Memory-mapped I/O ports

8FFF0000 - Unknown

Possibly power-management related. In the OS, all code that uses these registers is copied to SRAM and run from there, rather than running it directly from the SDRAM.

90000000 - General Purpose I/O (GPIO)

The GPIO registers are separated into 4 sections:

• Section 0: 90000000-9000003F
• Section 1: 90000040-9000007F
• Section 2: 90000080-900000BF
• Section 3: 900000C0-900000FF.

Each register is a word, and only bits 0-7 of each are used. There can be up to 32 devices accessed by this setup, each known as a GPIO. Each GPIO is defined by one of the 8 bits in one of the sections. The number of the GPIO is the section number times 8 plus the bit number. Each GPIO has a status bit and can cause interrupts.

The following addresses are offsets from the beginning of the GPIO section:

• +00 (R): Masked interrupt status ([+04] & [+08])
• +04 (R): Reads raw interrupt status (directly dependent on the GPIO input) or sticky interrupt status (becomes set when GPIO status changes) depending on bit in [+20]
• +04 (W): Write 1 to the bit to reset the sticky interrupt status.
• +08 (R): Reads current interrupt mask bit.
• +08 (W): Write 1 to the bit to enable interrupt (set mask bit to 1)
• +0C (W): Write 1 to the bit to disable interrupt (set mask bit to 0)

The following addresses should be changed with a read-modify-write pattern so as to not disturb other bits in the register.

• +10 (R/W): Direction (set bit to 0 for output, 1 for input)
• +14 (R/W): GPIO output bit
• +18 (R): Reads GPIO input bit.
• +1C (R/W): Setting bit to 1 will invert raw interrupt status (?)
• +20 (R/W): Setting bit to 1 will use sticky interrupt status in [+04], or setting to 0 will use raw interrupt status.
• +24 (R/W): (?)

Currently known GPIOs:

• GPIO 1 (Section 0, bit 1): Touchpad "clock" (protocol is like I²C)
• GPIO 2 (Section 0, bit 2): Input bit is 0 if battery door open, 1 if closed (TI-Nspire CAS)
• GPIO 3 (Section 0, bit 3): Touchpad data
• GPIO 8 (Section 1, bit 0): Input bit is 0 if Reset Button is pressed.
• GPIO 24 (Section 3, bit 0): Input bit is 1 if a keypad is not plugged in. (TI-Nspire non-cas only?)

90010000 - Fast timer

The same interface as 900C0000/900D0000, but runs at the speed of the APB clock (22.5MHz) rather than 32kHz. See Second timer for more info.

90020000 - Serial UART

Used to communicate with the RS232 serial port. The register interface is like that of 16550 UART used in PCs:

• 90020000 (R): Receiver Buffer Register
• 90020000 (W): Transmitter Holding Register
• 90020004 (R/W): Interrupt Enable Register
• 90020008 (R): Interrupt Identification Register
• 90020008 (W): FIFO Control Register
• 9002000C (R/W): Line Control Register
• 90020010 (R/W): Modem Control Register
• 90020014 (R): Line Status Register
• 90020018 (R): Modem Status Register
• 9002001C (R/W): Scratch Register

90060000 - Watchdog timer

Possibly an ARM SP805 or compatible.

90090000 - Real-Time Clock (RTC)

Might be an ARM PrimeCell PL031.

• 90090000 (R): Current time in seconds
• 90090004 (W): ?
• 90090008 (R/W): Sets the value of 90090000 (clock will not read new time until a couple seconds later). Reads last value written.
• 9009000C (W): ?
• 90090010 (W): ?
• 90090014 (R): ?

