Datasheet
Processor Configuration Registers
36 Datasheet, Volume 2
locations can be accessed only during I/O address wrap-around when address bit 16 is
asserted. Address bit 16 is asserted on the processor bus whenever an I/O access is
made to 4 bytes from address 0FFFDh, 0FFFEh, or 0FFFFh. Address bit 16 is also
asserted when an I/O access is made to 2 bytes from address 0FFFFh.
A set of I/O accesses are consumed by the internal graphics device if it is enabled. The
mechanisms for internal graphics IO decode and the associated control is explained
later.
The I/O accesses are forwarded normally to the DMI Interface bus unless they fall
within the PCI Express I/O address range as defined by the mechanisms explained
below. I/O writes are NOT posted. Memory writes to PCH or PCI Express are posted.
The PCI Express devices have a register that can disable the routing of I/O cycles to the
PCI Express device.
The processor responds to I/O cycles initiated on PCI Express or DMI with an UR status.
Upstream I/O cycles and configuration cycles should never occur. If one does occur, the
request will route as a read to Memory address 000C_0000h so a completion is
naturally generated (whether the original request was a read or write). The transaction
will complete with an UR completion status.
QPI I/O reads that lie within 8-byte boundaries but cross 4-byte boundaries are issued
from the processor as 1 transaction. The processor will break this into 2 separate
transactions. I/O writes that lie within 8-byte boundaries but cross 4-byte boundaries
will be split into 2 transactions by the processor.
2.2.9.1 PCI Express* I/O Address Mapping
The processor can be programmed to direct non-memory (I/O) accesses to the PCI
Express bus interface when processor initiated I/O cycle addresses are within the PCI
Express I/O address range. This range is controlled using the I/O Base Address
(IOBASE) and I/O Limit Address (IOLIMIT) registers in processor Device 1 or Device 6
(if a 2
nd
PEG port is enabled) configuration space.
Address decoding for this range is based on the following concept. The top 4 bits of the
respective I/O Base and I/O Limit registers correspond to address bits A[15:12] of an
I/O address. For the purpose of address decoding, the processor assumes that lower 12
address bits A[11:0] of the I/O base are zero and that address bits A[11:0] of the I/O
limit address are FFFh. This forces the I/O address range alignment to 4 KB boundary
and produces a size granularity of 4 KB.
The processor positively decodes I/O accesses to PCI Express I/O address space as
defined by the following equation:
I/O_Base_Address processor I/O Cycle Address I/O_Limit_Address
The effective size of the range is programmed by the plug-and-play configuration
software and it depends on the size of I/O space claimed by the PCI Express device.
The processor also forwards accesses to the Legacy VGA I/O ranges according to the
settings in the Device 1 configuration registers BCTRL (VGA Enable) and PCICMD1
(IOAE1), unless a second adapter (monochrome) is present on the DMI Interface/PCI
(or ISA). The presence of a second graphics adapter is determined by the MDAP
configuration bit. When MDAP is set, the processor will decode legacy monochrome IO
ranges and forward them to the DMI Interface. The IO ranges decoded for the
monochrome adapter are 3B4h, 3B5h, 3B8h, 3B9h, 3Bah and 3BFh.