GABE

GABE -- The System Control, I/O, and Sound Chip

"GABE" is the name of the system controller chip of the C256 Foenix RevC board, and performs several functions including system bus management, math coprocessing, I/O, and sound. The RevB version of the C256 had two chips "Gavin" and "Beatrix" that were combined into a single master chip called "GABE".

GABE has several key functions:

• System start up
• Debug interface control and bus mastering
• I/O interfacing
• Math coprocessor
• Sound chip interfacing and emulation

GABE System Startup Process

When the C256 Foenix powers up, GABE takes control and coordinates the startup process. It first copies the flash data out of flash and into system RAM, starting at bank $38 on the FMX ($18 on the RevB). GABE also copies the first bank of flash data down to bank $00 of system RAM before control is given to the CPU and the CPU's RESET is triggered. This process ensures that the kernel, hardware vectors, and interrupt handlers are all in place prior to the CPU taking control and resetting the first time on power up.

The mapping of flash memory locations to system RAM is thus:

Flash Range	RAM Range
$F8:0000...$FF:FFFF	$38:0000...$3F:FFFF
$F8:0000...$F8:FFFF	$00:0000...$00:FFFF

GABE Integer Math Coprocessor ($00:0100 – $00:012B)

GABE provides an easy to use integer math coprocessor in the form of two 16-multipliers (one signed, one unsigned), two 16-bit dividers (one signed, one unsigned), and a single 32-bit adder. Using the coprocessor is straight-forward: the operands are written to the correct registers, and the result is read from the output registers. The operations are all performed within a single CPU clock cycle, so no waiting is required.

Start Address	End Address	Register Name	Additional Info
$00:0100	$00:0101	UNSIGNED_MULT_A	A operand for unsigned multiplication (16-bit)
$00:0102	$00:0103	UNSIGNED_MULT_B	B operand for unsigned multiplication (16-bit)
$00:0104	$00:0107	UNSIGNED_MULT_RESULT	Result for unsigned multiplication (32-bit)
$00:0108	$00:0109	SIGNED_MULT_A	A operand for signed multiplication (16-bit)
$00:010A	$00:010B	SIGNED_MULT_B	B operand for signed multiplication (16-bit)
$00:010C	$00:010F	SIGNED_MULT_RESULT	Result for signed multiplication (32-bit)
$00:0110	$00:0111	UNSIGNED_DIV_DEM	Denominator for unsigned division (16-bit)
$00:0112	$00:0113	UNSIGNED_DIV_NUM	Numerator for unsigned division (16-bit)
$00:0114	$00:0115	UNSIGNED_DIV_QUO	Quotient for unsigned division (16-bit)
$00:0116	$00:0117	UNSIGNED_DIV_REM	Remainder for unsigned division (16-bit)
$00:0118	$00:0119	SIGNED_DIV_DEM	Denominator for signed division (16-bit)
$00:011A	$00:011B	SIGNED_DIV_NUM	Numerator for signed division (16-bit)
$00:011C	$00:011D	SIGNED_DIV_QUO	Quotient for signed division (16-bit)
$00:011E	$00:011F	SIGNED_DIV_REM	Remainder for signed division (16-bit)
$00:0120	$00:0123	ADDER32_A	A operand for 32-bit addition (32-bits)
$00:0124	$00:0127	ADDER32_B	B operand for 32-bit addition (32-bits)
$00:0128	$00:012B	ADDER32_R	Result of 32-bit addition (32-bits)

GABE Keyboard Ports

Address	R/W	Name	7	6	5	4	3	2	1	0	Description
$AF:1060	X	KBD_DATA	D7	D6	D5	D4	D3	D2	D1	D0	Keyboard data port
$AF:1061	X	PORTB	D7	D6	D5	D4	D3	D2	D1	D0
$AF:1064	R	KBD_STATUS	PAR_E	RX_TO	TX_TO	KBD_INH	CMD	SYS	IN_F	OUT_F	Keyboard Status
$AF:1064	W	KBD_CMD	CMD7	CMD6	CMD5	CMD4	CMD3	CMD2	CMD1	CMD0	Keyboard Command