900A0000 - Miscellaneous

• 900A0000 (R): ?
• 900A0004 (R/W): Set bit 0x20 to enable TI-84+ keypad link port. Other bits likely control functions of peripherals as well.
• 900A0008 (W): Write a 2 to reset the CPU
• 900A0010 (R/W): Fast timer interrupt status/acknowledge (6-bit). Write "1" bits to reset the corresponding interrupt requests.
• 900A0014 (R/W): Fast timer interrupt mask (6-bit). Set bits to 1 if the corresponding bits in [900A0010] should trigger an IRQ.
• 900A0018 (R/W): Timer 1 interrupt status/acknowledge (6-bit). Write "1" bits to reset the corresponding interrupt requests.
• 900A001C (R/W): Timer 1 interrupt mask (6-bit). Set bits to 1 if the corresponding bits in [900A0018] should trigger an IRQ.
• 900A0020 (R/W): Timer 2 interrupt status/acknowledge (6-bit). Write "1" bits to reset the corresponding interrupt requests.
• 900A0024 (R/W): Timer 2 interrupt mask (6-bit). Set bits to 1 if the corresponding bits in [900A0020] should trigger an IRQ.
• 900A0028-900A002C (R): These registers together give a 64-bit number (28 is low, 2C is high) which comprises 56 data bits and 8 parity checking bits, allowing any single-bit error in it to be detected and corrected.
  • Parity bit 0: Check of all data bits
  • Parity bits 1, 2, 4, 8, 16, and 32: Checks of the data bits whose positions, expressed in binary, have that respective bit set.
  • Data bits 3, 5-7, 9-15, 17-31, and 33-55: Middle part of the calculator's Product ID
  • Data bits 56-57: Unknown
  • Data bits 58-62: "ASIC user flags"; must match the 80E0 field in an OS image. 01 = CAS, 00 = non-CAS.
  • Parity bit 63: Check of parity bits 1, 2, 4, 8, 16, and 32.

900B0000 - Power management

• 900B0000 (R/W): Clock speed load value
  • Bits 1-7: Multiply by 2 to get base/CPU ratio
  • Bit 8: If set, base clock is 27 MHz, else see bits 16-20
  • Bits 12-14: Add 1 to get CPU/AHB ratio
  • Bits 16-20: If bit 8 is clear, base clock is (300 - 6*this) MHz
• 900B0004 (W): ?
• 900B0008 (R/W): ?
• 900B000C (R/W): Clock speed control (write 4 to set the clock speed according to the value in 900B0000)
• 900B0010 (R/W): ON interrupt mask (1-bit). 1 if ON interrupt should be serviced or 0 if not.
• 900B0014 (R/W): Bit 0 is set if ON interrupt is requested. Bit 1 also causes an interrupt, but the cause is unknown (and it is not masked by [900B0010]). Write "1" bits to reset the requests.
• 900B0018 (R/W): ?
• 900B0020 (R/W): ?
• 900B0024 (R): Reads current clock speed value (see 900B0000 for details)
• 900B0028 (R): Bit 4 (0x10) clear when ON key pressed

900C0000 - First timer

Configured as ~488 Hz timer by the OS. Same port structure as Second timer.

900D0000 - Second timer

Two timers are located here, but only the first can generate IRQs. IRQ status/mask is located at [900A0020]. Configured as a ~100 Hz timer by the OS.

• 900D0000 (R/W): Current IRQ timer value (16-bit). Set to 32 by the OS. Increases/decreases each [900D0004] ticks. The value written to this port is saved internally and can be reloaded automatically upon timer completion, depending on settings in [900D0008].
• 900D0004 (R/W): IRQ timer divider (16-bit). Ticks per count - 1. Set to 9 by the OS. Ticks are approximately 32 kHz.
• 900D0008 (R/W): IRQ timer control (5-bit).
  • Bit 4: Set to 1 to freeze timer.
  • Bit 3: Set to 1 for increasing timer, or to 0 for decreasing timer.
  • Bits 2-0: If 0, timer will count to zero and stop. If 1-6, timer will complete when it reaches the corresponding timer completion value and then reload with original timer value (see [900D0018] to [900D002C]). If 7, timer will never complete and runs infinitely.
• 900D000C (R/W): Current timer value (16-bit). Increases/decreases each [900D0010] ticks. The value written to this port is saved internally and can be reloaded automatically upon timer completion, depending on settings in [900D0014].
• 900D0010 (R/W): Timer divider (16-bit). Ticks per count - 1. Ticks are approximately 32 kHz.
• 900D0014 (R/W): Timer control (5-bit).
  • Bit 4: Set to 1 to freeze timer, or 0 to run timer.
  • Bit 3: Set to 1 for increasing timer, or to 0 for decreasing timer.
  • Bits 2-0: If 0, timer will count to zero and stop. If 1-6, timer will complete when it reaches the corresponding timer completion value (see [900D0018] to [900D002C]). If 7, timer will not complete and runs infinitely.
• 900D0018 (R/W): Timer completion value 1 (16-bit). If IRQ timer equals this value, bit 0 of the interrupt status becomes set.
• 900D001C (R/W): Timer completion value 2 (16-bit). If IRQ timer equals this value, bit 1 of the interrupt status becomes set.
• 900D0020 (R/W): Timer completion value 3 (16-bit). If IRQ timer equals this value, bit 2 of the interrupt status becomes set.
• 900D0024 (R/W): Timer completion value 4 (16-bit). If IRQ timer equals this value, bit 3 of the interrupt status becomes set.
• 900D0028 (R/W): Timer completion value 5 (16-bit). If IRQ timer equals this value, bit 4 of the interrupt status becomes set.
• 900D002C (R/W): Timer completion value 6 (16-bit). If IRQ timer equals this value, bit 5 of the interrupt status becomes set.
• 900D0030 (R/W): Unknown 6-bit value.

