82801BA Datasheet, PDF (89/498 Pages) Intel Corporation – Intel 82801BA I/O Controller Hub 2 (ICH2) and Intel 82801BAM I/O Controller Hub 2 Mobile

Functional Description
5.4.2
5.4.3
Fixed Priority
The initial fixed priority structure is as follows:
High priority.....Low priority
(0, 1, 2, 3) (5, 6, 7)
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.
Rotating Priority
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) form 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. This is compatible with the 82C37 and Page Register
implementation used in the PC-AT. This mode is set after CPURST is valid.
Summary of DMA Transfer Sizes
Table 5-9 lists each of the DMA device transfer sizes. The column labeled "Current Byte/Word
Count Register" indicates that the register contents represents either the number of bytes to transfer
or the number of 16-bit words to transfer. The column labeled "Current Address Increment/
Decrement" indicates the number added to or taken from the Current Address Register after each
DMA transfer cycle. The DMA Channel Mode Register determines if the Current Address Register
is incremented or decremented.
82801BA ICH2 and 82801BAM ICH2-M Datasheet
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