GABE Keyboard Status Flags

PAR_E

Parity error

RX_TO

Time out reading data

TX_TO

Time out transmitting data

CMD

If 0, then data written to input buffer is data for PS/2 device. If 1, then data written to input buffer is data for PS/2 controller command

SYS

Flag to the firmware that the keyboard has past POST

IN_F

Input buffer is full

OUT_F

Output buffer is full

GABE Floating Point Math Coprocessor ($AF:E200 – AFE20F)

GABE provides a floating point math coprocessor, providing addition, subtraction, multiplication, and division for 32-bit floating point numbers. Additionally, the coprocessor allows for conversion between a fixed point format and 32-bit IEEE 754 floating point format. The floating point coprocessor is a little more complex than the integer coprocessor, with each math unit having input multiplexers that control where the inputs come from, and with the output registers having their own multiplexer. Finally, the output of the multiplier and divider units can be used directly as inputs to the adder/subtracter.

Start Address	End Address	Register Name	Additional Information
$AF:E200		FP_MATH_CTRL0	Controls input multiplexers, Adder function, and Adder multiplexers
$AF:E201		FP_MATH_CTRL1	Controls output multiplexers
$AF:E202		FP_MATH_CTRL2	Unused: reserved
$AF:E203		FP_MATH_CTRL3	Unused: reserved
$AF:E204		FP_MATH_MULT_STAT	Status of the multiplier
$AF:E205		FP_MATH_DIV_STAT	Status of the divider
$AF:E206		FP_MATH_ADD_STAT	Status of the adder/subtracter
$AF:E207		FP_MATH_CONV_STAT	Status of the converters
$AF:E208	$AF:E20B	FP_MATH_INPUT0	Input 0 (write only)
$AF:E20C	$AF:E20F	FP_MATH_INPUT1	Input 1 (write only)
$AF:E208	$AF:E20B	FP_MATH_OUTPUT_FP	Result in IEEE 754 format (read only)
$AF:E20C	$AF:E20F	FP_MATH_OUTPUT_FIXED	Result in 20.12 fixed point format (read only)

GABE Register FP_MATH_CTRL0 ($AF:E200)

7	6	5	4	3	2	1	0
FP_MATH_CTRL0_ADD_IN1_MUX		FP_MATH_CTRL0_ADD_IN0_MUX		FP_MATH_CTRL0_ADD_SUB		FP_MATH_CTRL0_INPUT1_MUX	FP_MATH_CTRL0_INPUT0_MUX

FP_MATH_CTRL0_INPUT0_MUX

If 0, Input0 is assumed to be in IEEE format. If 1, it is assumed to be in 20.12 fixed point format and will be converted to IEEE.

FP_MATH_CTRL0_INPUT1_MUX

If 0, Input1 is assumed to be in IEEE format. If 1, it is assumed to be in 20.12 fixed point format and will be converted to IEEE.

FP_MATH_CTRL0_ADD_SUB

If 0, the adder will perform a subtract operation. If 1, it will perform an addition.

FP_MATH_CTRL0_ADD_IN0_MUX

Selects the source for input 0 of the adder unit.

FP_MATH_CTRL0_ADD_IN1_MUX

Selects the source for input 1 of the adder unit.

