Intel Extensible Firmware Interface - Manual (Page 441)

Protocols — PCI Bus Support
Version 1.10 12/01/02 12-23
Description
The Io.Read() , and Io.Write() functions enable a driver to access PCI controller registers in
the PCI root bridge I/O space.
The I/O operations are carried out exactly as requested. The caller is responsible for satisfying any
alignment and I/O width restrictions that a PCI root bridge on a platform might require. For
example on some platforms, width requests of
EfiPciWidthUint64 do not work.
If Width is EfiPciWidthUint8 , EfiPciWidthUint16 , EfiPciWidthUint32 , or
EfiPciWidthUint64 , then both Address and Buffer are incremented for each of the
Count operations performed.
If Width is EfiPciWidthFifoUint8 , EfiPciWidthFifoUint16 ,
EfiPciWidthFifoUint32 , or EfiPciWidthFifoUint64 , then only Buffer is
incremented for each of the Count operations performed. The read or write operation is
performed Count times on the same Address .
If Width is EfiPciWidthFillUint8 , EfiPciWidthFillUint16 ,
EfiPciWidthFillUint32 , or EfiPciWidthFillUint64 , then only Address is
incremented for each of the Count operations performed. The read or write operation is
performed Count times from the first element of Buffer .
All the PCI transactions generated by this function are guaranteed to be completed before this
function returns.
Status Codes Returned
EFI_SUCCESS The data was read from or written to the PCI root bridge .
EFI_INVALID_PARAMETER Width is invalid for this PCI root bridge.
EFI_INVALID_PARAMETER Buffer is NULL .
EFI_OUT_OF_RESOURCES The request could not be completed due to a lack of resources.