Intel Extensible Firmware Interface - Manual (Page 766)

Extensible Firmware Interface Specification
18-6 12/01/02 Version 1.10
Description
The DEVICE_IO.Mem() , .Io() , and .Pci() functions enable a driver to access device
registers in the appropriate memory or I/O space.
The I/O operations are carried out exactly as requested. The caller is responsible for any alignment
and I/O width issues which the bus, device, platform, or type of I/O might require. For example on
IA-32 platforms, width requests of
IO_UINT64 do not work.
For Mem() and Io() , the address field is the bus relative address as seen by the device on the bus.
For Mem() and Io() the caller must align the starting address to be on a proper width boundary.
For Pci() , the address field is encoded as shown in Table 18-1. The caller must align the register
number being accessed to be on a proper width boundary.
Table 18-1. PCI Address
Mnemonic Byte
Offset Byte
Length
Description
Register 0 1 The register number on the function.
Function 1 1 The function on the device.
Device 2 1 The device on the bus.
Bus 3 1 The bus.
Segment 4 1 The segment number.
Reserved 5 3 Must be zero.
Status Codes Returned
EFI_SUCCESS The data was read from or written to the device.
EFI_INVALID_PARAMETER Width is invalid.
EFI_OUT_OF_RESOURCES The request could not be completed due to a lack of resources.