Adder Source Selector Codes

Control Bits	Input
00	Input 0
01	Input 1
10	Output of the multiplier
11	Output of the divider

GABE Register FP_MATH_CTRL1 ($AF:E201)

7	6	5	4	3	2	1	0
						FP_MATH_CTRL1_OUTPUT_MUX0

FP_MATH_CTRL1_OUTPUT_MUX0

Selects which functional block provides the output result:

Result Source Selector Codes

Control Bits	Output Source
00	Result of multiplier is the routed to the output registers
01	Result of divider is the routed to the output registers
10	Result of adder is the routed to the output registers
11	1.0 is routed to the output registers

GABE Registers: FP Status Registers ($AF:E204 – $AF:E207)

These registers contain the status flags for the various functional units. The status flags are all roughly similar across the four units:

Status Codes

Code	Meaning
$01	Result was not a number (NAN)
$02	Overflow
$04	Underflow
$08	Result was 0.0
$10	Division by 0.0 was attempted

GABE Timer Registers ($00:0160 – $00:016F)

(Bring in from Kernel_FMX/src/timer_def.asm)

• Timer0 and Timer1 count system clock transitions (14318180Hz)
• Timer2 is clocked from VICKY's Start-of-Frame output.

GABE Interrupt Control Registers ($00:0140 – $00:014B)

There are four types of interrupt control register that GABE provides: pending, polarity, edge detection, and mask. Each interrupt that is supported has a bit position in each of the 24 or 32 bits provided by the register types.

Pending

The pending registers indicate if an interrupt of a particular type has been triggered and needs processing. An interrupt handler should also write to this register to clear the pending flag, once the interrupt has been processed.

Polarity

This register indicates if the interrupt is triggered by a high or low signal on the input to GABE.

Edge

This register indicates if the interrupt is triggered by an transition (edge) or by a high or low value.

Mask

This register indicates if the associated interrupt will trigger an IRQ to the processor. Interrupt signals with a mask bit of 0 will be ignored, while those with a mask bit of 1 will trigger an interrupt to the CPU.

Start	Register Description	Additional Info
$00:0140	INT_PENDING_REG0	Interrupt pending #0
$00:0141	INT_PENDING_REG1	Interrupt pending #1
$00:0142	INT_PENDING_REG2	Interrupt pending #2
$00:0143	INT_PENDING_REG3	Interrupt pending #3---FMX Model only
$00:0144	INT_POL_REG0	Interrupt polarity #0
$00:0145	INT_POL_REG1	Interrupt polarity #1
$00:0146	INT_POL_REG2	Interrupt polarity #2
$00:0147	INT_POL_REG3	Interrupt polarity #3---FMX Model only
$00:0148	INT_EDGE_REG0	Enable Edge Detection #0
$00:0149	INT_EDGE_REG1	Enable Edge Detection #1
$00:014A	INT_EDGE_REG2	Enable Edge Detection #2
$00:014B	INT_EDGE_REG3	Enable Edge Detection #3---FMX Model only
$00:0148	INT_MASK_REG0	Enable Interrupt #0
$00:0149	INT_MASK_REG1	Enable Interrupt #1
$00:014A	INT_MASK_REG2	Enable Interrupt #2
$00:014B	INT_MASK_REG3	Enable Interrupt #3---FMX Model only

There are several interrupts the GABE can handle, distributed over four blocks:

