4. Addressing > I/O Space
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PEB383 User Manual
July 25, 2011
Integrated Device Technology, Inc.
Confidential - NDA Required
The Bus Master Enable bit must be set for any I/O transaction to be forwarded upstream. If this bit is
not set, all I/O transactions on the PCI bus are ignored.
If ISA Enable bit is set, the bridge does not forward any I/O transactions downstream that are in the top
768 bytes of each 1-KB block within the first 64 KB of address space. Only transactions in the bottom
256 bytes of each 1-KB block are forwarded downstream. If the ISA Enable bit is clear, then all
addresses within the range defined by the I/O base and limit registers are forwarded downstream. I/O
transactions with addresses above 64 KB are forwarded according to the range defined by the I/O base
and limit registers. If the ISA Enable bit is set, the bridge forwards upstream any I/O transactions on
the PCI bus that are in the top 768 bytes of each 1-KB block within the first 64 KB of address space,
even if the address is within the I/O base and limit. All other transactions on the PCI bus are forwarded
upstream if they fall outside the range defined by the I/O base and limit registers. If the ISA Enable bit
is clear, then all PCI bus I/O addresses outside the range defined by the I/O base and limit registers are
forwarded upstream.
A bridge uses the I/O Base and I/O Limit registers to determine whether to forward I/O transactions
across the bridge, as shown in
. The I/O address range defined by these registers is always
aligned to a 4-KB boundary and has a size granularity of 4 KB. A bridge forwards I/O read and I/O
write transactions from its PCIe Interface to its PCI Interface if the address is in the range defined by
the I/O base and I/O limit registers (when the base is less than or equal to the limit). Conversely, I/O
transactions on the PCI bus in the address range defined by these registers are not forwarded upstream
by the bridge. I/O transactions on the PCI bus that are outside the defined address range are forwarded
upstream.