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 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 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 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