Block	Bit	Name	Function
0	$01	FNX0_INT00_SOF	Start of Frame @60Hz
0	$02	FNX0_INT01_SOL	Start of Line (programmable)
0	$04	FNX0_INT02_TMR0	Timer 0
0	$08	FNX0_INT03_TMR1	Timer 1
0	$10	FNX0_INT04_TMR2	Timer 2
0	$20	FNX0_INT05_RTC	Real Time Clock
0	$40	FNX0_INT06_FCC	Floppy Drive Controller
0	$80	FNX0_INT07_MOUSE	Mouse Interrupt (INT12 in SuperIO IOspace)
1	$01	FNX1_INT00_KBD	Keyboard Interrupt
1	$02	FNX1_INT01_SC0	VICKY_II (INT2) Sprite 2 Sprite Collision
1	$04	FNX1_INT02_SC1	VICKY_II (INT3) Sprite 2 Tiles Collision
1	$08	FNX1_INT03_COM2	Serial Port 2 (Internal)
1	$10	FNX1_INT04_COM1	Serial Port 1 (External)
1	$20	FNX1_INT05_MPU401	Midi Controller Interrupt
1	$40	FNX1_INT06_LPT	Parallel Port
1	$80	FNX1_INT07_SDCARD	SD Card Controller Interrupt (CH376S, if present)
2	$01	FNX2_INT00_OPL3	OPL3
2	$02	FNX2_INT01_GABE_INT0	GABE (INT0) - TBD
2	$04	FNX2_INT02_GABE_INT1	GABE (INT1) - TBD
2	$08	FNX2_INT03_SDMA	VICKY_II (INT4)
2	$10	FNX2_INT04_VDMA	VICKY_II (INT5) -- Vicky DMA completion
2	$20	FNX2_INT05_GABE_INT2	GABE (INT2) - TBD
2	$40	FNX2_INT06_EXT	External Expansion
2	$80	FNX2_INT07_SDCARD_INS	SDCARD Insertion
3	$01	FNX3_INT00_OPN2	OPN2
3	$02	FNX3_INT01_OPM	OPM
3	$04	FNX3_INT02_IDE	HDD IDE Interrupt
3	$08	FNX3_INT03_TBD	TBD
3	$10	FNX3_INT04_TBD	TBD
3	$20	FNX3_INT05_TBD	TBD
3	$40	FNX3_INT06_TBD	TBD
3	$80	FNX3_INT07_TBD	TBD

GABE SID Emulation ($AF:E400 – $AF:E41F)

GABE emulates the SID sound chip. Currently, the FMX provides one SID unit. Although the real SID chip provides analog to digital conversion for two potentiometer inputs for paddles, the C256's paddle inputs are provided through a different chip, so the POT registers listed below are not useful.

For complete information on how to use these registers, please consult documentation on the SID chips.

All registers are write-only, except where noted otherwise.

Start Address	End Address	Register Name	Additional Information
$AF:E400	$AF:E401	SID0_V1_FREQ	SID0 V1 Frequency (16-bits)
$AF:E402	$AF:E403	SID0_V1_PW	SID0 V1 Pulse Width (12-bits)
$AF:E404		SID0_V1_CTRL	SID0 V1 Control (8-bits)
$AF:E405		SID0_V1_ATCK_DECY	SID0 V1 Attack/Decay (8-bits)
$AF:E406		SID0_V1_SSTN_RLSE	SID0 V1 Sustain/Release (8-bits)
$AF:E407	$AF:E408	SID0_V2_FREQ	SID0 V2 Frequency (16-bits)
$AF:E409	$AF:E40A	SID0_V2_PW	SID0 V2 Pulse Width (12-bits)
$AF:E40B		SID0_V2_CTRL	SID0 V2 Control (8-bits)
$AF:E40C		SID0_V2_ATCK_DECY	SID0 V2 Attack/Decay (8-bits)
$AF:E40D		SID0_V2_SSTN_RLSE	SID0 V2 Sustain/Release (8-bits)
$AF:E40E	$AF:E40F	SID0_V3_FREQ	SID0 V3 Frequency (16-bits)
$AF:E410	$AF:E411	SID0_V3_PW	SID0 V3 Pulse Width (12-bits)
$AF:E412		SID0_V3_CTRL	SID0 V3 Control (8-bits)
$AF:E413		SID0_V3_ATCK_DECY	SID0 V3 Attack/Decay (8-bits)
$AF:E414		SID0_V3_SSTN_RLSE	SID0 V3 Sustain/Release (8-bits)
$AF:E415		SID0_FC_LO	SID0 Filter Frequency LOW (3 bits)
$AF:E416		SID0_FC_HI	SID0 Filter Frequency HIGH (8 bits)
$AF:E417		SID0_RES_FILT	SID0 Resonance / Filter (8-bits)
$AF:E418		SID0_MODE_VOL	SID0 Mode / Volume (8-bits)
$AF:E419		SID0_POT_X	SID0 POT X (Read only, C256 - NOT USED)
$AF:E41A		SID0_POT_Y	SID0 POT Y (Read only, C256 - NOT USED)
$AF:E41B		SID0_OSC3_RND	SID0 OSC3 / RANDOM (Read only, 8-bits)
$AF:E41C		SID0_ENV3	SID0 ENV3 (Read only, 8-bits)

