2610 when a macro is finished.
2.
The SSG register common to all channels must not be updated at
every tick. For instance, when doing so with the envelope channel,
this resets the envelope shape on every note, which is not the
behaviour implemented by Furnace.
Split macros into two subparts: one is only called once when the
macro is set up for a SSG channel. The other parts contains the
remaining data that must be run on every tick while a note is
playing.
Furnace stores the detune parameter of a FM instrument in
a different way as what is natively supported by the YM2610.
Convert the value in the generated instrument data to fix
the discrepancy.
