Processor Configuration Registers
2.3.11
I/O Address Space
The system agent generates either DMI Interface or PCI Express bus cycles for all
processor I/O accesses that it does not claim. Configuration Address Register
(CONFIG_ADDRESS) and the Configuration Data Register (CONFIG_DATA) are used to
generate PCI configuration space access.
The processor allows 64K+3 bytes to be addressed within the I/O space. The upper 3
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 I/O 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
transaction will complete with an UR completion status.
I/O reads that lie within 8-byte boundaries but cross 4-byte boundaries are issued from
the processor as 1 transaction. It will be divided 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.
2.3.11.1
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 Device 1 Functions 0, 1, 2 or
Device 6 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 device 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.
38
Datasheet, Volume 2
INTEL [ INTEL ]