GABE Trinity Joystick/Gamepad and DIP Switch Registers ($AF:E800 – $AF:E80E)

GABE provides access to the Trinity chip, which provides access to the joystick ports and the DIP switches. The joystick ports can be used in two modes: default mode, where they work like Atari style joysticks, and NES mode where they can interface with either NES or SNES game pads.

Address	Name	7	6	5	4	3	2	1	0	Description
$AF:E800	JOYSTICK0	BUTTON2	BUTTON1	0	BUTTON0	RIGHT	LEFT	DOWN	UP	Right-most joystick port
$AF:E801	JOYSTICK1	BUTTON2	BUTTON1	0	BUTTON0	RIGHT	LEFT	DOWN	UP	Next to joystick port 0
$AF:E802	JOYSTICK2	0	0	0	BUTTON0	RIGHT	LEFT	DOWN	UP	Next to joystick port 1
$AF:E803	JOYSTICK3	0	0	0	BUTTON0	RIGHT	LEFT	DOWN	UP	Left-most joystick port
$AF:E804	JOYSTICK_MODE	NES_TRIG	0	0	0	NES_DONE	NES_JOY	NES_EN1	NES_EN0	Control SNES compatibility
$AF:E805	REVOFPCB_C	"C"								Board identification code byte (expect "C")
$AF:E806	REVOFPCB_4	"4"								Board identification code byte (expect "4")
$AF:E807	REVOFPCB_A	"A"								Board identification code byte (expect "A")
$AF:E808	NES_SNES0_DAT_LO	see below								Port 0: NES or SNES gamepad data (low byte)
$AF:E809	SNES0_DAT_HI	see below								Port 0: NES or SNES gamepad data (highbyte)
$AF:E80A	NES_SNES1_DAT_LO	see below								Port 1: NES or SNES gamepad data (low byte)
$AF:E80B	SNES1_DAT_HI	see below								Port 1: NES or SNES gamepad data (highbyte)
$AF:E80C	CFP9301_REV	revision								Trinity revision code
$AF:E80D	DIP_USER	user								Switches 3 through 5. User defined meaning.
$AF:E80E	DIP_BOOTMODE	HD_INSTALLED	GAMMA_ON	0	0	0	0	BM1	BM0	Boot control switches.

SNES mode

Address	Name	7	6	5	4	3	2	1	0	Description
$AF:E808	NES_SNES0_DAT_LO	B	Y	SELECT	START	UP	DOWN	LEFT	RIGHT	NES data if NES_JOY = 1 and NES_EN0 = 1
$AF:E809	SNES0_DAT_HI	0	0	0	0	A	X	L	R	NES data if NES_JOY = 1 and NES_EN0 = 1)
$AF:E80A	NES_SNES1_DAT_LO	B	Y	SELECT	START	UP	DOWN	LEFT	RIGHT	NES data if NES_JOY = 1 and NES_EN1 = 1
$AF:E80B	SNES1_DAT_HI	0	0	0	0	A	X	L	R	NES data if NES_JOY = 1 and NES_EN1 = 1

NES mode

Address	Name	7	6	5	4	3	2	1	0	Description
$AF:E808	NES_SNES0_DAT_LO	A	B	SELECT	START	UP	DOWN	LEFT	RIGHT	NES data if NES_JOY = 0 and NES_EN0 = 1
$AF:E809	SNES0_DAT_HI	N/A		Unused if NES_EN0 = 0
$AF:E80A	NES_SNES1_DAT_LO	A	B	SELECT	START	UP	DOWN	LEFT	RIGHT	NES data if NES_JOY = 0 and NES_EN1 = 1
$AF:E80B	SNES1_DAT_HI	N/A		Unused in NES mode

NES Control Bits

NES_TRIG

Set to one to trigger the serializer to read the gamepad.

