Advanced Configuration and Power Interface Specification

ACPI Software Programming Model
IPMI operation regions require that all field elements be declared at command value granularity.
This means that each virtual register cannot be broken down to its individual bits within the field
definition.
Access to sub-portions of virtual registers can be done only outside of the field definition. This
limitation is imposed both to simplify the IPMI interface and to maintain consistency with the
physical model defined by the IPMI specification.
Since the system interface used for IPMI communication is determined by the _IFT object under the
IPMI device, there is no need for using of the AccessAs term within the field definition. In fact its
usage will be ignored by the operation handler.
For example, the register at command value 0xC1 for the power meter network function might
represent the command to set a BMC enforced power limit, while the register at command value
0xC2 for the same network function might represent the current configured power limit. At the same
time, the register at command value 0xC8 might represent the latest power meter measurement.
The following ASL code shows the use of the OperationRegion, Field, and Offset terms to represent
these virtual registers:
OperationRegion(POWR, IPMI, 0x3000, 0x100) // Power network function
Field(POWR, BufferAcc, NoLock, Preserve)
{
Offset(0xC1),
// Skip to command value 0xC1
SPWL, 8,
// Set power limit [command value 0xC1]
GPWL, 8,
// Get power limit [command value 0xC2]
Offset(0xC8),
// Skip to command value 0xC8
GPMM, 8
// Get power meter measurement [command value 0xC8]
}
Notice that command values are equivalent to the field element’s byte offset (for example,
SPWL=0xC1, GPWL=0xC2, GPMM=0xC8).
5.5.2.4.3.2 Declaring and Using IPMI Request and Response Buffer
Since each virtual register in the IPMI operation region represents an individual IPMI command, and
the operation relies on use of bi-directional buffer, a common buffer structure is required to
represent the request and response messages. The use of a data buffer for IPMI transactions allows
AML to receive status and data length values.
The IPMI data buffer is defined as a fixed-length 66-byte buffer that, if represented using a ‘C’-
styled declaration, would be modeled as follows:
typedef struct
{
BYTE Status;
BYTE Length;
BYTE[64]Data;
}
// Byte 0 of the data buffer
// Byte 1 of the data buffer
// Bytes 2 through 65 of the data buffer
Where:
• Status (byte 0) indicates the status code of a given IPMI command. See Section 5.5.2.4.3.3,
“IPMI Status Code,” for more information.
• Length (byte 1) specifies the number of bytes of valid data that exists in the data buffer. Valid
Length values are 0 through 64. Before the operation is carried out, this value represents the
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