900E0000 - Keypad

• 900E0000 (R/W): Unknown
• 900E0004 (R/W):
  • Bits 0-7: Number of rows to read (later rows are not updated in 900E0010-900E002F, and just read as whatever they were before being disabled)
  • Bits 8-15: Number of columns to read (later column bits in a row are set to 1 when it is updated)
• 900E0008 (R/W): Keypad interrupt status/acknowledge (3-bit). Bit 1 is set when a key is pressed or released. Bits 0 and 2 have an unknown cause. Write "1" bits to acknowledge.
• 900E000C (R/W): Keypad interrupt mask (3-bit). Set each bit to 1 if the corresponding event in [900E0008] should cause an interrupt.
• 900E0010-900E002F (R): Keypad data (one halfword per row; see below for map)
• 900E0030-900E0048 (R/W): Keypad GPIOs. Each register is 20 bits, with one bit per GPIO. The role of each register is unknown.

900E0010-900E001F - Keypad map

Each bit represents a key. If bit is cleared, the key is being pressed. Only bits 0 to 10 are used in each halfword. The mapping depends on the currently used keypad.

Clickpad keypad map:

offset	bit 0	bit 1	bit 2	bit 3	bit 4	bit 5	bit 6	bit 7	bit 8	bit 9	bit 10
0010	ret	enter	space	(-)	Z	.	Y	0	X	on	theta
0012	,	+	W	3	V	2	U	1	T	e^x	pi
0014	?	-	S	6	R	5	Q	4	P	10^x	EE
0016	:	*	O	9	N	8	M	7	L	x^2	i
0018	"	/	K	tan	J	cos	I	sin	H	^	>
001A	'	cat	G	)	F	(	E	var	D	caps	<
001C	flag	click	C	home	B	menu	A	esc	|	tab
001E	up	u+r	right	r+d	down	d+l	left	l+u	clear	ctrl	=

TI-84+ keypad map:

offset	bit 0	bit 1	bit 2	bit 3	bit 4	bit 5	bit 6	bit 7	bit 8	bit 9	bit 10
0010	down	left	right	up
0012	enter	+	-	*	/	^	clear
0014	(-)	3	6	9	)	tan	vars
0016	.	2	5	8	(	cos	prgm	stat
0018	0	1	4	7	,	sin	apps	X
001A		sto	ln	log	x^2	x^-1	math	alpha
001C	graph	trace	zoom	wind	y=	2nd	mode	del
001E

Touchpad keypad map:

offset	bit 0	bit 1	bit 2	bit 3	bit 4	bit 5	bit 6	bit 7	bit 8	bit 9	bit 10
0010	ret	enter		(-)	space	Z	Y	0	?!	on
0012	X	W	V	3	U	T	S	1	pi	trig	10^x
0014	R	Q	P	6	O	N	M	4	EE	x^2
0016	L	K	J	9	I	H	G	7	/	e^x
0018	F	E	D		C	B	A	=	*	^
001A		var	-	)	.	(	5	cat	frac	del	scratch
001C	flag		+	doc	2	menu	8	esc		tab
001E									shift	ctrl	,

900F0000 - HDQ/1-Wire and LCD contrast

The HDQ/1-Wire registers resemble those on the TI OMAP processors, and are possibly used to communicate with the wireless cradle. There is no conceivable reason for the LCD contrast register to be part of the same module, but here it is. :-(