NES_DONE

Poll to see if the serializer has finished reading the gamepad.

NES_JOY

If 1, sets SNES compatibility. If 0, sets NES compatibility.

NES_EN1

If 1, enables NES/SNES compatibility on port 1. If 0, joysticks work like Atari joysticks

NES_EN0

If 1, enables NES/SNES compatibility on port 0. If 0, joysticks work like Atari joysticks

Boot Mode

Switches 0 and 1 allow you to select the boot mode for the FMX. Note that the switches are active low: a switch in the "On" position registers as a 0.

BM1	BM0	Boot Mode
0	0	Boots from the IDE drive DIP switch 8 must be set (HD_INSTALLED).
0	1	Boots from the SD card
1	0	Boots from the floppy drive
1	1	Boots to BASIC

GABE IDE Registers ($AF:E830 – $AF:E839)

GABE provides an IDE interface to a hard drive (if installed). This is done through the Unity chip and uses standard IDE interface command, error, and status codes.

Address	R/W	Name	7	6	5	4	3	2	1	0	Description
$AF:E830	x	IDE_DATA	D7	D6	D5	D4	D3	D2	D1	D0	8-bit data read/write
$AF:E831	R	IDE_ERROR	BBK	UNC	MC	IDNF	MCR	ABRT	TK0NF	AMNF	Error code for the transaction
$AF:E832	W	IDE_SECT_CNT	SC7	SC6	SC5	SC4	SC3	SC2	SC1	SC0	Sector count
$AF:E833	W	IDE_SECT_SRT	SS7	SS6	SS5	SS4	SS3	SS2	SS1	SS0	Sector start (0 = 256), start at 1
$AF:E834	W	IDE_CLDR_LO	C7	C6	C5	C4	C3	C2	C1	C0	Cylinder Number (low)
$AF:E834	W	IDE_CLDR_HI	C15	C14	C13	C12	C11	C10	C9	C8	Cylinder Number (high)
$AF:E835	W	IDE_HEAD	1	0	1	M/S	H3	H2	H1	H0	Head number and Master (0) / Slave(1) device
$AF:E836	W	IDE_CMD_STAT	CMD7	CMD6	CMD5	CMD4	CMD3	CMD2	CMD1	CMD0	Send a command to the drive
$AF:E836	R	IDE_CMD_STAT	BSY	DRDY	DF	DSC	DRQ	CORR	IDX	ERR	Get the status of device. Reading this will clear the Interrupt Registers
$AF:E837	X	IDE_DATA_LO	D7	D6	D5	D4	D3	D2	D1	D0	16-bit data transfer registers (low byte)
$AF:E838	X	IDE_DATA_HI	D15	D14	D13	D12	D11	D10	D9	D8	16-bit data transfer registers (high byte)

GABE IDE Status Flags

BSY

Device is busy if set

DRDY

Device is ready for a command if set

DF

Device fault

DSC

Device seek complete. If set, the head has stopped moving and is over a track.

DRQ

Device is ready to transfer a data byte (either read or write) if set.

CORR

If set, indicates that data was corrected automatically.

IDX

A vendor specific status

ERR

If set, an error occurred with additional data in the error register.

