21152 datasheet(64/148 Pages) INTEL

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5-2

21152 PCI-to-PCI Bridge Preliminary Datasheet

Address Decoding

5.2.1

I/O Base and Limit Address Registers

The 21152 implements one set of I/O base and limit address registers in configuration space that define

an I/O address range for downstream forwarding. The 21152 supports 32-bit I/O addressing, which

allows I/O addresses downstream of the 21152 to be mapped anywhere in a 4 GB I/O address space.

I/O transactions with addresses that fall inside the range defined by the I/O base and limit registers

are forwarded downstream from the primary PCI bus to the secondary PCI bus. I/O transactions

with addresses that fall outside this range are forwarded upstream from the secondary PCI bus to

the primary PCI bus.

The I/O range can be turned off by setting the I/O base address to a value greater than that of the

I/O limit address. When the I/O range is turned off, all I/O transactions are forwarded upstream,

and no I/O transactions are forwarded downstream.

Figure 5-1 illustrates transaction forwarding within and outside the I/O address range.

The 21152 I/O range has a minimum granularity of 4 KB and is aligned on a 4 KB boundary. The

maximum I/O range is 4 GB in size.

The I/O base register consists of an 8-bit field at configuration address 1Ch, and a 16-bit field at

address 30h. The top 4 bits of the 8-bit field define bits <15:12> of the I/O base address. The

bottom 4 bits read only as 1h to indicate that the 21152 supports 32-bit I/O addressing. Bits <11:0>

of the base address are assumed to be 0, which naturally aligns the base address to a 4 KB

boundary. The 16 bits contained in the I/O base upper 16 bits register at configuration offset 30h

define AD<31:16> of the I/O base address. All 16 bits are read/write. After primary bus reset or

chip reset, the value of the I/O base address is initialized to 0000 0000h.

Figure 5-1.

I/O Transaction Forwarding Using Base and Limit Addresses

LJ-04636.AI4

I/O Limit

Primary

Interface

Secondary

Interface

4KB

Multiple

I/O Base

I/O Address Space

21152 Datasheet(PDF) 64 Page - Intel Corporation