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)
$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 - Channel	Key On/Off for each channel.

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.

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)
$A9	$AA	$A9	$AA	Bit 7 6 5 4 3 2 1 0 Def 2CH * F-Num 1				2CH - 2 Slot F-Numbers/Block (1/2)
$AD	$AE	$AD	$AE	Bit 7 6 5 4 3 2 1 0 Def - 2CH * Block 2CH * F-Num 2				2CH - 2 Slot F-Numbers/Block (1/2) (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)

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	2	3	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)