FM

The FM (Frequency Modulation) is part of the YM2610 sound chip. It provides 4 channels, each having their own set of 4 operators, panning and amplitude values. It's the most used way of producing music in games.

Common registers

Address (Z80 port 4)	Data (Z80 port 5)
$21	Bit 7 6 5 4 3 2 1 0 Def ?	Test register. Ignore or set to $00 for normal operation.
$22	Bit 7 6 5 4 3 2 1 0 Def - On Control	LFO control and frequency (see below).
$28	Bit 7 6 5 4 3 2 1 0 Def Slot (OP4/OP3/OP2/OP1) - Channel	Key On/Off for each channel, with operators control.

LFO frequency values are as follows:

• 0 – 3.98Hz
• 1 – 5.56Hz
• 2 – 6.02Hz
• 3 – 6.37Hz
• 4 – 6.88Hz
• 5 – 9.63Hz
• 6 – 48.1Hz
• 7 – 72.2Hz

Channel numbering:

FM Channel	Binary Code
CH1	001
CH2	010
CH3	101
CH4	110

This strange numbering seems to be due to the fact YM2610 is a YM2610B with 2 removed FM channels (000 and 100).

Channel registers

Depending on which channel you want to write to, the Z80 ports used are different:

• Channels 1 & 2: Ports 4/5
• Channels 3 & 4: Ports 6/7

Address				Data
CH1	CH2	CH3	CH4	CH1	CH2	CH3	CH4
Port 4		Port 6		Port 5		Port 7
$A1	$A2	$A1	$A2	Bit 7 6 5 4 3 2 1 0 Def F-Num 1				F-Numbers and Block (1/2)
$A5	$A6	$A5	$A6	Bit 7 6 5 4 3 2 1 0 Def - Block F-Num 2				F-Numbers and Block (2/2) (must set this first)
$B1	$B2	$B1	$B2	Bit 7 6 5 4 3 2 1 0 Def - FB ALGO				Feedback (FB) and Algorithm (ALGO)
$B5	$B6	$B5	$B6	Bit 7 6 5 4 3 2 1 0 Def L R AMS - PMS				Left (L)/Right (R) output, AM Sense (AMS), and PM Sense (PMS)

2CH mode additional operator frequencies:

• OP1 frequency is stored in the usual CH2 frequency registers ($A2/$A6)
• write to Z80 ports 4/5

Address			Data
OP2	OP3	OP4
$A8	$A9	$AA	Bit 7 6 5 4 3 2 1 0 Def 2CH * F-Num 1				2CH mode F-Num LSB
$AC	$AD	$AE	Bit 7 6 5 4 3 2 1 0 Def - 2CH * Block 2CH * F-Num 2				2CH mode F-Num MSB & Block (must set this first)

Operator registers

The ranges given for the address represent all of the parameter values. Each channel's operators are laid out as follows:

Operator	1	3	2	4
Channels 1, 3	$x1	$x5	$x9	$xD
Channels 2, 4	$x2	$x6	$xA	$xE

Address				Data
CH1	CH2	CH3	CH4	CH1	CH2	CH3	CH4
Port 4		Port 6		Port 5		Port 7
$31-$3E				Bit 7 6 5 4 3 2 1 0 Def - DT MUL				Detune (DT) and Multiple (MUL)
$41-$4E				Bit 7 6 5 4 3 2 1 0 Def - Total Level				Total Level (Volume)
$51-$5E				Bit 7 6 5 4 3 2 1 0 Def KS - AR				Key Scale (KS) and Attack Rate (AR)
$61-$6E				Bit 7 6 5 4 3 2 1 0 Def AM - DR				AM On (AM) and Decay Rate (DR)
$71-$7E				Bit 7 6 5 4 3 2 1 0 Def - SR				Sustain Rate (SR)
$81-$8E				Bit 7 6 5 4 3 2 1 0 Def SL RR				Sustain Level (SL) and Release Rate (RR)
$91-$9E				Bit 7 6 5 4 3 2 1 0 Def - SSG-EG				Envelope generator (not to be confused with the SSG one)