Advanced Configuration and Power Interface Specification

Device Configuration
Offset Field Name
Byte 2 Bit [7] Reserved (must be 0)
Bits [6:5] DMA channel speed supported, _TYP
00 Indicates compatibility mode
01 Indicates Type A DMA as described in the EISA
10 Indicates Type B DMA
11 Indicates Type F
Bits [4:3] Ignored
Bit [2] Logical device bus master status, _BM
0 Logical device is not a bus master
1 Logical device is a bus master
Bits [1:0] DMA transfer type preference, _SIZ
00 8-bit only
01 8- and 16-bit
10 16-bit only
11 Reserved
See Section 19.6.32, “DMA (DMA Resource Descriptor Macro),” for a description of the ASL
macro that creates a DMA descriptor.
6.4.2.3 Start Dependent Functions Descriptor
Type 0, Small Item Name 0x6, Length = 0 or 1
Each logical device requires a set of resources. This set of resources may have interdependencies
that need to be expressed to allow arbitration software to make resource allocation decisions about
the logical device. Dependent functions are used to express these interdependencies. The data
structure definitions for dependent functions are shown here. For a detailed description of the use of
dependent functions refer to the next section.
Table 6-194 Start Dependent Functions Descriptor Definition
Offset Field Name
Byte 0 Value = 0x30 or 0x31 (0011000nB) – Type = 0, small item name = 0x6, Length = 0 or 1
Start Dependent Function fields may be of length 0 or 1 bytes. The extra byte is optionally used to
denote the compatibility or performance/robustness priority for the resource group following the
Start DF tag. The compatibility priority is a ranking of configurations for compatibility with legacy
operating systems. This is the same as the priority used in the PNPBIOS interface. For example, for
compatibility reasons, the preferred configuration for COM1 is IRQ4, I/O 3F8-3FF. The
performance/robustness performance is a ranking of configurations for performance and robustness
reasons. For example, a device may have a high-performance, bus mastering configuration that may
not be supported by legacy operating systems. The bus-mastering configuration would have the
highest performance/robustness priority while its polled I/O mode might have the highest
compatibility priority.
If the Priority byte is not included, this indicates the dependent function priority is ‘acceptable’. This
byte is defined as:
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