• 900F0004 (W): Transmitted data
• 900F0008 (R): Received data
• 900F000C (R/W): Control/status
• 900F0010 (R): Interrupt status (automatically acknowledged when read)
• 900F0020 (R/W): LCD contrast. Valid values range from 0x6B to 0x95; normal value is 0x80

90100000 - TI-84 Plus link port

• 90100000 (W): Write to bits 0-1 to hold I/O link lines low or let them go high. 1=high, 0=low.
• 90100000 (R): Bits 0-1 hold the status of the I/O link lines. 1=high, 0=low. Bits 4-5 hold the last value outputted to bits 0-1.
• 90100004 (R/W): Unknown. The OS writes 0x80 here during initialization.
• 90100008 (R): Unknown. Bit 0x40 is set if any link lines are low.

90110000 - LED

• 90110B00 (R/W): Control register
  • Bit 0: Set this bit to enable green light blink data. If green blink data iteration is not on, the green light state is read from bit 0 of green blink data.
  • Bit 1: Set this bit and bit 6 to enable green blink data iteration.
  • Bit 2: Set this bit to force green light off. Overrides bit 4.
  • Bit 3: Set this bit to force red light off. Overrides bits 5 and 13.
  • Bit 4: Set this bit to force green light on.
  • Bit 5: Set this bit to force red light on.
  • Bit 6: See this bit and bit 1 to enable green blink data iteration. Reset before modifying green blink data or delay.
  • Bit 9: Set this bit to enable red light blink data. If red blink data iteration is not on, the red light state is read from bit 0 of red blink data.
  • Bit 10: Set this bit and bit 12 to enable red blink data iteration.
  • Bit 12: Set this bit and bit 10 to enable red blink data iteration. Reset before modifying red blink data or delay.
  • Bit 13: Forces red light on if bit 4 is 0, or red light off if bit 4 is 1. (?)
• 90110B04 (R/W): Green blink data. 32 bits of on and off state, represented by 1 and 0. Iteration is done from bit 31 to bit 0 repeatedly.
• 90110B08 (R/W): Green blink delay (negative). OS sets this to -2048.
• 90110B0C (R/W): Red blink data. 32 bits of on and off state, represented by 1 and 0. Iteration is done from bit 31 to bit 0 repeatedly.
• 90110B10 (R/W): Red blink delay (negative). OS sets this to -2048.

Note: If red and green lights are on at the same time, the color becomes yellow.

A0000000 - Unknown

The OS's page table has an entry to allow this to be accessed, but no known code does so.

A4000000 - Internal SRAM

A4000100-A40096FF is used by the OS as the LCD screen buffer. The upper left corner is the first byte. Each grayscaled pixel is 4-bit long. 1111 is white, 0000 is black.

A9000000 - SPI

Possibly used to communicate with the wireless cradle.

• A9000008 (R/W): Bits 0-3 = clock speed divider - 1
• A900000C (R/W): Bit 0 = set to 1 to start transfer; bits 16-21 = number of bits to transfer - 1
• A9000010 (R): Bit 0 set when transfer complete
• A9000014 (R/W): Unknown
• A9000018 (R/W): Unknown
• A900001C (R/W): Data register low
• A9000020 (R/W): Data register high

AC000000 - SD Host Controller

See http://www.sdcard.org/developers/tech/host_controller/simple_spec/

B0000000 - USB OTG controller

Appears similar to the USB interface on Freescale Semiconductor's microcontrollers.

