Gyruss: Routine at A31D

A31D: Send attenuation and frequency bytes to PSG

Used by the routine at tone_data_to_psg. Note: The value is actually 111,861 / frequency, so higher numbers give lower notes

Input
C	Attenuation operation in upper nybble (1CC1XXXX)
D	Tone operation in upper nybble (1CC0XXXX)
(IX+$03)	Frequency LSB
(IX+$04)	Attenuation/frequency MSB (AAAA00FF)

attn_and_freq_to_psg

A31D

CALL attn_or_noise_to_psg

Set attenuation

A320

LD A,(IX+$03)

Frequency LSB

A323

AND $0F

Isolate bits 0-3 of frequency

A325

OR D

Apply operation to bits 4-7

A326

OUT ($FF),A

Send to PSG

A328

LD A,(IX+$03)

Frequency LSB (FFFFFFFF)

A32B

AND $F0

Isolate bits 4-7 of frequency (FFFF0000)

A32D

LD D,A

Save it

A32E

LD A,(IX+$04)

Get frequency MSB (AAAA00FF)

A331

AND $0F

Isolate lower nybble with bits 8-9 of frequency (000000FF)

A333

OR D

Now FFFF00FF in wrong order

A334

RRCA

FFFFF00F

A335

RRCA

FFFFFF00

A336

RRCA

0FFFFFF0

A337

RRCA

00FFFFFF

A338

OUT ($FF),A

Send to PSG

A33A

RET