NH82801DB S L8DE Intel, NH82801DB S L8DE Datasheet - Page 93

• Manufacturer Part Number: NH82801DB S L8DE
• Manufacturer: Intel

Specifications of NH82801DB S L8DE

• Lead Free Status / RoHS Status: Compliant

5.4.1
5.4.1.1
5.4.1.2
5.4.2
Intel
®
82801DB ICH4 Datasheet
Rotating Priority
Channel Priority
For priority resolution, the DMA consists of two logical channel groups: channels 0–3 and
channels 4–7. Each group may be in either fixed or rotate mode, as determined by the DMA
Command Register.
DMA I/O slaves normally assert their DREQ line to arbitrate for DMA service. However, a
software request for DMA service can be presented through each channel's DMA Request Register.
A software request is subject to the same prioritization as any hardware request. See the detailed
register description for Request Register programming information in
Fixed Priority
The initial fixed priority structure is as follows:
The fixed priority ordering is 0, 1, 2, 3, 5, 6, and 7. In this scheme, channel 0 has the highest
priority, and channel 7 has the lowest priority. Channels 3–0 of DMA-1 assume the priority
position of channel 4 in DMA-2, thus taking priority over channels 5, 6, and 7.
Rotation allows for "fairness" in priority resolution. The priority chain rotates so that the last
channel serviced is assigned the lowest priority in the channel group (0–3, 5–7).
Channels 0–3 rotate as a group of 4. They are always placed between channel 5 and channel 7 in
the priority list.
Channel 5–7 rotate as part of a group of 4. That is, channels 5–7 form the first three positions in the
rotation, while channel group 0–3 comprises the fourth position in the arbitration.
Address Compatibility Mode
When the DMA is operating, the addresses do not increment or decrement through the High and
Low Page Registers. Therefore, if a 24-bit address is 01FFFFh and increments, the next address
will be 010000h, not 020000h. Similarly, if a 24-bit address is 020000h and decrements, the next
address will be 02FFFFh, not 01FFFFh. However, when the DMA is operating in 16-bit mode, the
addresses still do not increment or decrement through the High and Low Page Registers but the
page boundary is now 128 KB. Therefore, if a 24-bit address is 01FFFEh and increments, the next
address will be 000000h, not 010000h. Similarly, if a 24-bit address is 020000h and decrements,
the next address will be 03FFFEh, not 02FFFEh. This is compatible with the 82C37 and Page
Register implementation used in the PC-AT. This mode is set after CPURST is valid.
High priority.....Low priority
(0, 1, 2, 3)
(5, 6, 7)
Section
Functional Description
9.2.
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