• Module identification registers
  • B0000000 (R): ID - Identification register
  • B0000004 (R): HWGENERAL - General hardware parameters
  • B0000008 (R): HWHOST - Host hardware parameters
  • B000000C (R): HWDEVICE - Device hardware parameters
  • B0000010 (R): HWTXBUF - TX buffer hardware parameters
  • B0000014 (R): HWRXBUF - RX buffer hardware parameters
• Capability registers
  • B0000100 (R): CAPLENGTH - Capability registers length
  • B0000102 (R): HCIVERSION - Host controller interface version
  • B0000104 (R): HCSPARAMS - Host controller structural parameters
  • B0000108 (R): HCCPARAMS - Host controller capability parameters
  • B0000120 (R): DCIVERSION - Device controller interface version
  • B0000124 (R): DCCPARAMS - Device controller capability parameters
• Operational registers
  • B0000140 (R/W): USBCMD - USB command
  • B0000144 (R/W): USBSTS - USB status
  • B0000148 (R/W): USBINTR - USB interrupt enable
  • B000014C (R/W): FRINDEX - USB frame index
  • B0000150 (R): CTRLDSSEGMENT - 4G segment selector (always 0?)
  • B0000154 (R/W): PERIODICLISTBASE - Frame list base address
  • B0000158 (R/W): ASYNCLISTADDR - Next asynchronous list address
  • B000015C (R/W): TTCTRL - TT status and control
  • B0000160 (R/W): BURSTSIZE - Programmable DMA burst size
  • B0000164 (R/W): TXFILLTUNING - Host TT Xmit pre-buffer packet tuning
  • B0000180 (R): CONFIGFLAG - Configured flag register (always 1?)
  • B0000184 (R/W): PORTSC - Port status and control
  • B00001A4 (R/W): OTGSC - On-The-Go status and control
  • B00001A8 (R/W): USBMODE - USB device mode
  • B00001AC (R/W): ENDPOINTSETUPSTAT - Endpoint setup status
  • B00001B0 (R/W): ENDPTPRIME - Endpoint initialization
  • B00001B4 (R/W): ENDPTFLUSH - Endpoint de-initialize
  • B00001B8 (R): ENDPTSTATUS - Endpoint status
  • B00001BC (R/W): ENDPTCOMPLETE - Endpoint complete
  • B00001C0 (R/W): ENDPTCTRL0 - Endpoint control 0
  • B00001C4 (R/W): ENDPTCTRL1 - Endpoint control 1
  • B00001C8 (R/W): ENDPTCTRL2 - Endpoint control 2
  • B00001CC (R/W): ENDPTCTRL3 - Endpoint control 3

C0000000 - LCD controller

Probably an ARM PrimeCell PL110 or something compatible.

• C0000010 (R/W): Frame Base Address. Holds the address to read pixel data from. Set to A4000100 by the OS.
• C0000018 (R/W): Interrupt mask.
• C000001C (R/W): LCD Control
  • Bit 0: LCD controller enable.
  • Bits 1-3: LCD bits per pixel.
    • 000 = 1 bpp
    • 001 = 2 bpp
    • 010 = 4 bpp (default setting of OS)
    • 011 = 8 bpp
    • 100 = 16 bpp (holds the literal 16-bit palette value instead of a palette index)
    • 101 = 24 bpp (not applicable to TI-Nspire's STN LCD)
    • 110,111 = reserved
  • Bit 4: Set to 1 if STN LCD is monochrome, or 0 if color. Should be set to 1 on TI-Nspire.
  • Bit 5: Set to 1 if LCD is TFT, or 0 if STN. Should be set to 0 on TI-Nspire.
  • Bit 6: Set to 1 if monochrome STN LCD has a 8-bit interface, or 0 if 4-bit. Should be set to 1 on TI-Nspire.
  • Bit 7: Set to 1 if LCD is dual panel STN, or 0 if single-panel. Should be set to 0 on TI-Nspire.
  • Bit 8: Set to 1 if the palettes are read as BGR, or 0 if RGB. Since monochrome displays use only the R color, it is possible to store a secondary palette in the B color and switch instantly by flipping this bit.
  • Bit 9: Set to 1 if the bytes in each word are to be read as big-endian, or 0 if little-endian. Set to 0 by the OS.
  • Bit 10: Set to 1 if the pixels within each byte are to be read as big-endian, or 0 if little-endian. Only affects 1,2,4 bpp modes. Set to 1 by the OS.
  • Bit 11: LCD power enable.
  • Bits 12-13: Vertical compare interrupt region.
    • 00 = start of vertical synchronization
    • 01 = start of back porch
    • 10 = start of active video
    • 11 = start of front porch
  • Bits 14-15: Reserved.
  • Bit 16: LCD DMA FIFO Watermark Level.
  • Bits 17-31: Reserved.
• C0000020 (R): Raw interrupt status.
  • Bit 1: FIFO Underflow
  • Bit 2: LCD next address base update. Signifies that a new Frame Base Address value can be loaded for double-buffering.
  • Bit 3: Vertical compare. Set when one of four vertical regions (specified by the LCD Control register) is reached.
  • Bit 4: AHB Master bus error.
• C0000024 (R): Masked interrupt status.
• C0000028 (W): Interrupt clear. Write a 1 to each bit to clear.
• C000002C (R): Address that is currently being read from (approximate)
• C00000200-C00003FF (R/W): 256 color palettes (each is a half-word).
  • Bits 0-4: Red palette data (bits 1-4 are the grayscale data on the Nspire screen)
  • Bits 5-9: Green palette data (unused on Nspire)
  • Bits 10-14: Blue palette data (bits 11-14 are the grayscale data on the Nspire screen if BGR mode is set)
  • Bit 15: Intensity (unused on Nspire)