GABE IDE Error Flags

BBK

Bad block

UNC

Uncorrectable data error

MC

Media changed

IDNF

ID mark not found

MCR

Media change requested

ABRT

Command aborted

TK0NF

Track 0 not found

AMNF

Address mark not found

GABE Control Registers ($AF:E880 – $AF:E887)

Start Address	Ending Address	Register Description	Additional Info
$AF:E880		GABE_MSTR_CTRL
$AF:E881		Reserved
$AF:E882		GABE_RST_AUTH0	Must Contain the BYTE $AD for Reset to Activate
$AF:E883		GABE_RST_AUTH1	Must Contain the BYTE $DE for Reset to Activate
$AF:E884	$AF:E885	GABE_RNG_DATASEED	On read: 16-bit random data. On write, set 16-bit RNG seed.
$AF:E886		GABE_RNG_STATCTRL	On read: 8-bit status. On write: 8-bit control
$AF:E887		GABE_SYS_STAT	8-bit system status

GABE Master Control Register ($AF:E880)

7	6	5	4	3	2	1	0
GABE_CTRL_WRM_RST			GABE_CTRL_BUZZER			GABE_CTRL_SDC_LED	GABE_CTRL_PWR_LED

GABE_CTRL_PWR_LED

Turns the power LED (next to the reset button) on or off.

GABE_CTRL_SDC_LED

Turns the SDC activity LED (next to the SDC slot) on or off.

GABE_CTRL_BUZZER

Turns the built-in piezo buzzer on or off.

GABE_CTRL_WRM_RST

Triggers a warm reset of the board (GABE_RST_AUTH0 must be set to $AD and GABE_RST_AUTH1 to $DE in order to trigger the reset).

GABE System Status Register ($AF:E887)

7	6	5	4	3	2	1	0
GABE_SYS_STAT_CPUX	GABE_SYS_STAT_CPUA			GABE_SYS_STAT_EXP		GABE_SYS_STAT_MID1	GABE_SYS_STAT_MID0

GABE_SYS_STAT_CPUX

Indicates if the CPU's index registers are 8-bits or 16-bits wide.

GABE_SYS_STAT_CPUA

Indicates if the CPU's accumulator is 8-bits or 16-bits wide.

GABE_SYS_STAT_EXP

Indicates if the and expansion card is present (0).

GABE_SYS_STAT_MID1 and GABE_SYS_STAT_MID0

These two bits show the machine ID:

GABE_SYS_STAT_MID1	GABE_SYS_STAT_MID0	Machine
0	0	FMX - Development Platform
0	1	C256 Foenix - Dev Platform
1	0	C256 Foenix - User Version (65C816)
1	1	Reserved

GABE SN76489 Interface ($AF:F100)

GABE provides the interface to the SN76489 sound chip on the Foenix FMX. This chip uses only a single write-only address: $AF:F100. The SN76489 has four voices: three tone generators, and one noise generator. Each voice has a frequency and an attenuation setting. For full details on how to use the SN76489 chip, please consult the official documentation, but here is a brief summary.

Playing a Tone

To change the frequency of a tone generator, two bytes are sent to the chip in order:

	7	6	5	4	3	2	1	0
First	1	Channel		0	F3	F2	F1	F0
Second	0	x	F9	F8	F7	F6	F5	F4

The actual frequency played depends on the clock driving the chip: freq = N / (32 * F), where N = 3575800 (for the FMX), and F is the 10-bit number sent to the chip. The "Channel" number above must be 0, 1, or 3.

Play a Noise

To play a noise, only a single byte is written to the chip:

7	6	5	4	3	2	1	0
1	1	1	0	x	NF	NF1	NF0

The NF bit controls the type of noise generated, and the NF bits control the base frequency of the noise.

Setting the Volume

The volumes of each of the four voices can be set independently. To set the volume of a single voice, a single byte is sent to the chip:

7	6	5	4	3	2	1	0
1	Channel		1	A3	A2	A1	A0

The "Channel" bits are the number of the voice to change: 0, 1, 2, or 3. The four bit value A is the level of attenuation, where 0 is fully on, and 15 is fully off.