ALT Access Mode

Intel® Core™ Ultra 200H and 200U Series Processors

Datasheet, Volume 1 of 2

ID	Date	Version	Classification
842704	05/27/2025		Public

Before entering a low power state, several registers from powered down parts may need to be saved. In the majority of cases, this is not an issue, as registers have read and write paths. However, several of the ISA compatible registers are either read only or write only. To get data out of write-only registers, and to restore data into read-only registers, the processor implements an ALT access mode.

Write Only Registers with Read Paths in ALT Access Mode

The registers described in below table have read paths in ALT access mode. The access number field in the table indicates which register will be returned per access to that port.

Write Only Registers with Read Paths in ALT Access Mode

Restore Data
I/O Addr	# of Rds	Access	Data
20h	12	1	PIC ICW2 of Primary controller
		2	PIC ICW3 of Primary controller
		3	PIC ICW4 of Primary controller
		4	PIC OCW1 of Primary controller¹
		5	PIC OCW2 of Primary controller
		6	PIC OCW3 of Primary controller
		7	PIC ICW2 of Secondary controller
		8	PIC ICW3 of Secondary controller
		9	PIC ICW4 of Secondary controller
		10	PIC OCW1 of Secondary controller¹
		11	PIC OCW2 of Secondary controller
		12	PIC OCW3 of Secondary controller
40h	7	1	Timer Counter 0 status, bits [5:0]
		2	Timer Counter 0 base count low byte
		3	Timer Counter 0 base count high byte
		6	Timer Counter 2 base count low byte
		7	Timer Counter 2 base count high byte
42h	1		Timer Counter 2 status, bits [5:0]
70h	1		Bit 7 = Read value is ‘0’. Bits [6:0] = RTC Address
Notes: The OCW1 register must be read before entering ALT access mode. Bits 5, 3, 1, and 0 return 0.

PIC Reserved Bits

Many bits within the PIC are reserved, and must have certain values written in order for the PIC to operate properly. Therefore, there is no need to return these values in ALT access mode. When reading PIC registers from 20h and A0h, the reserved bits shall return the values listed in table below.

PIC Reserved Bits Return Values

PIC Reserved Bits	Value Returned
ICW2(2:0)	000
ICW4(7:5)	000
ICW4(3:2)	00
ICW4(0)	0
OCW2(4:3)	00
OCW3(7)	0
OCW3(5)	Reflects bit 6
OCW3(4:3)	01