C4000000 - Analog-to-Digital Converter (ADC)

Used to check various voltages. Channels 1 ("VBATT"), 2 ("VSYS"), and 4 ("B12") are used to check the battery status; channel 3 is used to determine which keypad is in use.

• C4000000 (R): Active interrupts (4 bits per channel: bits 0-3 are for channel 0, etc)
• C4000004 (W): Interrupt acknowledge
• C4000008 (R/W): Interrupt mask
• C4000100-C40001FF: Per-channel registers (channel 0 starts at C4000100, channel 1 at C4000120, etc.)
  • +00 (R/W): Set bit 0 to start measurement; interrupt 1 will be set when complete
  • +10 (R): Read measured voltage. Scale for channels 1 and 2 is 155 units = 1 volt; scale for other channels is 310 units = 1 volt

C8010000 - Unknown

The only code using this is in an apparently unused function in BOOT1.

CC000000 - SHA-256 hash generator

Implements the SHA-256 hash algorithm, which is used in cryptographic signatures.

• CC000000 (R): Busy if bit 0 set
• CC000000 (W): Write 0x10 and then 0x0 to initialize. Write 0xE to process first block, 0xA to process subsequent blocks
• CC000008 (R/W): ?
• CC000010-CC00004F (W): 512-bit block
• CC000060-CC00007F (R): 256-bit state

DC000000 - Interrupt controller

See Interrupts.

Registers that operate on sets of IRQs are bitmaps, with bit 0 corresponding to IRQ 0, and so on.

• DC000000 (R): Masked IRQ status (always equal to [DC000004] & [DC000008])
• DC000004 (R): Raw interrupt status or sticky interrupt status, depending on bitfield in DC000204
• DC000004 (W): Resets a set of sticky interrupts
• DC000008 (R): Current set of enabled IRQs
• DC000008 (W): Enable a set of IRQs
• DC00000C (R): Mirror of DC000008
• DC00000C (W): Disable a set of IRQs
• DC000020 (R): Reads current IRQ number (no side effects)
• DC000024 (R): Reads current IRQ number, copies the value in DC00002C to DC000028, writes the priority of the current IRQ to DC00002C
• DC000028 (R): Reading this register will reset the IRQ request. The value read is whatever was last copied by reading DC000024
• DC00002C (R/W): 4-bit value. IRQs with priority greater than or equal to this value will not be requested
• DC000100 (R): Masked FIQ status (always equal to [DC000104] & [DC0000108])
• DC000104 (R): Raw interrupt status or sticky interrupt status, depending on bitfield in DC000204
• DC000104 (W): Resets a set of sticky interrupts
• DC000108 (R): Current set of enabled FIQs
• DC000108 (W): Enable a set of FIQs
• DC00010C (R): Mirror of DC000108
• DC00010C (W): Disable a set of FIQs
• DC000120 (R): Reads current FIQ number (no side effects)
• DC000124 (R): Reads current FIQ number, copies the value in DC00012C to DC000128, writes the priority of the current IRQ to DC00012C
• DC000128 (R): Reading this register will reset the FIQ request. The value read is whatever was last copied by reading DC000124
• DC00012C (R/W): 4-bit value. FIQs with priority greater than or equal to this value will not be requested
• DC000200 (R/W): Bits that are 0 will invert the corresponding raw interrupt status bit. Typically this register should hold 0xFFFFFFFF.
• DC000204 (R/W): Bits that are 1 will cause the corresponding bit in DC000004 and DC000104 to read the sticky interrupt status. Bits that are 0 will cause the corresponding bit to read the raw interrupt status.
• DC000208 (R/W): ?
• DC000300-DC0003FF (W): IRQ priority (0-7). One register per IRQ. Lower values indicate higher priority.